too much return air?

[RobFarns]

Is there such a thing as having too much return air?... I have a story and a half home and the 2 bedrooms upstairs are pretty cold. The return in one of the rooms works but the other dosn't draw at all. just wanted to know if you can have too much return air before i tried to put a working return in the one room.
Thanks,

[beenthere]

Its possible, but hard to have.

Remember, a return is not suppose to draw hard.

[George2]

Unless the doors are closed, the problem (cold rooms) is not because of lack of return air. Note: Assuming there is enough returns elsewhere.

[Shophound]

It's possible to have a return pull more air out of a room than is delivered to it, but not easy. And likely not the cause of your problem.

You can't heat a room comfortably if you never get enough heat into it from the start. Return air outlets don't heat rooms. Supply air inlets do via delivering the right amount of heated air to the room. The return is a way for mixed air in the room to get back to the furnace so it can be heated again. That is why it is called a "return".

[skippedover]

To answer your question, no, you normally cannot supply too much return air. If installed in a certain fashion, with bedroom doors closed, it is possible to put a single room or two under negative pressure but that won't affect the amount of heat entering the room substantially.

To answer your problem, I'd recommend a complete and thorough load analysis be done (room-by-room) followed by an equipment and duct analysis. I suspect that one of two issues exists. Either the supply outlets are inappropriately sized for one or more rooms (especially near the t-stat) or the main trunk is improperly designed and/or installed. Or it could be both, which would not surprise me in the slightest. But that's the only way to get a solid guarantee on fixing the problem, if the ducts are accessible.

[RobFarns]

I was just thinking that maybe the pressure in the ducts could be too high without one of the returns working and with the higher pressure the furnace couldn't push the air to the second floor?? sounds reasonable to me..lol anyway I guess if you guys don't think it will help I won't bother going through the work.. I guess the space heaters are working fine... Thanks everyone for the input!

[air2spare]

Is there such a thing as having too much return air?... I have a story and a half home and the 2 bedrooms upstairs are pretty cold. The return in one of the rooms works but the other dosn't draw at all. just wanted to know if you can have too much return air before i tried to put a working return in the one room.
Thanks,

are the rooms too cold when the A/C runs or heating?

[RoBoTeq]

Contrary to popular belief; returns don't suck. They are called "returns" because they are the accesses in which the higher pressure of an area due to supply air being pushed into the area allow the air to "return" to the blower.

There are situations where not having enough return static pressure can reduce air volume, but that is normally only if no return ducting at all is attached to the system.

[barbar]

Contrary to popular belief; returns don't suck. They are called "returns" because they are the accesses in which the higher pressure of an area due to supply air being pushed into the area allow the air to "return" to the blower.

There are situations where not having enough return static pressure can reduce air volume, but that is normally only if no return ducting at all is attached to the system.

Now that is a play on words!
You are correct energy flow is from positive to negative, so as the no such thing as negative energy (just less) "suck" is the wrong choice of word to describe energy flow. However we use the word "suck", to describe an area of pressure caused by an external force (fan suction, vacuum "hoover") less than the surroundings, so return ducts do "Suck"

[beenthere]

hihat123223, this is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise here. Please apply to the AOPC today, thank you.

You can find the rules for posting and qualifications here (http://hvac-talk.com/vbb/showthread.php?t=957002).


Further infractions may result in loss of posting privileges.

[genduct]

Now that is a play on words!
You are correct energy flow is from positive to negative, so as the no such thing as negative energy (just less) "suck" is the wrong choice of word to describe energy flow. However we use the word "suck", to describe an area of pressure caused by an external force (fan suction, vacuum "hoover") less than the surroundings, so return ducts do "Suck"

And the useless returns that don't suck...really suck!
Seems that obsevation is in the same vein as your negative energy obsevation

[motoguy128]

I'm not sure you can, within reason, have too much return air. Only unbalanced return distribution. That might be what the OP is experiencing. Returns, are only a return path to the blower. The supplies genrate the air movement and velocity needed to air distribution and mixing. Returns generate a slight negative air pressure in hte immediate airea they are located and can influence supply balance and gross air movement in a home.

[hearthman]

The blower creates a zone of pressure in the return plenum lower than that of the supply air causing ambient air such as that in the return plenum to enter the blower. There is no driving force pushing air into the return plenum. Instead you must make sufficient air available to the blower to feed the air being entrained into the blower. If there is a constriction that reduces the volume-not pressure of the return air, the blower could eventually cavitate. On the converse, making the entire outdoors available to the blower would not place any strain on the blower or change its performance. Blowing air back into the blower would.

The problem with various returns is balancing where you are drawing air from so you don't get pressurization of rooms and air stagnation. Returns should provide a point where the stale air can exit the room to be allowed to ooze back to the blower to get reconditioned.

[woody19]

Contrary to popular belief; returns don't suck. They are called "returns" because they are the accesses in which the higher pressure of an area due to supply air being pushed into the area allow the air to "return" to the blower.

There are situations where not having enough return static pressure can reduce air volume, but that is normally only if no return ducting at all is attached to the system.

Hmmmm, may have to disagree with you on that one. Took care of a sound stage at Warner Studios that used 100% osa and if you stood at the intake on the outside of the building you could feel the air being "sucked" in. In fact it would collect paper on the grille that had to be cleaned all the time. Sound stage was pressurized and air loss occurred whenever the doors were opened or through barometric dampers.
But I do understand what you are saying.

[RoBoTeq]

Now that is a play on words!
You are correct energy flow is from positive to negative, so as the no such thing as negative energy (just less) "suck" is the wrong choice of word to describe energy flow. However we use the word "suck", to describe an area of pressure caused by an external force (fan suction, vacuum "hoover") less than the surroundings, so return ducts do "Suck"
Vacuum systems are completely different then the forward/backward incline blower wheels that are used in HVAC.

It is all about pressure differentual. The difference in pressure in an HVAC system is due to the positive side of the system pushing air into the space. That same pressure is what pushes air in the space into the opening that is "returning" to the blower.

[RoBoTeq]

And the useless returns that don't suck...really suck!
Seems that obsevation is in the same vein as your negative energy obsevation
The only reason that air is not moving in any given return source is that there is no supply air pushing into that space.

[motoguy128]

If there is a constriction that reduces the volume-not pressure of the return air, the blower could eventually cavitate. On the converse, making the entire outdoors available to the blower would not place any strain on the blower or change its performance. Blowing air back into the blower would.

Might want to double check that. Cavitation only applies to water to hydraulic pump. A "cavity" or pocket of air is formed and that air gets compressed and superheated.

IF given unrestricted return air, the load on the blower is then only dependant on the restriction fo the supply duct.

A blower with less restriction has MORE load on it and consumes more power. A blower with a lot of restriction is "freewheeling" and has minimum restruction. Ever ridden a stationary bicycle wiht a little blower for the resistance? To increase resistance, you open up a slide plate on the return. To reduce resistance you block off the return.

The blower is less efficient when restricted, especially if it's
falls off" the fan curve. But it does consume less power. The reason it's understaood that restriction increases power consumption, is because you have ot increase the RPM's ot get the same CFM. At soem point you'll reach the blwoers maximum static pressure based on either availale motor HP or maximum RPM. The ability to generate static pressure is dependant on the vane clearances to the housing and the angle and overall design of hte blwoer.

[barbar]

Vacuum systems are completely different then the forward/backward incline blower wheels that are used in HVAC.

It is all about pressure differentual. The difference in pressure in an HVAC system is due to the positive side of the system pushing air into the space. That same pressure is what pushes air in the space into the opening that is "returning" to the blower.
Flow of any energy type is always about differential.
But you are wrong about your HVAC statement, and easy to prove.
No fan running the static pressure is atmospheric pressure 0 PSIG, if you like.
Our reference point "standard/normal"
We now have a fan discharging air into the atmosphere, air is drawn from a space (not totally air tight), the pressure is lower in the space than the standard.
The air that is being discharged has no influence on the standard pressure of the atmosphere, therefore the fan has to be sucking (lower than standard pressure) for flow to occur.
The pressure of the space does NOT need to be at a higher pressure than standard for flow to occur. Tha fan is undertaking mechanical work, to promote flow.

[RoBoTeq]

Flow of any energy type is always about differential.
But you are wrong about your HVAC statement, and easy to prove.
No fan running the static pressure is atmospheric pressure 0 PSIG, if you like.
Our reference point "standard/normal"
We now have a fan discharging air into the atmosphere, air is drawn from a space (not totally air tight), the pressure is lower in the space than the standard.
The air that is being discharged has no influence on the standard pressure of the atmosphere, therefore the fan has to be sucking (lower than standard pressure) for flow to occur.
The pressure of the space does NOT need to be at a higher pressure than standard for flow to occur. Tha fan is undertaking mechanical work, to promote flow.
OK, let's try it this way, if you have proper return air ducting throughout the house and add more return ducting to one room without increasing the supply in that room, will that oversized return now suck in more air then before it was increased in size?

[barbar]

OK, let's try it this way, if you have proper return air ducting throughout the house and add more return ducting to one room without increasing the supply in that room, will that oversized return now suck in more air then before it was increased in size?

This may seem to be a similar question, but it is not, your first statement was based upon energy flow, this one is based around system "equilibrium"
So you have increased the return duct size, you have the "potential" to increase the air mass flow rate at the same pressure drop or the same air mass flow rate at a smaller pressure drop. The limitation to the performance is directly related to the system as a whole. Because we are a loop "circular system" we have to make a stab at a start point "increased return duct" so less assume for easy we now have more air flow, the fan is doing what it does, the air passes to the supply ducting (this has not been changed), as we have more mass flow, the resistance will increase, causing a increased pressure drop. The pressure drop will cause the mass flow to drop. A point of equilibrium is reached.
Which has the greatest change on mass flow, the reduced resistance of the return system or the increased resistance on the supply system. This becomes system specific.
Because a fan is not a positive displacment mass mover, limitations are made upon pressure development (suction/discharge)

[RoBoTeq]

This may seem to be a similar question, but it is not, your first statement was based upon energy flow, this one is based around system "equilibrium"
So you have increased the return duct size, you have the "potential" to increase the air mass flow rate at the same pressure drop or the same air mass flow rate at a smaller pressure drop. The limitation to the performance is directly related to the system as a whole. Because we are a loop "circular system" we have to make a stab at a start point "increased return duct" so less assume for easy we now have more air flow, the fan is doing what it does, the air passes to the supply ducting (this has not been changed), as we have more mass flow, the resistance will increase, causing a increased pressure drop. The pressure drop will cause the mass flow to drop. A point of equilibrium is reached.
Which has the greatest change on mass flow, the reduced resistance of the return system or the increased resistance on the supply system. This becomes system specific.
Because a fan is not a positive displacment mass mover, limitations are made upon pressure development (suction/discharge)
JUST STOP!:gah:

The OP has larger then needed returns in an area. Does the larger return dictate the air flow in that area from the system by sucking the air into the system or does the supply air being distributed to that room determine how much air that room receives?

Nice try on the "baffle em with bullcrap" though....:grin2:

[barbar]

JUST STOP!:gah:

The OP has larger then needed returns in an area. Does the larger return dictate the air flow in that area from the system by sucking the air into the system or does the supply air being distributed to that room determine how much air that room receives?

BOTH and it is called "equilibrium"

Nice try on the "baffle em with bullcrap" though....:grin2:

Now Now Now, I only correct your mis-information:bump:

I only show you, your error of your ways!

Next thing you will be telling me is that your indoor fan has to be left on during a defrost! (heat pump cycle).

[genduct]

The only reason that air is not moving in any given return source is that there is no supply air pushing into that space.

RoBo, tho the extent that the slightly positive pressure created by the supply air (door is closed I guess) and the room walls etc aren't leaky, yes it helps "push the air into the return grill BUT
I think you need to agree with Barbar that the algebra suggest it is mostly the negative in the return duct that "pulls it back.

I suggest to the apprentices that the essential concept to understand the fan which is the heart/ lungs of the system is that the fan Sucks AND Blows.
Too much suck and not enough left over for blow

And yes the fan is more circulator then positive displacement pump

Hows that for being a referee?

[genduct]

The land where the water drains the "other way"

[genduct]

Might want to double check that. Cavitation only applies to water to hydraulic pump. A "cavity" or pocket of air is formed and that air gets compressed and superheated.

IF given unrestricted return air, the load on the blower is then only dependant on the restriction fo the supply duct.

A blower with less restriction has MORE load on it and consumes more power. A blower with a lot of restriction is "freewheeling" and has minimum restruction. Ever ridden a stationary bicycle wiht a little blower for the resistance? To increase resistance, you open up a slide plate on the return. To reduce resistance you block off the return.

The blower is less efficient when restricted, especially if it's
falls off" the fan curve. But it does consume less power. The reason it's understaood that restriction increases power consumption, is because you have ot increase the RPM's ot get the same CFM. At soem point you'll reach the blwoers maximum static pressure based on either availale motor HP or maximum RPM. The ability to generate static pressure is dependant on the vane clearances to the housing and the angle and overall design of hte blwoer.

THE HEARTHMAN IS RIGHT!~ The fan Surge area on the fan curve is what happens when the fan is starved for air just like with less than the net positive head you need for the intake of the other circulator with water

[beenthere]

Blowers don't suck. There is no such thing as negative pressure. Even a vacuum is nothing more then a lowered atmospheric pressure. A perfect vacuum is considered 30" of mercury, which is 0" PSIA.

[genduct]

No push and pull?
No ying and yang?
No suck and blow?

Maybe it is more scientific to think of cold as the absence of heat OR evil as the absence of good, craziest one is positive is the absence of those negative electrons but I think us regular guys believe in Evil or cold and think that fans suck. That's my story and I am sticking to it

I probably am beyond redemption even right after Easter, so pray for me

[beenthere]

Keep in mind. that in commercial apps where an economizer is used on a RTU. While its operating in economizer mode, the blower is bringing in outside air, so the returns have the air simply being pushed into them with no aid from the blower providing a "negative" pressure. And the rooms still have the same air circulation. So returns don't suck, and neither do blowers. LOL...

Stay sane Mike.

[barbar]

I have asked an independent, the wife "she who must be obeyed" about this suck issue, and got a smack around the head.
I did not bring up the blow issue, as i can not afford the health care insurance.

[genduct]

I have asked an independent, the wife "she who must be obeyed" about this suck issue, and got a smack around the head.
I did not bring up the blow issue, as i can not afford the health care insurance.

You have everything backwards! Winter in Summer (Christmas). drains going the wrong way. When the earth's magnetic core shifts again in 2012 Well you'll be OK and we will be suffering from confusion.
Only thing that doesn't change is the dominate Gender! Yes Dear! You're Right!
The only way any of us guys gets married is when the women decide to lower their standards

Cheers!

[energy_rater_La]

I LOVE THIS FORUM.
sorry for yelling..had to get past
the roaring in my brain.

[woody19]

Keep in mind. that in commercial apps where an economizer is used on a RTU. While its operating in economizer mode, the blower is bringing in outside air, so the returns have the air simply being pushed into them with no aid from the blower providing a "negative" pressure. And the rooms still have the same air circulation. So returns don't suck, and neither do blowers. LOL...

Stay sane Mike.

I agree that in this application the air is being "pushed" into the return ducts but if the fan isn't sucking the osa into the blower where is the air coming from? In my provious post I mentioned a sound stage where 100% of the air comes from outside of the building and it is sucked into the fan room and there are no return ducts. Just a thought for more discussion.

[beenthere]

I agree that in this application the air is being "pushed" into the return ducts but if the fan isn't sucking the osa into the blower where is the air coming from? In my provious post I mentioned a sound stage where 100% of the air comes from outside of the building and it is sucked into the fan room and there are no return ducts. Just a thought for more discussion.

Its not being sucked into the fan room. The higher air pressure from outside is "pushing" into the fan room.

[barbar]

A fan is a mechanical device that causes a "lower" energy area (Suckion or Suction) and uses an external force (electricity for example) to produce a "higher" energy area (Discharge "blow")
Without both no flow will occur.
The Lower and Higher are referenced to a standard/or application. Standard normally being PSIG, application is normally "absolute" PSIA. No difference to when you put gauges on a refrig system. Just a play on words.
Energy flows from positive to negative. So infact sucking does not occur, so sucking really describes production of a low pressure area.

[RoBoTeq]

I have asked an independent, the wife "she who must be obeyed" about this suck issue, and got a smack around the head.
I did not bring up the blow issue, as i can not afford the health care insurance.
Is it possible that because you are on the other side of the globe from us that your air sucks while our air blows?

A lowered pressure caused by the removal of air that is being pushed into a system does not constitue suction. Blowers remove gallons of air, push that air into rooms where the added air in the rooms find their way back to the blower to fill the hole left by the blower removing the gallon of air and creating an area to be filled back up. Once the first volume of air that the blower can scoop up and push has been executed, the cycle of air pushing by the blower is only moving the amount of air that the blower can scoop up and move each minute. As the air is scooped up and pushed into the supply ducts, the same amount of air that is being pushed by the blower is forced out of the rooms where the air is being pushed into and pushed into the area where the original amount of air was scooped out of.

[RoBoTeq]

Its not being sucked into the fan room. The higher air pressure from outside is "pushing" into the fan room.
Exactly. The blower is removing a chunk of air from the return side of the blower and air from the outside pours into the opening that is left.

[RoBoTeq]

A fan is a mechanical device that causes a "lower" energy area (Suckion or Suction) and uses an external force (electricity for example) to produce a "higher" energy area (Discharge "blow")
Without both no flow will occur.
The Lower and Higher are referenced to a standard/or application. Standard normally being PSIG, application is normally "absolute" PSIA. No difference to when you put gauges on a refrig system. Just a play on words.
Energy flows from positive to negative. So infact sucking does not occur, so sucking really describes production of a low pressure area.
If you are in a boat that has a hole in it below the water line, and water is entering that hole as fast as you can bail the water out, is that water being sucked into the hole in the boat?

[barbar]

If you are in a boat that has a hole in it below the water line, and water is entering that hole as fast as you can bail the water out, is that water being sucked into the hole in the boat?

Pressure equilibrium is being reach with the driving force being gravity.
Energy travels from positive to negative, the sea has greater pressure than the driving force of gravity.

Care of Wiki;

Suck may refer to:
Suction, the creation of a partial vacuum or region of low pressure

So a simple question is the inlet to a fan at a pressure (when flow is occurring) Multichoice (can not go wrong on this)
1 Lower than surroundings
2 Equal to the surroundings
3 Higher than the surroundings
4 Non of the above

[barbar]

Is it possible that because you are on the other side of the globe from us that your air sucks while our air blows?

A lowered pressure caused by the removal of air that is being pushed into a system does not constitue suction. Blowers remove gallons of air, push that air into rooms where the added air in the rooms find their way back to the blower to fill the hole left by the blower removing the gallon of air and creating an area to be filled back up. Once the first volume of air that the blower can scoop up and push has been executed, the cycle of air pushing by the blower is only moving the amount of air that the blower can scoop up and move each minute. As the air is scooped up and pushed into the supply ducts, the same amount of air that is being pushed by the blower is forced out of the rooms where the air is being pushed into and pushed into the area where the original amount of air was scooped out of.

You are simply describing the production of a low pressure region, if the low pressure region is made mechanically, then it becomes a suck.
If the area is already at a lower pressure (standard). Think of a balloon, the balloon is at higher pressure, the air that exits the balloon blows.
A vacuum creates a lower pressure than standard, the air that is drawn into the vacuum is "sucked" A fan is no different to a vacuum, it produces an area of lower pressure.
Removing a scoop, leaving a void "lower pressure"

[beenthere]

Think of a balloon, the balloon is at higher pressure, the air that exits the balloon blows.
A vacuum creates a lower pressure than standard, the air that is drawn into the vacuum is "sucked" A fan is no different to a vacuum, it produces an area of lower pressure.


The air "drawn" into a vacuum is no more "sucked" in then the air coming out of a balloon is "sucked" out. The air is blown out of a balloon, and air is blown into the area that has a lower pressure. It is not "sucked" into that area.

[barbar]

The air "drawn" into a vacuum is no more "sucked" in then the air coming out of a balloon is "sucked" out. The air is blown out of a balloon, and air is blown into the area that has a lower pressure. It is not "sucked" into that area.
If the word "Suck" is not a word, then you are correct. But if the word Suck is to be used, then if a lower pressure is "created", then Suck it does.
It is the dropping of pressure by means of mechanics, then the machine sucks.
I have clearly stated from dot one , that energy flows from positive to negative.
OK lets ask you a question, when does anything "SUCK", in this sense! Not "it is not very good" "it sucks"

[beenthere]

If the word "Suck" is not a word, then you are correct. But if the word Suck is to be used, then if a lower pressure is "created", then Suck it does.
It is the dropping of pressure by means of mechanics, then the machine sucks.
I have clearly stated from dot one , that energy flows from positive to negative.
OK lets ask you a question, when does anything "SUCK", in this sense! Not "it is not very good" "it sucks"

Never.

Its just a term used because most people can't understand the concept that a higher pressure flows toward a lower pressure.

The blower pushes the air out of it, that air that was in the blower has to be replaced. Its replaced by air pushing into its inlet/inlets.

Put a tissue against a return grille while the system is running. It is not held there by suction. Its held there by pressure pushing it against the grille.

A water pump in a basement doesn't suck water out of the well. The water pressure in the well pushes the water up to the pump.

[barbar]

Never.

Its just a term used because most people can't understand the concept that a higher pressure flows toward a lower pressure.

The blower pushes the air out of it, that air that was in the blower has to be replaced. Its replaced by air pushing into its inlet/inlets.

Put a tissue against a return grille while the system is running. It is not held there by suction. Its held there by pressure pushing it against the grille.

A water pump in a basement doesn't suck water out of the well. The water pressure in the well pushes the water up to the pump.

That being the case, and suck can not be used as term, neither can the word blow!
If the low pressure is drawn, then the high pressure displaces.
So the air into a room is not blown in, it displaces the air via higher pressure, or the room draws air because it is at a lower pressure.
The fact is fan has 2 points during flow, an area of lower pressure and an area of higher pressure, both are required to to achieve flow! Suck/Blow can be used together, as this is an understanding and referenced against the environment to which it is to be applied.
Check my all my posts, on agreement on energy flow. Positive to Negative

[RoBoTeq]

Let's use the term "air pressure". A blower scoops up a blower wheel full of air and pushes it away from the blower. This creates pressure in the air area where the air scooped up and pushed winds up at. At the same time, a void has been created in the area of the blower where the air was scooped up from. We refer to the pressure in that void "negative pressure". The physics behind what is going on has to do with pressure, not an absence of pressure. The void where the air that the blower scooped up is becomes filled with air that is directly or indirectly pushed into the void. No sucking has occured.

[genduct]

If you are in a boat that has a hole in it below the water line, and water is entering that hole as fast as you can bail the water out, is that water being sucked into the hole in the boat?

"leaky boat....this sucks!" I guess that's why we thought that was true

On a more serious note, the fan creates a pressure difference that allows or forces the air to more from one space to another

As far as the economizer example goes, may I point out that with a single fan (no return fan) that the economizer doesn't work very well if there is an extensive, ducted return. Also that a power exhauster is no a return fan "in the old days" barometric relief didn't allow a substantial amount of that free cooling

[Shophound]

Flow of any energy type is always about differential.
But you are wrong about your HVAC statement, and easy to prove.
No fan running the static pressure is atmospheric pressure 0 PSIG, if you like.
Our reference point "standard/normal"
We now have a fan discharging air into the atmosphere, air is drawn from a space (not totally air tight), the pressure is lower in the space than the standard.
The air that is being discharged has no influence on the standard pressure of the atmosphere, therefore the fan has to be sucking (lower than standard pressure) for flow to occur.
The pressure of the space does NOT need to be at a higher pressure than standard for flow to occur. Tha fan is undertaking mechanical work, to promote flow.

If the general rule of physics applies, in that greater always flows toward lesser, it applies in this case, of course. In a standard residential HVAC system, with one or more return openings and several supply openings, it can be said there is a "zone" of reduced pressure at each return provision. That said, the weight of air contained within the structure is the greater force that allows air to flow into the blower intake, in addition to the pressure difference created by the blower itself. It is not merely isolated to a low pressure void created by the blower at the blower intake due to a region of higher pressure created at the blower discharge. There always must be a greater force to seek equilibrium with an upset in the balance of things.

On the surface, this entire topic could fall into the "which came first; chicken or egg" syndrome, but I default to the order of operations I see everywhere in physics; i.e. greater moves toward lesser, in the never ending quest for equilibrium.

[woody19]

Its not being sucked into the fan room. The higher air pressure from outside is "pushing" into the fan room.

So if the fan is not operating the osa will "push" air into the fan room and papers can collect on the grill outside.

[Shophound]

So if the fan is not operating the osa will "push" air into the fan room and papers can collect on the grill outside.

If the fan is not running, and as a result paper and debris remain collected on the intake grill, there is a higher pressure holding them there, unless there is also adhesion that has occurred (the paper or debris has become "stuck" to the grill due to oils or moisture or dirt films or etc.) Absent an adhesion factor, it also means there is air movement into the intake even with the blower off. Meaning the weight of the atmosphere is holding the paper against the grill, and air is entering the intake to equalize an area within the building of lower air pressure.

Such could very well occur if the outdoor air fan room intake is near the bottom of the building. Natural stack effect will occur within the building, and the primary driver behind stack effect is atmospheric pressure combined with a temperature difference between outdoors and indoors (which creates an air density difference).

[beenthere]

That being the case, and suck can not be used as term, neither can the word blow!
If the low pressure is drawn, then the high pressure displaces.
So the air into a room is not blown in, it displaces the air via higher pressure, or the room draws air because it is at a lower pressure.
The fact is fan has 2 points during flow, an area of lower pressure and an area of higher pressure, both are required to to achieve flow! Suck/Blow can be used together, as this is an understanding and referenced against the environment to which it is to be applied.
Check my all my posts, on agreement on energy flow. Positive to Negative

I find using the phrase "the fan sucks in air" misleads most people on what is actually happening. And then gives them false notions on how air circulation works. leading them to think that return placement is critical.

[beenthere]

So if the fan is not operating the osa will "push" air into the fan room and papers can collect on the grill outside.

Yes its possible, if the air also has a way to escape the building.

On a mild temp day with a breeze blowing. Open 2 windows at your house, one on the windward side, and one on the opposite side.

Put a piece of tissue up to the screen, the air will hold it, and you don't have a fan running.

[barbar]

On the surface, this entire topic could fall into the "which came first; chicken or egg" syndrome, but I default to the order of operations I see everywhere in physics; i.e. greater moves toward lesser, in the never ending quest for equilibrium.

Eloquently written there Shophound.

I think the issues is not the understanding, but the choice of words, used to explain.
Should we use the term Suck or Suction!

[Shophound]

"Suction" is a term of convenience...does not take as long to say as "greater pressure entering intake to seek equilbrium with lesser pressure at intake of device". :)

Beenthere's water illustration...think on that a moment. You have a fairly deep tank and a pump at the top moving water out of the tank. As the water lowers, why does the pump not lose suction since the amount of water in the tank has decreased? Answer: atmospheric pressure. That is what keeps the pump primed on the intake side even as the water level drops (meaning the mass or weight of the water decreases).

A vacuum pump on a sealed refrigerant circuit does not "suck" air and non-condensables out of the tubing. The discharge of the pump, by creating a high pressure and then discharging it, creates a reduced pressure at the pump intake, which by comparison the higher pressure of the atmosphere within the tubing "pushes" vapors toward the vacuum pump inlet. When you hear a vacuum pump sales guy say his pump moves 8 CFM, suggesting it can pull down a system really fast, keep in mind that CFM number only happens when the pump is first opened to the system. After that it drops significantly.

[RoBoTeq]

Now I have to wonder if gravity is real or if the Earth sucks!

[Shophound]

Now I have to wonder if gravity is real or if the Earth sucks!


:LOL::LOL:

[barbar]

"Suction" is a term of convenience...does not take as long to say as "greater pressure entering intake to seek equilbrium with lesser pressure at intake of device". :)

Beenthere's water illustration...think on that a moment. You have a fairly deep tank and a pump at the top moving water out of the tank. As the water lowers, why does the pump not lose suction since the amount of water in the tank has decreased? Answer: atmospheric pressure. That is what keeps the pump primed on the intake side even as the water level drops (meaning the mass or weight of the water decreases).

A vacuum pump on a sealed refrigerant circuit does not "suck" air and non-condensables out of the tubing. The discharge of the pump, by creating a high pressure and then discharging it, creates a reduced pressure at the pump intake, which by comparison the higher pressure of the atmosphere within the tubing "pushes" vapors toward the vacuum pump inlet. When you hear a vacuum pump sales guy say his pump moves 8 CFM, suggesting it can pull down a system really fast, keep in mind that CFM number only happens when the pump is first opened to the system. After that it drops significantly.

I think I will stay with convenience in conversation!
We chose words that best explain to others what can be understood.
And the choice it most case is related to a reference point, so practically we feel that there is no pressure around us, so our reference point is Zero. A standard set of gauges. How often do state the correct term for preesure either being PSIG or PSIA (imperial), most just state a pressure PSI with an understanding that PSI actually means PSIG. I see the same with the suck and blow issue, the term is referenced to a set of conditions.
I am very much into those in our industry understanding Mass balance (or Energy Mass Balance) or equilibrium of a system (s). If we look at a refrig or AC system, we have many loops involved, some what similar in look to the Olympic rings, each riing could been seen as separate, but practically all are intertwined, and influence each other.
It is funny different industries use different terminology for similar devices.
In refrigeration we use swept volume, to determine mass flow, as the air industry uses a standard of "free air delivered FAD" , as you stated about the vac pump.

[barbar]

I find using the phrase "the fan sucks in air" misleads most people on what is actually happening. And then gives them false notions on how air circulation works. leading them to think that return placement is critical.
I think that return placement is critical, but I do not think this relates to what we are discussing.

[barbar]

Now I have to wonder if gravity is real or if the Earth sucks!
Did you not know that we have velcro on the bottom of our shoes, as being at the bottom of the world we might fall off.:gah:

[RoBoTeq]

Did you not know that we have velcro on the bottom of our shoes, as being at the bottom of the world we might fall off.:gah:
I had heard that about you guys. Then again, I also heard a rumor that you downunders think you are the ones who are rightside up and that we are the ones defying gravity. Just goes to show how silly some people can be.

[beenthere]

I think that return placement is critical, but I do not think this relates to what we are discussing.

Its not critical. And when people thing that the return/blower sucks air in, they tend to think that it is critical.

[barbar]

Ok , let move on this one.
I have explained the suck issue, or the use of the word in this application. No point going further on this.
Return air placement! Critical yes, random no! (not discounting the position of the supply system)
The air within area, is not just about flow but distribution, velocity and comfort, and the system as whole determines performance and efficiency.
If we take an extreme for example with a very high air change rate, temperture can be maintained evenly and with ease and placement of return is less important as the rooms internal velocity is the driving force for equilibrium, however such a room would be uncomfortable to be in. Practically most areas have dead spots and/or levels of stratification. We attempt to control the zone where the required conditions are used.
So to achieve greastest efficiency, it is best that the return air is as warm as possible for cooling and as cold as possible for heating (within the realms of the area). Optimization of correct return air position effect the Saturated Evap Pressure (cooling)
" Note that was quite a good shake"
and the saturated cond pressure (heating). Small changes in both can dramatically change both out put performance and efficiency.
So clearly correct positioning of the return air is critical.

[beenthere]

Return is not critical. Or else ductless mini splits wouldn't work. Nor would high velocity systems, which don't rely on exchanging the air in the house.

[barbar]

Same set of principles apply.
Working and working efficiently are 2 different things.
So you install a mini split for heating, where do you position it on a wall "Wall is 10 ft high"

[beenthere]

Same set of principles apply.
Working and working efficiently are 2 different things.
So you install a mini split for heating, where do you position it on a wall "Wall is 10 ft high"

Same height I do for cooling.

6 to 7' depending on whats on the opposite wall.

[barbar]

Same height I do for cooling.

6 to 7' depending on whats on the opposite wall.

Pretty normal.

Cooling first, as this what they are design for!
Discharge air blow horizontal, (velocity gives distance), density causes the cool air to fall. Air heated (the load) air rises. The return air is above the outlet, so warmest enters the unit. Warmest air highest SST, maximum performance and COP (EER I think you call it)

Heating
A bit info you may or may not know. Most mini splits have a temperature off set built into the algorithms, to compensate for what I am going to say. (If is set at 20C on the remote, the actual setting on the stat could be 25C, make specific)
When testing is undertaken, of such machines the constant is not room temp but the air onto the heat exchanger.

You are now in heating mode.
You want the heat at floor level, beacause this where we live. The air outlet is in a vertical position (close to with a bit of swing) The velocity pressure has to overcome the density change (gravity if you want), to reach the required area, as the air changes from laminar to turbulent, velocity is lost and we end up with a stratified area and poor/uneven heat distribution. (this why many swing functions). So far so good i hope.
Now get to the important bit about return air. AS the room is stratified, the warmest air of the room (within the airflow envelope), now enters the indoor unit. This is less dense so less mass flow goes through the unit, the reduced density increase the pressure drop through the coil (equilibrium is reach), and with the higher temperature, the saturated discharge temperature is high. Less efficient.
Now lest consider mounting at floor level (1 ft), the air now is set to exit horizontal again, we can get greater throw as we not fighting directly against gravity (holding at bay), The heat rise giving a more average temperature, more comfortable.
No the important bit, the return air is that enters is at close to the lowest possible temp (within the air flow envelope), this air is more dense, so more mass of air can pass through the heat exchanger, with less pressure drop, and the lower temp gives a much lower saturated discharge temp. Which uses less power, gives greater net refrigeration on the outdoor unit, which increase the total heat of rejection (more heat fro the area) Fundamental laws.
I fought the world over this issue and won.
Nowadays i would not do it as decent floor models are now available, at decent price.
The conclusion,
Better temp distribution
Better Heat Output
Lower Running cost
Greater comfort levels.
So can you see the importance of correct return air positioning. (the same principle is return air selection apply to ducted systems)

[beenthere]

On paper putting the unit close to the floor would look good. And make return position look important. but your example ignores the fact that you have now decreased your supply air's ability to cause movement at the higher level, and end up with stratification even while the unit is running.

Mounted up higher(between 6 and 7'), and the supply directed 45° down gives a good mixing of the air in the room, evening out the temp. The return position not mattering.

[barbar]

On paper putting the unit close to the floor would look good. And make return position look important. but your example ignores the fact that you have now decreased your supply air's ability to cause movement at the higher level, and end up with stratification even while the unit is running.

Mounted up higher(between 6 and 7'), and the supply directed 45° down gives a good mixing of the air in the room, evening out the temp. The return position not mattering.

Not a paper issue, Installed about 15,000 units this way (or i should say a CO-OP of engineers), had experts from Japan and a number of other countries visit, i was invited and went to Japan. To go through the process is more detail.
Air distribution is better on heating and more even, the same way that underfloor heating, is the most even heating system.
warm air raises naturally.
Air on temp always matters on the efficiency of a system..
Lets look at your argument of 45 degree angle.
If the unit is mounted at 7 ft, then your air will not hit the floor (if at all), until 7 ft out (of course we have some spread as it travel) so we end up with a very large dead spot between the wall and this point. Due to the angle the air hits the floor, the air will then deflect in a near vertical direction, velocity is removed. Little or no Coanda effect. So we end up with another dead spot of the other side of this intersection.
As the air raises in a more vertical direction we get increase stratification.
The reason for the stat of set on these types of unit.
Have you heard of the "cold feet syndrome" which very common with hiigh mounted mini split system. Why because high mounted mini split as cooling units.
The above issues are made worse with inverters in heating as the fan speed, normally follows the compressor speed, so as we get towards set point, the comp slow down, the fan slows, down, less air velocity, less chance of the air hitting low levels.
Most modern floor mounted consuls now have 2 different set of air distribution, low level exit for heating and high level exit for cooling (combination of both can also be chosen)
NOTE; If ambient does not drop much below 7C, then up high work OK, if ambient drops below 7C go for low mount.

[RoBoTeq]

Return is not critical. Or else ductless mini splits wouldn't work. Nor would high velocity systems, which don't rely on exchanging the air in the house.
While returns are not critical to system air displacement, there are many instances where return air placement can make a huge difference in the quality of the air movement.

For example, any multistoried house should have high returns on the upper level to allow the air that "returns" to the blower to be warmer air of stratification.

Also, for heat pump systems, it is good to have low wall returns in every room applicable when high supply regesters are used. Allowing the cooler air from lower in the rooms to "return" to the blower, the warmer air in the room is allowed to take the place of the cooler air that is "returned".

For the heating mode of lower temperature systems, such as heat pumps, it is also better to have the least amount of distance for the warmer supply air to have to travel to be returned to the blower in order to reduce drafts, which can be uncomfortable in the heating season.

[beenthere]

Not a paper issue, Installed about 15,000 units this way (or i should say a CO-OP of engineers), had experts from Japan and a number of other countries visit, i was invited and went to Japan. To go through the process is more detail.
Air distribution is better on heating and more even, the same way that underfloor heating, is the most even heating system.
warm air raises naturally.
Air on temp always matters on the efficiency of a system..
Lets look at your argument of 45 degree angle.
If the unit is mounted at 7 ft, then your air will not hit the floor (if at all), until 7 ft out (of course we have some spread as it travel) so we end up with a very large dead spot between the wall and this point. Due to the angle the air hits the floor, the air will then deflect in a near vertical direction, velocity is removed. Little or no Coanda effect. So we end up with another dead spot of the other side of this intersection.
As the air raises in a more vertical direction we get increase stratification.
The reason for the stat of set on these types of unit.
Have you heard of the "cold feet syndrome" which very common with hiigh mounted mini split system. Why because high mounted mini split as cooling units.
The above issues are made worse with inverters in heating as the fan speed, normally follows the compressor speed, so as we get towards set point, the comp slow down, the fan slows, down, less air velocity, less chance of the air hitting low levels.
Most modern floor mounted consuls now have 2 different set of air distribution, low level exit for heating and high level exit for cooling (combination of both can also be chosen)
NOTE; If ambient does not drop much below 7C, then up high work OK, if ambient drops below 7C go for low mount.


I wouldn't expect a VRV, inverter mini split or modulating gas furnace to do well for heating with high supplies, since the velocity of the supply air will often be too low to get a good throw and mix. In those cases, a low return is helpfull to cover up the low velocity problem. the poor supply air flow is the reason to need the return low. Other wise the return location wouldn't matter.

In a ducted system, the supply and return wouldn't be put close to each other, like a mini split is. The mini split is another example that return placement is not critical. With the exception of those units that decrease air flow that the supply can't throw and mix properly.

[barbar]

I wouldn't expect a VRV, inverter mini split or modulating gas furnace to do well for heating with high supplies, since the velocity of the supply air will often be too low to get a good throw and mix. In those cases, a low return is helpfull to cover up the low velocity problem. the poor supply air flow is the reason to need the return low. Other wise the return location wouldn't matter.

In a ducted system, the supply and return wouldn't be put close to each other, like a mini split is. The mini split is another example that return placement is not critical. With the exception of those units that decrease air flow that the supply can't throw and mix properly.

I agree with Robo, as far as displacement goes, positioning of either the supply or return is not important, "what goes in, has to come out" all we need to know for selection if the resistance to flow.
As far as positioning this does come down to the application and what are the exceptions are of the client.
So where I live heating is priority, and clients do not like drafts, so we can not use high velocity diffusers to mix the air evenly through a room(s), so by correct positioning of both supply and return, you end up with a more gentle and even room temp. So it is important to correctly select return air position for comfort and efficiency.
Our single biggest issue is Architects. There is never any allowance for air distribution services, so if client wants whole house heating, then all works have to be completed in the attic space (we do not have basements, or dedicated service areas)
95% of AC/Heat pumps sold down here are some form of variable refrigerant flow, either inverter or digital scroll.