Need another return duct in small house?

[Hot 'N Bothered]

The rule of thumb for properly sizing return air grilles is to multiply the H. x W. X 2.

400 cfm per ton. 3 X 400 = 1200 needed. 20 X 20 = 400 X 2 = 800.

I agree with the man that wanted to add the additional grille in the living room. Size accordingly.

Easiest and the best way. Also, there is absolutely nothing wrong with tranfer grilles if you want to add any.

If possible, try to avoid running ductwork in the (hot) attic.

Thanks -- this seems to make some sense, & I just noticed the formula you used on another website, as well. The salesman who recommended the additional 14x14 return in the living room was selling a 2-speed York unit, however -- not Carrier. Haven't heard much about York's reliability. So that may take me down a different road. In any case I do appreciate your advice.

[beshvac]

We buy an inexpensive 4" high filter rack that will take a 16 x 25 1" or 2" filter.

That will work for the 17.5" wide furnaces. They also make a 21" wide rack for 20 x 25 filters.

For that application that is great, we used to use the same thing but used the 4" pleated filter.

[Hot 'N Bothered]

Just out of curiosity, though -- still wondering if an AC system can be thrown out of balance by adding to a room a return duct that's larger than necessary (20x20 instead of 14x14). That's what one salesman claimed. To rephrase -- is it possible to have too much return in a room?

[Todd S 2]

Then the thumbrule for flow thru a filter grille of 2xhxw does not apply. A 20x20 Hart and Cooley 659 return grill will flow 1250 cfm at only a .028 static loss???.

* Tested without filters. Typical disposable 1-inch filter capacity is 2 cfm per square inch of gross filter area.
Recommended velocity is 300-400 fpm. Velocities higher than 500 fpm will decrease filter performance,
increase flow resistance, and possibly blow off agglomerates of collected dirt. Velocity measured 1” from face.

I have a H & C book also. My point is to try and reduce the air noise.

Can you move 1,200 cfm through a 20 x20? Sure. Quietly? Probably not.

The H & C book says you can move 1342 cfm at 700 fpm. Recommended fpm is 300-400 fpm. Unless that pertains to filters only. I would like to know.

[Hot 'N Bothered]

Just out of curiosity, though -- still wondering if an AC system can be thrown out of balance by adding to a room a return duct that's larger than necessary (20x20 instead of 14x14). That's what one salesman claimed. To rephrase -- is it possible to have too much return in a room?

Also -- if I don't add any returns at all, do I risk damaging the system or prematurely wearing out the system or ducts? Not really concerned about noise -- just maintenance problems.

[Todd S 2]

Just out of curiosity, though -- still wondering if an AC system can be thrown out of balance by adding to a room a return duct that's larger than necessary (20x20 instead of 14x14). That's what one salesman claimed. To rephrase -- is it possible to have too much return in a room?

A return system needs some restriction I've been told.

In other words it shouldn't just "free wheel"; i.e. without a blower door or something like that.

But I see so many homes that don't have enough return.

[Todd S 2]

Also -- if I don't add any returns at all, do I risk damaging the system or prematurely wearing out the system or ducts? Not really concerned about noise -- just maintenance problems.

With a E.C.M. motor (variable-speed) it will adjust to the restriction (both the supply and return).

As you take steps to reduce the restriction to the airflow, you'll hear the noise level decrease.

This will make the motor last longer and improve the efficiency.

[Hot 'N Bothered]

With a E.C.M. motor (variable-speed) it will adjust to the restriction (both the supply and return).

As you take steps to reduce the restriction to the airflow, you'll hear the noise level decrease.

This will make the motor last longer and improve the efficiency.

Thanks -- you've been very helpful.

You suppose there's a huge amount of difference between Carrier and York? I read on one thread someone saying that Carrier has better customer support in Florida.

[Todd S 2]

Thanks -- you've been very helpful.

You suppose there's a huge amount of difference between Carrier and York? I read on one thread someone saying that Carrier has better customer support in Florida.

I sell York but used to sell Carrier. I lean towards the Carrier (slightly).

That said, it ALWAYS depends on the company you're dealing with first and formost.

[Hot 'N Bothered]

Ok -- thanks a bunch. Huge help.

[airtight_tech]

Also -- if I don't add any returns at all, do I risk damaging the system or prematurely wearing out the system or ducts? Not really concerned about noise -- just maintenance problems.

Didn't notice if u mentioned wether it's a heat pump or gas furnace if it's a gas furnace high static can lead to burnt up motors and cracked heat exchangers (as little as 3 yrs depending on the severity). If it's a heat pump it could also have refrigeration problems.

[genduct]

Todd, the fan needs a "load" some duct hanging off of it...That's true Because as counter intuitive as it sounds the blower with out a load is moving Max air and will have Max amp draw even without any duct pressure And may even trip the breaker.
fan doesn't care where the duct resistance is coming from supply or return.

Opening up the return panel is not a good practice because it will give you the false impression that all's well When you put it back on the airflow characteristics change and hence you have not really understood anything

Make Sense?

[Todd S 2]

Todd, the fan needs a "load" some duct hanging off of it...That's true Because as counter intuitive as it sounds the blower with out a load is moving Max air and will have Max amp draw even without any duct pressure And may even trip the breaker.
fan doesn't care where the duct resistance is coming from supply or return.

Opening up the return panel is not a good practice because it will give you the false impression that all's well When you put it back on the airflow characteristics change and hence you have not really understood anything

Make Sense?

I had hear that 35 years ago from a guy that really knew his stuff.

But like you mentioned it's counter intuitive to logical thinking.

Thank you for confirming what I had been told. I think he called it "free wheeling".

[Hot 'N Bothered]

Didn't notice if u mentioned wether it's a heat pump or gas furnace if it's a gas furnace high static can lead to burnt up motors and cracked heat exchangers (as little as 3 yrs depending on the severity). If it's a heat pump it could also have refrigeration problems.

It's a heat pump. Outdoor line did freeze over once about a year and a half ago. Tech said evaporator coil leak and added freon.

[genduct]

counter intuitive Because everyone thinks the Horsepower is needed to create the pressure difference when the real work is "shoveling" that air that too many people don't realize has weight ? mass.
If air didn't have mass then Gravity wouldn't hold it down and we wouldn't have an atmosphere.

[Hot 'N Bothered]

Todd, the fan needs a "load" some duct hanging off of it...That's true Because as counter intuitive as it sounds the blower with out a load is moving Max air and will have Max amp draw even without any duct pressure And may even trip the breaker.
fan doesn't care where the duct resistance is coming from supply or return.

Opening up the return panel is not a good practice because it will give you the false impression that all's well When you put it back on the airflow characteristics change and hence you have not really understood anything

Make Sense?

Thanks genduct. Just to be sure I'm understanding your first paragraph -- you're confirming what Todd said, that it's best to have some restriction on the return -- i.e., it's possible to have a return that's too large?

And -- I'm not sure I understand your second paragraph about it being a bad idea to open up the return panel and then put it back on? (Apologies -- am very new to AC terminology).

[beshvac]

Thanks genduct. Just to be sure I'm understanding your first paragraph -- you're confirming what Todd said, that it's best to have some restriction on the return -- i.e., it's possible to have a return that's too large?

And -- I'm not sure I understand your second paragraph about it being a bad idea to open up the return panel and then put it back on? (Apologies -- am very new to AC terminology).

It is possible but not likely. Airhandlers are designed to move a certain amount of air at a certain amount of "backpressure". The same applies to a return and the path of least resistance. You could have a 20x20 return grill right next to the airhandler and when you run a duct to your master bedroom on the other side of the house....most of the return air is still going to come from the close return unless steps are taken to equalize the friction on both return paths.

[genduct]

You need a short ( not too short or it will be noisy) return path with a grill that is usually about 1 sf per ton of AC. You need a path for the air to get back to the unit but that can be transfer or jump ducts with grills and a properly sized supply duct, I like ductboard because it is insulated and sealed and if you like you can have short flex branches in areas that are and will always be accessible.
That about sums up my approach to residential duct

[Hot 'N Bothered]

You need a short ( not too short or it will be noisy) return path with a grill that is usually about 1 sf per ton of AC. You need a path for the air to get back to the unit but that can be transfer or jump ducts with grills and a properly sized supply duct, I like ductboard because it is insulated and sealed and if you like you can have short flex branches in areas that are and will always be accessible.
That about sums up my approach to residential duct

Hmm. Your approach (1 sf per ton of AC) would suggest I might not need another return -- a 21x21 return covers 3 tons, so my 20x20 might squeak by for a 3 ton variable system. Todd's formula, on the other hand, suggests I probably need an extra 14x14.

[Hot 'N Bothered]

It is possible but not likely. Airhandlers are designed to move a certain amount of air at a certain amount of "backpressure". The same applies to a return and the path of least resistance. You could have a 20x20 return grill right next to the airhandler and when you run a duct to your master bedroom on the other side of the house....most of the return air is still going to come from the close return unless steps are taken to equalize the friction on both return paths.

What kind of steps might equalize the friction?