Sorry but I have to say this, The Evolution control would give you the cfm.

That said, measuring cfm at the registers is not the preferred way to do this...what about duct loss? This is a problem that is difficult to deal with, even for most pros. The best way is to take a good static reading at the inlet and outlet of the furnace coil and compare to the manufacturers pressure drop chart. Even this is a good estimate, not necessarily a absolute true measurement.

You are too easy a customer. What do you mean your contractor will not balance the duct......make him do it.

 

If using a garbage were an accurate way to measure airflow then all of the professional's would not be spending thousands of dollars for a flow hood and instruments to do that.

 

>>Research so far indicates the "garbage bag" method of measuring system airflow at registers is actually comparable...

I've gotta ask you what is this research. I have heard of this method from a Canadian source but have never heard anything good supporting it from this forum. And I think whenever you have a concensus on this board, they tend to be right. For one thing you must measure the seconds to fill, and the precision of your stopwatch will limit the precision (and therefore accuracy) of your CFM figure. If you go ahead and do some measurements this way, I am sincerely eager to hear how it turns out! But in the meantime I have to wonder if there could be better methods.

The one super-amateur method I know of, would use a cardboard box placed over the supply vent, with a measured hole for air to exit. You need the hole large enough that it won't disrupt total airflow much, and small enough that you have a measurable pressure difference between the box and atmospheric. Then you might use a manometer to measure that pressure, and there are equations relating that to airflow. Everything has its drawbacks and this method would certainly be more complicated than the fill-a-bag method. And I have not done it myself, so who knows whether your method might be more practical.

Dwyer makes a pitot tube for use with its manometers, which could be used to measure air speed. By taking various measurements you could get a semi-good measurement of CFM, the pros have a method for this and I better let them describe it. If you are interested of course.

But to get really *good* measurements, the pros use flow hoods costing thousands and those must be calibrated frequently. I really don't think they would do this if a gas-bag method could be developed that would give good enough results.

Just expressing my questions on this. By all means I ask you to share your experiences and results on this board if it does not run afoul of the DIY rules.

Best of luck -- Pstu

 

But to get really *good* measurements, the pros use flow hoods costing thousands and those must be calibrated frequently. I really don't think they would do this if a gas-bag method could be developed that would give good enough results.

Exactly.

 

Are we talking Gas or Heat pump?

Easiest way to calculate air flow is to run the airhandler at the fan speed (e.g. cooling speed, etc.) you are curious about with the electric heat on. Measure the delta T across the equipment out of line of sight of the strips, the volts and amp draw of the electric heat only (not the blower).

Cfm = BTU out / 1.08 * delta T

To help populate the equation...

Watts = Amps * Volts
Electric strips put out 3.413 BTUs per Watt.

Same thing works on a gas furnace but you have to clock the gas meter and know the BTU value of the fuel gas. Or you can get close by making sure the gas pressure is correct and then going Input Btu's * AFUE but that's your call.

-Brent

 

Reading what comes out of the registers and what goes through the coil are generally two different numbers as a result of duct leakage.

The feedback from the ECM is based on blower perfromance curves, torque and RPM, it doesnt read CFM, but it doesnt need to either. As with any ECM, assuming it is working properly, as long as you are below an inch, you should get what you ask of it.

You can often calculate airflow with a PSC provided you knew the watts and had the blower performance charts to match. You can use the temperature rise method and get close. The static pressures will also tell you airflow if you have the charts. If you wanted a run by run calculation, then accurate velocity pressure and duct size will give you what you need. There are many ways to get close mesurements, and no good reason not to.

 

The feedback from the ECM is based on blower perfromance curves, torque and RPM, it doesnt read CFM, but it doesnt need to either.
.

While this is technically true, the Infinity/Evolution control can provide a CFM reading from this information. We have checked both the CFM reading and the static reading many times and have always found them to be very accurate.

The OPs furnace is compatible with this control and if he had it installed he could see the CFM output. Of course if he had the control it would automatically set the proper airflow settings for his system and he would not have to be worried about proper settings of the dip switches. Now duct balance is a different problem, so far I do not believe there is a control for that.

 

Re: Are we talking Gas or Heat pump?

Easiest way to calculate air flow is to run the airhandler at the fan speed (e.g. cooling speed, etc.) you are curious about with the electric heat on. Measure the delta T across the equipment out of line of sight of the strips, the volts and amp draw of the electric heat only (not the blower).

Cfm = BTU out / 1.08 * delta T

To help populate the equation...

Watts = Amps * Volts
Electric strips put out 3.413 BTUs per Watt.

Same thing works on a gas furnace but you have to clock the gas meter and know the BTU value of the fuel gas. Or you can get close by making sure the gas pressure is correct and then going Input Btu's * AFUE but that's your call.

-Brent

This is a fairly accurate method for calculating CFM on PSC motors, but not on variable speed motors.

 

Re: Re: Are we talking Gas or Heat pump?

Easiest way to calculate air flow is to run the airhandler at the fan speed (e.g. cooling speed, etc.) you are curious about with the electric heat on. Measure the delta T across the equipment out of line of sight of the strips, the volts and amp draw of the electric heat only (not the blower).

Cfm = BTU out / 1.08 * delta T

This is a fairly accurate method for calculating CFM on PSC motors, but not on variable speed motors.

I don't know a thing about carrier stuff but I do know that
physical laws don't change just because you have a variable speed drive. The equation always works because it is based on thermodynamics and independent of motor type unless you're leaving the door off the equipmet while you measure or some other obvious procedural mistake.

DocHoliday is exactly right about how variable speed drives work, they just look at a fan curve based on torque and rpm programmed into the black module sitting on the end of the motor. They're really great, but not magic.

 

Contractor keeps telling me he did balance the system, that holding his hand up to the register and feeling the air flow is the standard methodology used by all HVAC people in this area and I won't find anyone who would do it any differently. I simply have to wait until it gets real cold then run the heater then go sit in each room. If a room is cold I increase flow to that room. If a room is hot I decrease flow to that room.

Interestingly, when I then ask him why the end result of his "balancing" was that all dampers on all ducts are wide open his answer is that the installation of the ducting was obviously perfect. The ducting was perfectly sized and perfectly run. That argument then tells me that either the system is perfectly balanced or I'm screwed because I'll not be able to increase flow to one room without decreasing flow to another.

So I appear to only have one option, to figure out as close an estimate of the current state of balancing (what each register is putting out) based on Manual J and then show this to the contractor (at which time of course I will have balanced the system myself).

 

Re: Are we talking Gas or Heat pump?

Easiest way to calculate air flow is to run the airhandler at the fan speed (e.g. cooling speed, etc.) you are curious about with the electric heat on. Measure the delta T across the equipment out of line of sight of the strips, the volts and amp draw of the electric heat only (not the blower).

Cfm = BTU out / 1.08 * delta T

To help populate the equation...

Watts = Amps * Volts
Electric strips put out 3.413 BTUs per Watt.

Same thing works on a gas furnace but you have to clock the gas meter and know the BTU value of the fuel gas. Or you can get close by making sure the gas pressure is correct and then going Input Btu's * AFUE but that's your call.

-Brent

wouldn't it be easier to get a flow hood or pitot-traverse of the duct?
what is the % error of the temperature gradient 'method'?

 

First, some links for anyone else interested in violating the covenant and playing around with the "garbage bag" method of air flow measurements:

Here's a very good study as to the accuracy of slow measurements in general, including the method under discussion here. Interestingly, it proposes that all the many thousands of dollars being spent on high-tech flow hoods and such may well be little more than show-boating with little net worth (sorry for the long link, but it is worth it for those truly interested):
http://eetd.lbl.gov/ie/pdf/LBNL-51550.pdf#search=""garbage bag" "air flow" "homeowner""

This one is simply a useful place for when you want to figure out the volume of your garbage bag, convert cubics, etc.:
http://www.metric-conversions.org/volume

This is the "source" of the method:
http://www.cmhc.ca/en/co/maho/yohoyohe/inaiqu/inaiqu_003.cfm

 

You won't find any airflow hood's in our area either. It's done by feel if it's done at all. No damper's either! Should a HO complain a lot about a particular vent the contractor will grudgingly adjust it by increasing or decreasing the duct tape over the end of the boot. Damper's cost a buck(hypotheticly) so the contractors use the duct tape across the boot(or takeoff) to limit the airflow. Standard terms are 1/4, 1/2, 3/4 boot's. HO's don't have a clue & it doesn't help a tech either but the contractors do it anyway. Your lucky they at least use dampers there!

 

OK. So putting aside any discussion of the method itself for now ---

I could use a basic lesson on air flow in general. Assume for the point of discussion that the blower is capable of maintaining a set CFM output. I am gathering from these discussions that this is in fact the norm for a variable speed blower in a Bryant Plus90I furnace (and comparable ones).

So then if the blower is maintaining 1,200 CFM then does this imply that 1,200 CFM of air is going into the Plenum (attached at the output of the blower)? Logic tells me yes, but logic often fails.

Assume for now the answer is "yes", then where does the 1,200 CFM of air go?

Air does not compress well (the HVAC system certainly can't compress air), so therefore it is another logical assumption that 1,200 CFM of air is either exiting the plenum via the ductwork attached to it or it is sneaking back around the blower (a loop). Assume for now it isn't sneaking, therefore it is all going into the ductwork.

So now there is 1,200 CFM of air being pushed out into 'x' ducts. Where does the air go?

Assume for discussion that the ductwork is air-tight. So then (using same logic as before that air doesn't compress) it must be exiting the ducts via the registers.

So then assuming one could measure the air flow with total precision, why would one not expect to find a total of 1200 CFM of air being pushed out of the registers?

 

Re: Re: Are we talking Gas or Heat pump?

Easiest way to calculate air flow is to run the airhandler at the fan speed (e.g. cooling speed, etc.) you are curious about with the electric heat on. Measure the delta T across the equipment out of line of sight of the strips, the volts and amp draw of the electric heat only (not the blower).

Cfm = BTU out / 1.08 * delta T

wouldn't it be easier to get a flow hood or pitot-traverse of the duct?
what is the % error of the temperature gradient 'method'?

This is easiest for me b/c it uses the tools I carry to do my daily service work (thermocouple, & amp meter / voltmeter). As with everything, the quality of your measurement is what determines the accuracy of your result. Perfect measurements yield a perfect result, if you are using one of them $10 stick thermometers and have the filters out and the doors off then your result is junk.

 

OK. So putting aside any discussion of the method itself for now ---

I could use a basic lesson on air flow in general. Assume for the point of discussion that the blower is capable of maintaining a set CFM output. I am gathering from these discussions that this is in fact the norm for a variable speed blower in a Bryant Plus90I furnace (and comparable ones).

So then if the blower is maintaining 1,200 CFM then does this imply that 1,200 CFM of air is going into the Plenum (attached at the output of the blower)? Logic tells me yes, but logic often fails.

Assume for now the answer is "yes", then where does the 1,200 CFM of air go?

Air does not compress well (the HVAC system certainly can't compress air), so therefore it is another logical assumption that 1,200 CFM of air is either exiting the plenum via the ductwork attached to it or it is sneaking back around the blower (a loop). Assume for now it isn't sneaking, therefore it is all going into the ductwork.

So now there is 1,200 CFM of air being pushed out into 'x' ducts. Where does the air go?

Assume for discussion that the ductwork is air-tight. So then (using same logic as before that air doesn't compress) it must be exiting the ducts via the registers.

So then assuming one could measure the air flow with total precision, why would one not expect to find a total of 1200 CFM of air being pushed out of the registers?

Actually, air is quite compressable. It's a gas! You can compress it in a ballon with your lips dude. Why do you think fan curves are based on static pressure?

By the way, your Manual J will show you that the cfm requirements are difference for each room when heating versus cooling.

Ideally if the system is moving 1200 cfm, it's pulling 1200 cfm out of the space and putting 1200 cfm into the space. In reality your duct system leaks at least 10% and probably more like 15-20. This is a problem if the duct system is in an attic or crawl space and not as big a deal if it's inside the envelope of the home such as in the basement. So it is very easy to measure less than 1200 cfm at your return grill and less than 1200 cfm if you sum all your supply outlets but if the variable speed drive is set to 1200cfm and total system static is reasonable the box is pulling 1200 cfm total from somewhere and pushing 1200 cfm total too somewhere. Honest.

We don't have balancing dampers here either, they just make noise and add to your total system static anyhow. They won't fix a bad duct system b/c air is COMPRESSIBLE so unlike closing a water valve, closing an air damper just creates a high pressure zone in front of the damper, it doesn't neccessarily force the air to squirt out the other holes with more force.

For you techies out there I have ignored a balancing dampers effect on velocity pressure and static regain since the conversion from static pressure to velocity pressure and back is outside the scope of this rant and only happens in the even of an aerodyanimc take off anyhow.


[Edited by stafford on 09-21-2006 at 01:23 PM]

 

Re: Re: Re: Are we talking Gas or Heat pump?

This is a fairly accurate method for calculating CFM on PSC motors, but not on variable speed motors. [/B]

I don't know a thing about carrier stuff but I do know that
physical laws don't change just because you have a variable speed drive. The equation always works because it is based on thermodynamics and independent of motor type unless you're leaving the door off the equipmet while you measure or some other obvious procedural mistake.

DocHoliday is exactly right about how variable speed drives work, they just look at a fan curve based on torque and rpm programmed into the black module sitting on the end of the motor. They're really great, but not magic. [/B][/QUOTE]

You are correct, physical laws do not change. However the way variable speed motors are used do change. A standard PSC air handler at best uses 2 speeds for normal operation, one for cool and one for heat. Variable speed air handlers use many different speeds, cooling, heating, strip heat, dehumidification, and sometimes more. If you are not careful, you will test the cfm in the emergency heat mode and then if you put it into any other mode the cfm will be different so you test will not be of value. My point is if the cfm is calculated in the emergency heat mode, you can not say the cfm will be the same in the cooling mode like you often can with a PSC motor. If you are only wanting to check cfm in one mode, then yes the formula will work. But that is not what most people are attempting to do when they use this test.

 

Re: Re: Re: Re: Are we talking Gas or Heat pump?

This is a fairly accurate method for calculating CFM on PSC motors, but not on variable speed motors.

I don't know a thing about carrier stuff but I do know that
physical laws don't change just because you have a variable speed drive. The equation always works because it is based on thermodynamics and independent of motor type unless you're leaving the door off the equipmet while you measure or some other obvious procedural mistake.

DocHoliday is exactly right about how variable speed drives work, they just look at a fan curve based on torque and rpm programmed into the black module sitting on the end of the motor. They're really great, but not magic. [/B]

You are correct, physical laws do not change. However the way variable speed motors are used do change. A standard PSC air handler at best uses 2 speeds for normal operation, one for cool and one for heat. Variable speed air handlers use many different speeds, cooling, heating, strip heat, dehumidification, and sometimes more. If you are not careful, you will test the cfm in the emergency heat mode and then if you put it into any other mode the cfm will be different so you test will not be of value. My point is if the cfm is calculated in the emergency heat mode, you can not say the cfm will be the same in the cooling mode like you often can with a PSC motor. If you are only wanting to check cfm in one mode, then yes the formula will work. But that is not what most people are attempting to do when they use this test.

[/B][/QUOTE]

Plain Spoken, you are correct, which is why I stated in the first sentence to run the air handler at the speed you want to check. He said it's a furnace anyhow so he really needs to clock the meter and then multiply by the BTU value of the fuel gas. Checking the speed of a variable drive is silly anyhow since if the static is reasonable they'll deliver exactly what they're set to and the temp rise method is super easy on PSC stuff. Didn't mean to sound like a weenie.

 

Contractor keeps telling me he did balance the system, that holding his hand up to the register and feeling the air flow is the standard methodology used by all HVAC people in this area and I won't find anyone who would do it any differently. I simply have to wait until it gets real cold then run the heater then go sit in each room. If a room is cold I increase flow to that room. If a room is hot I decrease flow to that room.

Interestingly, when I then ask him why the end result of his "balancing" was that all dampers on all ducts are wide open his answer is that the installation of the ducting was obviously perfect. The ducting was perfectly sized and perfectly run. That argument then tells me that either the system is perfectly balanced or I'm screwed because I'll not be able to increase flow to one room without decreasing flow to another.

So I appear to only have one option, to figure out as close an estimate of the current state of balancing (what each register is putting out) based on Manual J and then show this to the contractor (at which time of course I will have balanced the system myself).

Ask your contractor when the last time was he had his hand calibrated?

 

We don't have balancing dampers here either, they just make noise and add to your total system static anyhow.

We install dampers on every system we install supply branches & return branches.

Can't say we have ever had any noise issues or high pressure drops either with them installed.

If you guys are having issues with them it might be a good time to have a talk with the install crews.

 

I would think that your branch velocities are too high if balancing dampers are noisy.

 

I would think that your branch velocities are too high if balancing dampers are noisy.

I just find they rattle and sometimes hum. When I design I figure branch velocities at 600 fpm or using flex to help damp noise but I've seen 'em rattle plenty in the field. Especially the big flappers at the supply plenum for trying to do a 2 story house with one system.

DavidR, Why would you put balancing dampers on a return? Are you trying to control where the return air comes from? Does that work? Are you an Asimov fan? (RDavid?)

 

Re:"Actually, air is quite compressable. It's a gas! You can compress it in a ballon with your lips dude. Why do you think fan curves are based on static pressure?"

I was just trying to keep things simple and eliminate discussion of pressure differentials, etc. (Joe HO like me gets lost). Basic point of confusion being that air (or anything else) can only be compressed to a certain degree before something else blows or additional air simply goes somewhere else (like out another register, out a leaky duct, etc.). I was hoping to aviod that thread in order to stick to my basic question (which was answered).

RE:"By the way, your Manual J will show you that the cfm requirements are difference for each room when heating versus cooling."

Good point! So then what does one do if running in "Auto" mode where you want HEAT when it is cold at night and COOL when it gets hot during the day? Balance for COOL and ***** at night or balance for HEAT and ***** during the day?

RE:"Ideally if the system is moving 1200 cfm, it's pulling 1200 cfm out of the space and putting 1200 cfm into the space. In reality your duct system leaks at least 10% and probably more like 15-20. This is a problem if the duct system is in an attic or crawl space and not as big a deal if it's inside the envelope of the home such as in the basement. So it is very easy to measure less than 1200 cfm at your return grill and less than 1200 cfm if you sum all your supply outlets but if the variable speed drive is set to 1200cfm and total system static is reasonable the box is pulling 1200 cfm total from somewhere and pushing 1200 cfm total too somewhere. Honest."

Thanks. It sounds pretty basic but these discussions get pretty deep (for Joe HO) and so I figured I should confirm that there wasn't any magic taking place.

RE: "We don't have balancing dampers here either, they just make noise and add to your total system static anyhow. They won't fix a bad duct system b/c air is COMPRESSIBLE so unlike closing a water valve, closing an air damper just creates a high pressure zone in front of the damper, it doesn't neccessarily force the air to squirt out the other holes with more force."

Interesting...setting aside my curiosity as to what happens once the air is no longer being compressed (can't compress infinitley)...would this be a likely explanation for why it is that when we (contractor and I) found one register with hardly any flow we dampered off the majority of the other ducts and yet still got hardly any flow from the suspect register? Or would it have been an indication that when we had all other ducts dampered off we probably had one heck of a leak somewhere in the attic (where the ducts are)?

 

RE: Basic point of confusion being that air (or anything >else) can only be compressed to a certain degree before >something else blows or additional air simply goes >somewhere else (like out another register, out a leaky >duct, etc.)

I find this to be a big misconception in home owners who compare a duct system to water pipes. For most branch ducts, the air is too compressible to be forced out another outlet by closing a damper.

RE:"By the way, your Manual J will show you that the cfm >requirements are difference for each room when heating >versus cooling."
>
>Good point! So then what does one do if running in "Auto" >mode where you want HEAT when it is cold at night and COOL >when it gets hot during the day? Balance for COOL and >***** at night or balance for HEAT and ***** during the >day?

Usually as long as people are reasonably comfortable in both modes they don't worry about it. If you ARE worried about it you need a variable air volume / modulating damper zoning system which will vary the cfm delivered to the space based on the load value of the space as determined by the zone sensors. But that is big money b/c the equipment is expensive and it squares the complexity of the duct design and cubes the installation effort.

>
>Interesting...setting aside my curiosity as to what >happens once the air is no longer being compressed (can't >compress infinitley)...would this be a likely explanation
>for why it is that when we (contractor and I) found one >register with hardly any flow we dampered off the majority >of the other ducts and yet still got hardly any flow from >the suspect register? Or would it have been an indication >that when we had all other ducts dampered off we probably >had one heck of a leak somewhere in the attic (where the >ducts are)?

When air is no longer being compressed it liquifies?
Seriously, you gotta drop the idea that you can push on air. I've never seen it work.

Probably you'll find the duct that is not delivering proper air flow has a take off that is improperly positioned on the trunk line, or like you said it's disconnected, or you have some crazy ductboard box & flex spider system up there and the run in question has an equivalent length of about 1000 feet. In anyevent, if it's in an attic it should be super easy to identify and fix. On that note, I have never ever seen balancing dampers in an attic, are you talking about the dampers on the diffuser faces in the space?

 

DavidR, Why would you put balancing dampers on a return? Are you trying to control where the return air comes from? Does that work? Are you an Asimov fan? (RDavid?)

We do our best to keep room pressures neutral this will many times involve dampering down a return.

We have had a great deal of success with it & have been doing it for a number of years now.
Too often the return side of the duct system is neglected & not given the attention it deserves.

As far as my name the r is for my last name initial, I just really suck at when to capitalize my letters.