td x 1.08/btu rating =cfm?

[onad]

has anyone compared this formula that i heard during a nate class to actual readings is it close?td times 1.08 divided by btu output of the furnace to get cfm.

[Senior Tech]

It will get you close.

[woody19]

1.08 is for sea level. The higher you go that constant becomes less...so at say 5000 feet it would be .896.

[derb]

What is the name of that 1.08 constant, or the theory behind it. What is it and how do I find it for different altitudes? Would it be better to compensate for humidity, maybe with dew point?

Thanks Dan

[beenthere]

Would it be better to compensate for humidity, maybe with dew point?

Thanks Dan

NO, its a sensible heat formula.
.68 is for latent.

[BigJon3475]

http://www.nrel.gov/docs/fy02osti/30152.pdf

page 203.

"For developing inputs the term K = "ρ (cp) 60" is used. In general for standard air ρ = 0.075 lb/ft3. For dry
air cp = 0.24 Btu/lb°F resulting in K = 1.08 and for moist air w ≈ 0.01 so that cp = 0.244 Btu/lb°F resulting
in K = 1.10, where K has units of (Btu*min)/(ft3*F*h). Also note that some references indicate that
standard air is dry (e.g. ASHRAE Terminology, Howell et al) while others only specify the density but
indicate the possibility that standard air can be moist (e.g. ANSI/ASHRAE 51-1985)."

[Kevin O'Neill]

What is the name of that 1.08 constant, or the theory behind it. What is it and how do I find it for different altitudes? Would it be better to compensate for humidity, maybe with dew point?

Thanks Dan

Converts Cubic Feet per Minute into Pounds per hour and multiplies times specific heat of air.

1 CFM * 60 minutes per hour = 60 CFH * .075 pounds per cubic foot (Standard Air) = 4.5 * 0.24 Specific heat of air = 1.08. That is where that number comes from. It is only constant at standard air. As temperature of air or altitude changes, the density of the air changes and the 1.08 becomes something else. But 1.08 is usually close enough for the kind of girls I go out with. :D

Instead of multipying be .075, divide by 13.33 which is the specific volume of air at standard conditions. You will get the same results. The specific volume of air at sea level can be found on the psychrometric chart. Those lines slope steeply diagonally down to the right (not the enthalpy lines). That will increase the accuracy of your calculations.

[lynn comstock]

Well done Kevin.

[Shophound]

Would it be better to compensate for humidity, maybe with dew point?

Thanks Dan

For factoring in humidity (latent heat), there is the total heat formula, which is:

4.5 x CFM x delta E (enthalpy difference)

This formula is the one to be used to perform an actual capacity performance check on an operating a/c system. It requires that actual CFM be known. Actual CFM can be obtained via a manometer and comparing actual external static pressure (ESP) to manufacturer's data. There's other means to obtain CFM if the manufacturer's data is unavailable, such as obtaining a velocity pressure reading, converting that reading to velocity, and then further converting that to CFM:

Velocity (FPM) = 4005 x (square root of velocity pressure)

CFM = (Area of duct in square feet) x (velocity in FPM)


A bit of number crunching, for certain, but short of sticking a hot wire anemometer in the supply plenum and getting a ballpark velocity reading that way, it's better than mere guessing.

Back to the total heat formula. Not only does it require CFM to be known, it also requires wet bulb/dry bulb temperatures of the supply and return air conditions. These readings are then converted to enthalpy on a psychrometric chart or converter to obtain the numbers for the enthalpy delta.

It looks like a lot of work but it really doesn't take all that long, and combined with other data gathering done on a system, can tell the technician a lot on how that system is performing, how much latent heat removal vs. sensible heat removal is being done, etc.

[BigJon3475]

For factoring in humidity (latent heat), there is the total heat formula, which is:

4.5 x CFM x delta E (enthalpy difference)

This formula is the one to be used to perform an actual capacity performance check on an operating a/c system. It requires that actual CFM be known. Actual CFM can be obtained via a manometer and comparing actual external static pressure (ESP) to manufacturer's data. There's other means to obtain CFM if the manufacturer's data is unavailable, such as obtaining a velocity pressure reading, converting that reading to velocity, and then further converting that to CFM:

Velocity (FPM) = 4005 x (square root of velocity pressure)

CFM = (Area of duct in square feet) x (velocity in FPM)


A bit of number crunching, for certain, but short of sticking a hot wire anemometer in the supply plenum and getting a ballpark velocity reading that way, it's better than mere guessing.

Back to the total heat formula. Not only does it require CFM to be known, it also requires wet bulb/dry bulb temperatures of the supply and return air conditions. These readings are then converted to enthalpy on a psychrometric chart or converter to obtain the numbers for the enthalpy delta.

It looks like a lot of work but it really doesn't take all that long, and combined with other data gathering done on a system, can tell the technician a lot on how that system is performing, how much latent heat removal vs. sensible heat removal is being done, etc.

Sweet ShopHound...I been scratching my head on that one since you posted in the thread "leaking valves lowering watts" or whatever the name of that one was.

[davidr]

Using that formula on a fuel fired furnace could get you in trouble.

Too many assumptions being made from the start.

[Shophound]

Sweet ShopHound...I been scratching my head on that one since you posted in the thread "leaking valves lowering watts" or whatever the name of that one was.

I was hoping you'd catch this post since I did broach the subject on another thread you were involved with the other evening.

Go try it out when you can. It'll open your eyes to new things regarding how these systems perform.

[derb]

Thanks for the help Kevin and Shophound. I just have to know what is behind the numbers. I still have some learning to do. Thanks.

Dan

[lynn comstock]

Velocity (FPM) = 4005 x (square root of velocity pressure)

CFM = (Area of duct in square feet) x (velocity in FPM)
In the real world of non-laminar flow ductwork, where do you measure the velocity pressure?

As you know, the answer is a pattern of many readings to obtain an average. I just thought we ought to let the others in on the pitfalls of a single reading. In some locations, the reading might even be negative due to turbulence.

[Shophound]

In the real world of non-laminar flow ductwork, where do you measure the velocity pressure?

As you know, the answer is a pattern of many readings to obtain an average. I just thought we ought to let the others in on the pitfalls of a single reading. In some locations, the reading might even be negative due to turbulence.

True. This is what a duct traverse would accomplish. Multiple readings averaged = average velocity pressure.

Maybe one of these days someone will make and sell a flow hood sized for residential/light commercial that is not obscenely priced, and technicians might be willing to make a little room for one in their tool budgets. Meanwhile,I want to know more than just looking at a coil and sticking my hand over a supply and looking at my gauges and declaring, "Hmm...airflow seems good." So at the very least I'll stick some kind of measuring device into the ductwork and obtain the best readings I can with the conditions given.

[Jim Davis]

has anyone compared this formula that i heard during a nate class to actual readings is it close?td times 1.08 divided by btu output of the furnace to get cfm.

The real answer to this question is it won't even be close! The 1.08 is too high for heating and the BTU rating of equipment is over stated. Two wrong will not make a right!!

[davidr]

The real answer to this question is it won't even be close! The 1.08 is too high for heating and the BTU rating of equipment is over stated. Two wrong will not make a right!!


Usually comes out to about 1.00 at sea level if fired properly doesn't it captain?

[beenthere]

Usually comes out to about 1.00 at sea level if fired properly doesn't it captain?


Ah, about 1.01


Ooops, wrong captaan :)

[davidr]

Ah, about 1.01


Ooops, wrong captaan :)


120º supply temperature at sea level comes out to 0.98064 if you want to get anal. :D


130º comes out to 0.96336. ;)

[beenthere]

120º supply temperature at sea level comes out to 0.98064 if you want to get anal. :D


130º comes out to 0.96336. ;)


I don't have to want to get anal, I am anal. :)

[BigJon3475]

Are you not actually looking at the temp before it's heated. This way you know what the air is before it heated. What your actually trying to find out is how much heat is gained into the return air as it is opposed to resistance heaters. It works like amass air flow sensor. If you send voltage through a small metal wire then have airflow run over it. (Not the supply air) it changes the resistance...a change in resistance will change amperage draw if voltage is the same. True you need to change for your particular application when you plot your conditions on the psychrometric chart. This is how cars have been accurately mixing air fuel ratios since 1982 when Bosch came out with it. I may be wrong this is posted more as a question then stating facts.


http://www.kemparts.com/TechTalk/tt06.asp

very similar idea here.

[davidr]

I don't have to want to get anal, I am anal. :)


Lol....Birds of a feather. ;)

[Shophound]

Are you not actually looking at the temp before it's heated. This way you know what the air is before it heated. What your actually trying to find out is how much heat is gained into the return air as it is opposed to resistance heaters. It works like amass air flow sensor. If you send voltage through a small metal wire then have airflow run over it. (Not the supply air) it changes the resistance...a change in resistance will change amperage draw if voltage is the same. True you need to change for your particular application when you plot your conditions on the psychrometric chart. This is how cars have been accurately mixing air fuel ratios since 1982 when Bosch came out with it. I may be wrong this is posted more as a question then stating facts.


http://www.kemparts.com/TechTalk/tt06.asp

very similar idea here.

The overarching challenge, Jon, is establishing an accurate CFM reading for any of the formulas disclosed here to render any useful data. In the case of the OP's formula, others have disclosed the inherent drawbacks regarding actual conditions to derive the needed data to plug into the formula. Natural gas does not always have the same heat content, nor is there any guarantee that what a manufacturer says a furnace will output in BTUH, that is what actually is coming out of it and entering the supply plenum.

Even the formulas I included in this post need good CFM data to get you good information. I do agree with Lynn that just poking manometer into a supply plenum at one spot may or may not render a good velocity pressure reading, hence the need for a duct traverse. Airflow in ducts is not easy to measure accurately, but it is very doable to obtain data that will render a reasonable outcome.

As for the process you mentioned, a mass air flow sensor, it is likely similar to a hot wire anemometer, a tool available for HVAC techs to use.

[BigJon3475]

As for the process you mentioned, a mass air flow sensor, it is likely similar to a hot wire anemometer, a tool available for HVAC techs to use.

that's exactly what it is. It actually even uses that terminology. It has a platinum wire that is hot and when air passes over it know resistances are set in the data base for know CFM that removes heat from the wire creating different resistance. Except the mass air flow actually adjust the signal to the hot wire to stay constant and the amount it has to add back is how it figures mass quantity. It does this accurately enough to follow fuel tables to keep air fuel ratios @ stoich 14.7:1 (gasoline)


The tool may work on that exact same principle and most likely does. But I don't know enough about it to speak intelligently.....although that's never stopped me before :D

[Shophound]

The tool may work on that exact same principle and most likely does. But I don't know enough about it to speak intelligently.....although that's never stopped me before :D

I have one on order and will soon be able to speak with whatever level of intelligence I can muster regarding how useful and effective this tool is.

[BigJon3475]

Will be looking forward to the field test results....so many tools.....so little money.

[Kevin O'Neill]

I have one on order and will soon be able to speak with whatever level of intelligence I can muster regarding how useful and effective this tool is.


I have the TSI velocicalc. It has pressure ports for measuring static pressure across a system or coil, down to 0.001 IWC. It can also be used with a pitot tube. It measures air velicity with a hot wire anemometer. It also reads %RH, wet bulb, dry bulb. It you enter the duct size and do a traverse, it will read out in CFM.

I bought it from the National Comfort Institute in Ohio. They teach a residential/small commercial air balance class at locations all around the country. Recommended.

By the way, I also have the TSI Accu-balance flow hood. I carry both instruments on my truck (along with a junk yard dog to protect them:D ). Don't leave home without them!:o

[Shophound]

Mine's also coming from National Comfort Institute, but it is a "velocity stick" made by Testo. I already have their "humidity stick" and it's a handy little jewel. Shows what an HVAC geek I am; I'll carry it into a restaurant and while seated start taking stealth readings of the indoor air temperature, humidity, and wet bulb temps. :D One of my favorite lunch spots has a pretty lame a/c system, I've found. :D

I once considered joining the National Comfort Institute. Once I saw the price of admission I about fell out of my chair. Combined, my yearly RSES and ASHRAE dues barely equal half of what NCI wants to get in the door. I think they're a good outfit teaching very valuable material; just wish they'd be a bit less proud about it.

[Kevin O'Neill]

I have the humidity stick also. Sure beats swinging that old Bacharach sling till my arm gets tired. Great tool!

[Jim Davis]

120º supply temperature at sea level comes out to 0.98064 if you want to get anal. :D


130º comes out to 0.96336. ;)

Right on David. Rob and I were going over those charts yesterday.