Does Altitude the correction factor of specific heat?

[genduct]

It gave some good advice on adjusting the Fan (not ecm or vfd) but only said that the "constant was not constant and didn't continue the thought.

There is a remarks email for the site, so I will reach out to them when I get a chance. unless you do it first

MIKE

[billrae93]

Great Info guys but my head is completely swimming. Going back to the original question, If I am installing the furnace and I want to measure the amount of BTU's it is outputing taking into account my elevation what do I need to do? An example with numbers would be great. Can we use a 900 ft elevation? Thanks very much.

[genduct]

There is not enough difference to make a difference IMO
The margin of error in measuring low quanities of air is greater than the adjustments

[ACFIXR]

Great Info guys but my head is completely swimming. Going back to the original question, If I am installing the furnace and I want to measure the amount of BTU's it is outputing taking into account my elevation what do I need to do? An example with numbers would be great. Can we use a 900 ft elevation? Thanks very much.

ACCA has the most detailed information on altitude and correction factors that us "simple minded" techs can understand. Bulletin 2008-01 written by Hank Rutowski is one. Another one is this ancient thing written a few years ago.

Here is a simple ACF chart

[second opinion]

Great Info guys but my head is completely swimming. Going back to the original question, If I am installing the furnace and I want to measure the amount of BTU's it is outputing taking into account my elevation what do I need to do? An example with numbers would be great. Can we use a 900 ft elevation? Thanks very much.

No correction factor is needed for 900 feet.

[ACFIXR]

There is enough information in "all" of the posts for you to do your own equations.Good luck.

[billrae93]

Once again, thanks for all of your help. I have seen that no correction is needed for anything under 2000 feet. My whole point for these questions is that there is a company that has software that says it calculates all this for you. I am trying to determine what formulas they are using. The closest I have been able to come to their results is to average the supply and return air densities. If I am below 2000 sq ft I still need to take in to account the air temperature, correct?

[billrae93]

Sorry for the multiple replys. If I am at 900ft elevation and testing a furnace, I can use the standard formula of 1.08 X CFM X Delta T? With out messing with Air Density based on temp at all?

[second opinion]

Yes

[billrae93]

Now, given that there is no boundary work being done by the air in our closed air loop, and that at steady state the volume of air in the house is constant, why are we using Cp (0.24btu/lb*°F ) in our equations rather than Cv (0.172btu/lb*°F)?

Were you saying that you should use .172 instead of .24 or was this an actual question? I am trying to understand this but failing. If I use the standard formula 1.08 X CFM X Delta T I get a btu output. If I use the specific volume of the return drop air X .24 X 60 I get a BTUH of roughly 2000-3000 less. If I use the average specific volumes of the supply and return air I get a number roughly 5000 Btuh less than the standard formula. Which is the right one to use? Or maybe I should just call myself stupid and quit trying to understand this.

[ACFIXR]

Were you saying that you should use .172 instead of .24 or was this an actual question? I am trying to understand this but failing. If I use the standard formula 1.08 X CFM X Delta T I get a btu output. If I use the specific volume of the return drop air X .24 X 60 I get a BTUH of roughly 2000-3000 less. If I use the average specific volumes of the supply and return air I get a number roughly 5000 Btuh less than the standard formula. Which is the right one to use? Or maybe I should just call myself stupid and quit trying to understand this.

More info to numb your mind.

[hvacrmedic]

Were you saying that you should use .172 instead of .24 or was this an actual question? I am trying to understand this but failing. If I use the standard formula 1.08 X CFM X Delta T I get a btu output. If I use the specific volume of the return drop air X .24 X 60 I get a BTUH of roughly 2000-3000 less. If I use the average specific volumes of the supply and return air I get a number roughly 5000 Btuh less than the standard formula. Which is the right one to use? Or maybe I should just call myself stupid and quit trying to understand this.

It was a hypothetical question and was part of an argument. The 2000-3000 BTU error that you derived is correct, assuming a 100,000 btuh furnace. The 5000 BTU error was not, since supply air density is immaterial.

[billrae93]

Just to make 100% sure then. To figure Btuh then I should be using (specific volume of return air X.24 X 60) X CFM X Delta T to find BTUH. If I am correct can you point me to a text book that states this. I need to reference something other than this forum unfortunately. I think these forums should be required reading for Techs everywhere.

Thanks

[hvacrmedic]

Just to make 100% sure then. To figure Btuh then I should be using (specific volume of return air X.24 X 60) X CFM X Delta T to find BTUH. If I am correct can you point me to a text book that states this. I need to reference something other than this forum unfortunately. I think these forums should be required reading for Techs everywhere.

Thanks

Specific density x 0.24 x 60 x cfm x ΔT

or

0.24 x 60 x cfm x ΔT / specific volume

I don't have a textbook reference, sorry.

[hvacrmedic]

Specific density x 0.24 x 60 x cfm x ΔT

or

0.24 x 60 x cfm x ΔT / specific volume

I don't have a textbook reference, sorry.

Should have been "dry air density" rather than "specific density" in the first equation.

[ACFIXR]

A VS ECM would move more CFM at 7,000 foot. As it wants to move its set mass air flow, and doesn't know a CFM from a BTU.

Beenthere, I thought that ecm's are constant rpm motors therefore moving the same cfm's at any elevation. Can you explain?

[ACFIXR]

Never mind

I found this very informative post.

http://hvac-talk.com/vbb/showpost.ph...2&postcount=19

Jim Bergman states;
"ECM motors are not constant airflow but constant mass flow fan systems. ECM fans automatically correct for changes in air density (mass flow rate) by increasing or decreasing the RPM based upon the torque (as created by the fan) that the measured by sensors (as current) in the motor system. As the air density decreases, the motor and or fan will speed up because of the lower torque. In turn, a higher CFM will be measured through the system. The mass flow rate however will remain constant allowing the coil or heat exchanger to operate at its optimal efficiency. Where a standard fan operates at a constant volume and variable mass flow rate, an ECM operates with a constant mass flow rate and varies the CFM as the air density changes."

So.......an ecm has a constant rpm based on torque.Learned something new today.

[dijit]

Sorry one last question. When dealing with cooling we use 4.5 X cfm X delta H

Can the same correction factors be applied to the 4.5 or am I asking a completly stupid question? If I am wrong could you please give me an example?

Thanks

Yes, but you do not use 4.5 as the value you correct. the value 4.5 comes from specific density (0.075 lb/ft3 at standard conditions) times 60min/hr so you correct only the .075 value and not the 60 seconds if that makes sense. You correct first then multiply by 60 not vice versa.

[beenthere]

So.......an ecm has a constant rpm based on torque.Learned something new today.

Closer to, it has a constant torque. It increases or decreases its RPM to maintain that constant torque.

[ACFIXR]

Just to make 100% sure then. To figure Btuh then I should be using (specific volume of return air X.24 X 60) X CFM X Delta T to find BTUH. If I am correct can you point me to a text book that states this. I need to reference something other than this forum unfortunately. I think these forums should be required reading for Techs everywhere.

Thanks

Reference this.

http://hvac-talk.com/vbb/showpost.ph...1&postcount=59

Closer to, it has a constant torque. It increases or decreases its RPM to maintain that constant torque.

Thanks!