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Minimum CFM for 10KW heat strip (Goodman 2 ton ARUF air handler with 2 ton 14 SEER HP

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5.2K views 6 replies 6 participants last post by  Restaurant Mech  
C
#1 ·
Hello all.

Question from a Canadian in Southern Ontario...

Heat pumps aren't popular up here due to cold temperatures during the winter, cheap natural gas that is readily available to most homes, and a lack of knowledge up here about how cost effective heat pumps can actually be even in relatively colder temperatures; but an installer of ours just installed his first heat pump ever (2 ton ARUF PSC) and asked me my opinion on something.

The installation literature appears to have a kind of discrepancy between two charts. One chart lists the expected heat rise of the various size heat strips at various CFMs. The other chart lists the MINIMUM CFM required for the various heat strip sizes on various air handler sizes. The seeming discrepancy is that the first chart lists a temperature rise of 35F for a 10KW heat strip at 800CFM, but the second chart says the MINIMUM CFM for a 10KW heat strip is 950... so while the first chart lists a temperature rise for a 10KW heat strip at 800CFM, the second chart says we're not even allowed to go as low as 800CFM.

We'd prefer to have the CFM around 800 CFM so as to keep the temperature rise as high as possible and lessen the chance of complaints of "the air coming out of the registers feels too cold".

I know I'm not providing much information but I'm not really requesting opinions on the particular data of this situation (although I'm certainly open to hearing any), but rather my question is more about how other installers/techs would approach any similar type of discrepancy. For comparison sake, would you feel comfortable choosing the 800 CFM (which one chart seemingly lists as an option) even though a second chart seems to contradict it?

Thank-you in advance for any input you have.
 
H
#6 ·
If a manufacturer says 950, there is probably a reason and they're not shooting from the hip.
The first chart simply shows you the temperature rise at 800 cfm, it doesn't imply you should run it at 800. The other chart clearly states run it at 950 as a minimum. If I do the math, that should give you a supply temperature of 108ºf (42ºC) if your return is about 70. I doubt they will think 42 out of the registers feels cold. Are the ducts running through unconditioned space?
 
R
#7 ·
As always, follow manufacturer recommendations.
However, heat strips usually don't care if the airflow is a little lower or a little higher.
Most of the time I try to measure the temperature right at the heat strips when setting up the fan for heat.
That way you know you aren't going to be tripping the high limit.
 
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