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  1. #1
    Join Date
    Jul 2008
    Posts
    551

    General CFM effects and setting questions.....

    1) What effects does changing CFM on your blower have on a system with a TXV? Like, if you changed the CFM for a 3 ton heat pump or AC from 1260 CFM to 1200 CFM, would that severely affect the refrigerant pressures or would you have to change from 1200 CFM to like 1000 CFM before the system would need to be recalibrated? (I'm not asking how, why, or what is better. I'm just curious as to a cause/effect answer)

    2) I know with cooling, CFMs are set typically by the rule of 400 CFM per ton. How does that work for heating? How do you set the CFMs for a furnace? Is it by BTUs, comfort, static pressure, noise? I realize that setting the CFMs for a unit too low could cause the heat exchanger to get too hot, but I'm not sure what else governs the CFM settings for a furnace and what exactly is that danger zone.

  2. #2
    Join Date
    Jul 2008
    Posts
    551
    Also, does a furnace use the cooling or heating mode CFM selection when the heat pump is in heating mode? I only ask because it seems like a typical furnace blower in heat mode would provide much more CFM than it supplies in cooling mode. Would this increased air flow detract from the heat pump's heating performance or do the rules for CFM requirements change for a heat pump when it is heating compared to cooling?

  3. #3
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,763
    It uses the cooling mode CFM when using the heat pump for heat.

    A change from 1260 to 1200 CFM has no great effect on the A/C, as far as charge.

    It will slightly increase moisture removal, and slightly decrease the A/Cs ability to lower temp.

    As your air filter loads up. You lose more then 60 CFM.
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  4. #4
    Join Date
    Jul 2008
    Posts
    551
    Interesting. Thanks.

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