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ASHP efficiency in the summer: cool or dry setting?

forcedexposure | Posted in General Questions on

Hi Folks,

As the humidity and the temperature is rising as we approach summer, there’s a question milling about in town, where a bunch of people recently installed ASHPs:

What is the better, more efficient heat pump setting:
Cool or Dry? 

Thanks in advance for your wise input!

best wishes, Kris

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  1. joshdurston | | #1

    Cool will use less energy to maintain the space temperature at setpoint, but often at a low latent capacity. If you need cool + dehum (space hum above 60%) then I think dry is likely more efficient than running in cool and an external dehumidifier.

    My approach is to use cool until my space humidity hits between 55-60%rh and then switch to dry. If my space temp is below 22C then I just rely on dehumidifier to avoid overcooling to the point of discomfort.

  2. walta100 | | #2

    My Rheem heatpump has a dehumidification mode with several setting
    1 Dehumidification mode on or off
    2 Dehumidification set point as a %
    3 Over-Cooling Amount in degrees
    4 Dehumidification Drain Timer

    I would assume others brands work in a similar way but may not give you as much control to adjust the variables.

    I set mine to 60% with a 2° of over cooling is allowed. If the humidity gets over 60% the system will run and cool the house 2° below set point, giving the system more run time allowing it to remove more moisture. Then the system stays off until the temp gets above the set point. I think the drain timer will keep the system off for the set amount of time allowing the coil to drain fully before the system can start again.

    The Dry mode will uses more energy it could make you feel more comfortable if high humidity bothers you more than the over-cooling does.


  3. Expert Member
    AKOS TOTH | | #3

    On most mini splits, dry mode lowers the fan speed and reduces the coil temperature. Both of these cost extra energy, so unless you need the extra dehumidification, keep the unit in COOL mode.

  4. Jon_R | | #4

    For reasons I don't understand, most mini-splits are not able to automatically adjust fan speeds (CFM/ton) as needed to meet a specified %RH and temperature. Even worse, the fan usually doesn't modulate as low as the compressor - which means poor-to-none dehumidification in low load conditions (when it's needed even more).

    Daikin Quaternity has features to provide better humidity performance, even dehumidifying without cooling (it can use part of the inside heat exchanger as a condenser).

    1. _Stephen_ | | #6

      This is why my next heat pump will be a Trane. Integrated variable control of temp, humidity across multiple zones simultaneously.

  5. forcedexposure | | #5

    Thanks for the good info, folks!

  6. johns3km | | #7

    I would run in dry mode as long as it's cooling the home comfortably- eventually the low fan speed won't be able to keep up and you'll have to switch to cool. I use dry in smaller rooms to try and get some longer run times to limit short cycling.

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