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Air to air heat pump sizing advice needed

I am working on a small space to be heated.  600 square feet.  R35 insulation in the floor, walls, ceiling.  3 new doors and one window, all excellent quality.  Assiduous attention was paid to tightening the envelope.  Climate controlled storage for now with the possibility of a living space down the road.

 I believe that I am in zone 5.  Bitter cold wind on a hill.  I have had no difficulty making the space comfortable with a 500/1000/1500 watt 3 setting electric space heater.  Works fine.  But that is only for working in the winter.

Plan is an air to air heat pump.  I think a 3/4 or 1 one ton unit would be fine.  

Any advice on size would be appreciated as a ball park guess.  

Given my experience with the portable heater, it seems like 5,000 btu an hour or 10000 btu an hour would do it, with back up proper electric heat for design day loads.  Design day around here is about 5 degrees.

Thank you in advance.


  • GGross
    GGross Member Posts: 968
    edited February 7
    Sounds like a good application for a small mini split perhaps. your resistance heater is putting out 5118btu/hr at max setting A small mini split would be in the 9k range, different brands will post different outputs at their respective low operating temperatures. For example the brand I deal with, LG, a 9k standard model would output 8,310 btu/hr at -4f which would be more heating than your resistance heater puts out, and would use about half as much electricity (0.71kw around 3.4 COP at full load at -4f outdoor temp). No harm in keeping the electric heater as backup but a mini split heat pump will heat that space below design day temperatures. The mini split would also ramp up or down as needed to maintain the temperature a bit more evenly
  • Daveinscranton
    Daveinscranton Member Posts: 148
    I am a bit excited to try a mini split.  I have never tried one.  I think that it is the perfect application.  Should be fun!

     I appreciate the help on sizing as I have zero experience with them.
  • ethicalpaul
    ethicalpaul Member Posts: 5,617
    But wait @GGross, I have been told that mini splits don't work below 32F or so! :wink:
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • Jamie Hall
    Jamie Hall Member Posts: 22,934

    But wait @GGross, I have been told that mini splits don't work below 32F or so! :wink:

    Depends on the mini-split. The Carrier in my wife's apartment seems to keep heating down to almost zero...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • pecmsg
    pecmsg Member Posts: 4,696
    But wait @GGross, I have been told that mini splits don't work below 32F or so! :wink:
    They all have cold climate heat pumps. 

    Like everything else sizing is critical. 
  • gyrfalcon
    gyrfalcon Member Posts: 150
    Just a reference for the original poster. 
    I have a 12k Mitsubishi hyper heat in a 300+ sq foot sunroom / entryway.  The ceiling and west wall have some insulation but the east wall and south wall are pretty much all glass. This room has Always been extremely hot or extremely cold,  now it is nice and tempered at whatever temperature I set it to. The Mitsubishi mini split modulates nicely as needed.  
    Slant Fin Galaxy GG100(1986) , 2 zone hot water baseboard, T87 Honeywell thermostats. 
  • worldclasshvac
    worldclasshvac Member Posts: 14
    The only real drawback with a ductless is the defrost cycles.  When it's your only heat source and it's very cold it will defrost often and blow some cold air around the space while it thaws the outside coil.  In a small space it's more noticable compared to an install where you are supplementing a room with insufficient heat from primary source.  The other caveat is they can have issues like any other system and dealing with them in below zero temps is tricky, especially the refrigerant side of the unit.