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mini split sizing for basement office (how good are todays’ “inverters”)

rob_s | Posted in General Questions on

I’m in NJ, finishing a roughly 350 square foot basement room. Added 2″ of continuous foam board, air sealed as good as I could. Attempted a load calculation with (but don’t really know what I’m doing!) and it tells me I need roughly 4,600 BTUH of cooling and heating. 

I’ve had a number of HVAC contractors all of which just eyeball the space and recommend a 9,000 BTU mini split. 

I’d also like to have the best shot of the mini split being able to dehumidify the room without over cooling. 

Looking at the specs of these mini split units they have a range of heating and cooling so it looks to me like a 9,000 BTUH nominal unit might be able to modulate down to say 3,000 BTUH or ramp up to 12,000 if needed. 

My question is, thanks to this modulating capability, do these units actually work pretty well even if a bit over sized? Should I not worry so much and just go with a 9,000 BTUH unit? I don’t want to be freezing or hot or have the unit cycle on and off all the time, but maybe these units are more robust than other technologies that don’t modulate? 

Thanks for any advice or sharing of similar experiences!


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  1. Deleted | | #1


  2. paul_wiedefeld | | #2

    You can get a 6,000 Btu one, that would modulate lower. The Mitsubishi 6kbtu I believe can heat from 1500 to 9000, or so. Is the space in need of cooling now? There’s a path where you can install electric baseboard and no cooling and save 90%.

  3. rob_s | | #3

    Thanks Paul. The temperature last week, when it was in the 80's outside peaked at 75 in the basement, not too bad. But the humidity was around 60%, would be nice to bring that down a bit. Could be that a space heater and a dehumidifier would be a better fit, but hard to say.

    1. paul_wiedefeld | | #4

      The smallest, highest efficiency ductless units are generally the lousiest at dehumidification.

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