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Ductless mini split sizing advice - Mitsubishi specific

We are going to have a Mitsubishi heat pump installed in our house (in progress) soon and would be interested in some thoughts on sizing.

The house is in Jackson, NH 03846 (climate zone 6a). It's cold and snowy, but we have great passive solar gain, open floor plan, tight construction, and high insulation values. We have wiring in place for regular electric resistance backup. Our Energy Star rater calculated our heat load at 11.2kBTU / hr at design temp (-6F, 65F inside, based on St Johnsbury VT).

Looking at the new Mitsubishi FH line, MSZ/MUZ units:

We are leaning towards the FH12NA for lower cost and greater efficiency. Its rated capacity at 17f is 8k BTU/hr and can operate at max capacity of 13.6k BTU/hr down to 5f.

The FH15NA has a rated capacity of 11k BTU/hr at 17f and operates at max capacity of 18k BTU/hr down to 5f.

I'm pretty sure either unit will work, we might just have to rely on the electric resistance backup a bit more with the smaller unit during the coldest parts of winter. I believe most of December through early March, it regularly gets below 10f at night.

Do you think the FH12NA would be working too hard during the colder months and does it make sense to go with the FH15NA? Or stick with the FH12NA because it's rated as more efficient and will work, just with the chance of a bit more electric resistance backup?



Asked by Brian P
Posted May 23, 2014 6:28 PM ET


3 Answers

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All of my units are oversized, and it is my opinion that you will be more comfortable with an undersized unit. I say this because the strategy of the unit is to never shut off but run at its lowest level, even if the setpoint has been met. Most of the time you will be well below capacity thus the unit will allow the temperature to wander perhaps more than you would like. A technically undersized unit will easily throttle down in the vast majority of the heating season and keep the temperature pretty constant. I find this true with heat and ac, the more heavily they are loaded the happier they are

I also say this because a simple heatloss frequently overstates actual use by a fair bit, and the amount of time spent at design values is very low.

It would surprise me with the smaller unit if you ever had to use you back up heat

Don't take this as any sort of hit on the mitsu units at all, it is just understanding the way they work. You can figure out ways to make them do what you want, such as manually controlling the fan so that it cannot over heat or overcool the space in the shoulder seasons

Answered by Keith Gustafson
Posted May 24, 2014 8:35 AM ET


The FE12NA has 10200BTU/h output at -5F, assuming 70F indoor. So the FH12NA should be VERY close if not exceeding your 11200BTU/h at -6F and 65F indoors. Personally, I would probably not upsize to the FH15NA. Spending the price difference on some insulation or air sealing would probably make more sense -- or just don't spend it, and save a few bucks.

One interesting thing I notice is that the bigger units have dramatically higher moisture removal. FH09 is 0.6pints/h, while FH12 is 1.9 and FH15 is 4.0. I'm not sure how this bears out in real life dehumidification.

Answered by Nick Welch
Posted May 28, 2014 4:08 PM ET
Edited May 28, 2014 4:11 PM ET.


Thanks for the input, we just put a deposit down on a FH12 installation yesterday.

Answered by Brian P
Posted May 28, 2014 4:15 PM ET

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