Help deciding on Mits Heat Pump design
I am trying to lower my electricity requirements by switching from electric resistance to heat pump. I do have a decent size wood fireplace insert but would like to reduce the amount of wood heat I use as well. I was originally thinking about an air to water system given they can be quite versatile (adding solar thermal, DHW, pool heat) but converting to radiant flooring isn’t really in the cards, space for duct work is very limited, and fan coils became cost prohibitive given the number I would need. Air to air seems like a better fit for our house. I have quotes from Mits and Fujitsu but have essentially ruled out Fujitsu because I am not very confident in the contractor. He insisted I needed about 7 heads in my house and I have read enough on here to know that is not a good idea. Mits contractor has been good to work with and I think we might be getting close but was hoping to get some feedback on what is the better choice.
A bit about the house first. Located near Peterborough, Ontario (climate zone 6 I think. -9F or -22C design temp). It was built in 1965 and has main floor and basement that is conditioned. Main floor is ~1200 sq ft and has a large open area (~790 sq ft) with two bedrooms and a bathroom off the one end. Basement is a little choppy but includes a rec room and bedroom (~140 sq ft each) plus storage, utility room, and an insulated crawl space that is ~380sqft. The house is built well and we have improved considerably on the insulation from what it was (R60 in ceilings – there is a section of cathedral we couldn’t do much with but it is quite air tight, R22 walls, all new triple pane windows, basement walls R20, uninsulated slab, crawlspace has R25 on walls and R8 on floor). I used load-calc.net and came up with the following:
Heating BTU – 31186, Cooling BTU – 13719. I’ll attach a floor plan that includes the room by room break down.
Blower test was done and was 4.5 air changes at 50psa, but we have replaced 4 leaky windows and insulated the crawl space since then so I think it will be a bit better than that now.
Option 1 – MXZ-5C42NAHZ with three zones. One head in the basement bedroom (MSZ-GL09NA), one in the main living area (MLZ-KP18NA-U1), and a ducted in the crawl space to feed the two bedrooms (SEZ-KD15NA) and put some heat into crawl space/utility room.
Option 2 – MXZ-3C24NAHZ with two zones. One head in the basement 09NA and the ducted at 15NA. A second unit, MUZ-FH12NAH w MSZ-FH12NA in the main living area.
Option 2 is what I proposed to him after reading all the articles about oversizing and issues with multi poor modulation leadings to lots of over cycling. I think I still need the multi as we would really like the ducted option for the bedrooms and there doesn’t appear to be a 1:1 ducted option for Mits in the cold climate version. With the single in the main living area, I thought I might be able to use just this in shoulder seasons and fire up the multi in the mid summer and winter periods and then I can better take advantage of the modulation of the 1:1. My contractor is all for option 2, but he doesn’t want to go as small as the 12 being concerned it won’t be enough. Based on my calculations, the 12 would do our living area until -18C, which seems pretty good to me given I have a 60000BTU woodstove in there if needed. If I do go with the 15 1:1 in the living room, the modulation doesn’t look that much better to me that the multi (15 ranges from 5150-24000, the Zuba Multi 36 ranges from 7200-45000 and that would be for the whole house rather than just the single, the 12 1:1 goes down to 3700). I would love to rip the baseboard heaters out if it did cover everything, but am okay with keeping them in if it means I can have a more efficient system.
Any advice would be appreciated. Are both oversized/undersized? Is one better than the other or will they have similar results. Cost isn’t really a factor, they are both coming in basically the same at 20k. The simplicity of option 1 is nice, but the redundancy and potentially increased efficiency of 2 is good as well.
Thanks a lot and please let me know if I left out any information.
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