Retrofit heat pump sizing
I am in the process of adding a solar array to a 4 unit building that I own. As part of that, I am planning to convert one or two of the 4 units to heat pumps (currently hydronic oil). It’s an old row house, but in 1900, insulation isn’t great, nor are the windows (30 y/o vinyl). The units are two story town house style, about 1300 sq ft each.
One of the companies I have quoting the solar also does HVAC. They initially spec’d a 2-ton ducted system for the second floor and a 2-ton, 2 head system non-ducted system for the first floor. The only justification they can offer for that spec is square footage.
I thought this spec sounded way oversize, so I started digging into utility bills from when I lived there. I am concerned with heating load, not cooling in general, given the property is in Massachusetts. I used oil consumption from a 5 week period in Jan-Feb 2014 (avg temp of about 16F) to calculate the actual heating load. Working through efficiencies, oil-btu conversions etc I came to about 2 tons for the whole unit (this includes hot water).
As has been suggested in other threads here, real data is better than a calculation. I proposed a Mitsu 29k 3 zone system (MXZ-3C30) with 2 6k units on the first floor and one 12k ducted air handler on the second. Of course the contractor balked and assured me it wouldn’t keep up.
All of this brings me to another question, can I rely on the max capacity rating at 5f? Per the submittal it’s still 28,600 at that temp, but the rated capacity at 17f is only 18k (there is no rate capacity at 5f).
Thanks in advance,
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