Help Pls: Mitsu Hyperheat final sizing decision
Info: Air sealing done (-50% infiltration, 11 ACH > 5!), Attic ccSPF flash + Timberfill loose to R-50. Data taken through winter shows an actual heat load of 35k @ Rochester NY design of 5F. Quotes from 5 installers, Mitsu PVA/PUZ combo to replace centrally ducted NG HVAC. Manual J 42k. We will have a good large wood burning insert for supplemental and emergency heat. 2700 ft2 1986 Ranch 2×6, Anderson Casements, oversized ducting, zoned, heats well and uniformly.
Decision to be made: From NEEP detailed sizing:
1) 3 ton unit w/ 38k max (109%), balance point 3 F, 98.9% load served, 26 hrs supplemental, only 10% AC load modulating COP indicated a touch better but strip heaters running part time will counterbalance.
2) 3.5 ton w/ 48k max (137%), balance point -5F, 99.9% load served, needs only 2 hrs supplemental, only 5% summer AC modulating. No supplemental need, just emergency.
Community here suggests taking smaller unit for more modulating time and run the wood stove when it gets colder. Installers firmly suggest taking the larger one, mainly so the unit is not running maxed out, noisy and stressing the equipment, every time it gets cold. Cost about a wash, 3.5 ton a good bit more expensive but delta mostly covered by NYSERDA rebate said to be not available with 3 ton “adequate” unit due to fat in required Manual J.
What say we, Folks?
Thanks in advance
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Replies
Like you say, it's a wash. Go with the 3.5ton. Maybe in a few years you won't want to saw any logs or lug any lumber and the efficiency difference is likely a wash as well.
3.5 ton.
Without diving deep into a manual J. I tend to err on the higher side. Especially with the turndown ratios of heat pumps, it gives you lots of wiggle room.
I tend to think pin point right sizing a unit is more important for gas units (but take that statement with a grain of salt, I don't do this everyday)
I should say I'm jaded, as years ago I spec'd a right sized system based on the manual J, and got burned. because the relentlessness of reality chips away (beats away) at theory and the install or simply general conditions might not turn out as perfect as we expected. Ie: obstructions for ductwork, the lack of a coating on the windows wasn't accounted for, an extra skylight got put in, or simply the system doesn't perform as perfectly as on paperwork etc. and then I ended up with an undersized system and it became a comfort issue.
Obviously don't go crazy, it needs to be reasonable but with redundancy factored in. I generally want a system to be mostly running in the sweet spot, and not be running mostly maxed out. There will be people smarter than me who disagree with this, So I could be wrong, These are complex systems.
Jamie