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getting conflicting recommendations for boiler replacement size - would like other opinions at this point.
this is for CI chimney vented standard efficiency hot water boiler with 40 gallon indirect DHW on priority zone
first contractor is saying weil mclain CGa3 with 70 MBH input, 59 DOE/MBH and 51 Net IBR
second contractor says we need the CGa4 with 105 MBH input, 88 DOE/MBH and 77 Net IBR
Both were given the same independent heat loss calc using manual J (done by energy audit company - a very detailed accounting room by room for windows/doors/construction etc)
The heat loss for DTD was 61k and they (the energy company) said there was some fudge factor even in the manual J calc so the 61K was probably still a bit higher than actual HL especially when we "winterize" the envelope with temporary measures every year.
-House is 1960 two story colonial with below grade unfinished/unheated (stays around 55-60 degrees year round) block basement
-Approx 1450 sq ft living space above
-Two copper fin baseboard zones ~90 ft each (finned) - total linear feet of radiating baseboard 180 feet
I understand why the second contractor is saying CGa4/105, because of the 77 net IBR, right? boiler in basement=unconditioned space, the net IBR is supposed to be equal or higher than the heat loss. 77 > 61
The first one is saying that the CGa3/70 is more efficient because the HL is DTD which accounts for less than two weeks of the heating season here, and that the DOE is is more appropriate to size on than the IBR. So he says 59 DOE is close enough to actual heat loss to be sufficient.
We're actually fine with a lower indoor temp on the design days if it means the boiler is more efficient year round.
Right sizing the boiler for 97% of the year is more important in my mind. Price difference of the two boilers isn't the issue as it's negligible anyway, sizing and efficiency is. What do you think?