
The idea of needing to replace your furnace can be cause for a few sleepless nights, given the potential expense. “User-171762” writes in this Q&A post that he wants to move to a dual-fuel heating and cooling system. He lives in a 104-year-old, 1-½-story house in Portland, OR., that has minimal insulation, original double-hung windows, and ducting that was updated in the 1970s.
The new system is intended for the 1300-sq.-ft. main floor. (The home’s second floor is a separate ADU with a dedicated minisplit system.) He wants to replace his old gas furnace with a ducted heat pump and a new gas furnace for backup heat and air handling. He likes the Carrier 38MURA compact heat pump, but two different vendors have recommended two different sizes; one specifies the 2-ton model, while the other says the 3-ton would be best. Whichever size he selects, it will be combined with a 70,000 Btu 80% efficient two-stage gas furnace.
To get an idea of his heating and cooling needs, User-171762 used an online HVAC sizing tool, which indicated a 31,000 Btu heating load and a 25,000 Btu cooling load. The vendor recommending the 3-ton unit calculated the heat load at 34,000 plus a duct-loss factor, resulting in 38,467 Btu.
If he were to rely on the heat-pump only, he says it would make sense to get the larger unit, but he plans to use the gas furnace when it makes economic sense to do so—based on utility rates and the heat pump’s COP (coefficient of performance). According to this calculator, that will be when temps are in the mid-30° range.
“I’ve read that oversizing heat pumps isn’t a good idea because of short cycling,” he notes. “Maybe this is why the second Carrier vendor proposed 2T. Three other vendors I…
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