More Q&A Spotlight
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…
Weekly Newsletter
Get building science and energy efficiency advice, plus special offers, in your inbox.
This article is only available to GBA Prime Members
Sign up for a free trial and get instant access to this article as well as GBA’s complete library of premium articles and construction details.
Start Free TrialAlready a member? Log in
2 Comments
I totally agree that using the sizing outlined in this article:https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler is probably the most accurate way of sizing as long has no changes to the envelope have taken place recently. One needs to get the accurate heating degree days for the monthly bills to maximize the accuracy. Although the design temperature for Portland is 24 degrees F I would also check the 17 degree efficiency of the heat pump to verify what it will do at design temperature. Some of the 2 stage units don't do well at low temperatures. Assuming 24k heat load, and $.21/kWh, and $15-20/ MCF(varies across the year) and using the 80% furnace and a ASHP with a COP of 3 it comes out about a wash with $1320 for all electric and $1260-1690 for all gas. The 70k nat gas furnace is oversized and even with 2 stages will cycle a lot and so may not be 80% efficient. If they can go all electric and eliminate monthly gas service charges it will even better. Personally, as a person who lives in a colder climate(Michigan) and has a heat pump as its only heating, I would certainly go the all heat pump. Also that comment about sizing for cooling because of lack of de-humidification if over sized doesn't hold true for most climates with the variable flow refrigerants and the "drying settings" that they have now. Although I have not mentioned it, the climate is another good reason to go all electric.
One additional note I looked at the Carrier specs on the NEEP list and some of those 2 ton units (there were several listed) only have 12-16K output at 5 degrees and so may not be quite enough on their own at 24 degrees F.
https://ashp.neep.org/#!/product_list/ plug in Carrier and 38MURA
Log in or become a member to post a comment.
Sign up Log in