Help sizing dual-fuel heat pump system
user-171762
| Posted in General Questions on
Looking for feedback on proposed options for replacing my 26 y.o. gas furnace with a dual-fuel system (ducted high-heat heat pump with gas furnace for backup heat and air handling). I’m leaning towards Carrier’s 38MURA compact heat pump as it ticks a lot of boxes for me. The dilemma is sizing. One Carrier vendor specified a 2T heat pump, the other a 3T (combined with 70k BTU 80% 2-stage furnace in both cases). What to choose?
FWIW I plugged my basic home data into an online HVAC sizing tool ( https://hvac.betterbuiltnw.com). It indicates about a 31,000 BTU heating load, 25,000 BTU cooling. The vendor who proposed the 3T system calculated the heat load at 34,000 plus a 1.1 duct loss factor, thus equipment sized for 38,467 BTUs. I’ve asked the second vendor to help me understand his calculations and rationale for a 2T but haven’t heard back yet.
I can see how if relying solely on the heat pump it could make sense to size at the higher end. But I plan to use the gas furnace when it makes economic sense to do so, based on utility rates and the heat pump’s COP. Currently that looks to be when temps are in the mid 30’s (per https://siecje.github.io/heatpump-cost/). Also, I’ve read that oversizing heat pumps isn’t a good idea because of short cycling, etc. Maybe this is is why the second Carrier vendor proposed a 2T. Three other vendors I met with (from Trane, Lennox and Ruud) proposed 2T, 2.5T and 3T heat pumps respectively. A friend who works in the industry and knows my house thinks a 3T would be overkill, but I’m worried that a 2T might be too small (for heating, less so for cooling). So I’m tempted to ask for a 2.5T. I feel like Goldilocks 🙂
Some salient points in case it matters:
* 104 y.o. 1 1/2 story house in Portland, Oregon. System is for ~1300 sq ft main floor (and one small supply register in basement), as upstairs floor is a separate ADU with its own mini-split.
* Roof insulation (above ADU) is about 50/50 R20 and R11. There’s also insulation between the main floor and ADU, about R11. No wall insulation.
* Original double-hung windows.
* Underneath floors is 1/2 crawlspace (with not too effective fiberglass batts), 1/2 basement. The unfinished basement has a supply register, but even with that closed the temp stays in low 60s.
* Ducting was updated maybe in the 70’s. Supply ducts sealed with tape, return ducts are mostly panned joists.
I’d appreciate any input folks can share. GBA has been a great resource as I educate myself.
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Replies
You can sanity-check your sizing based on your past gas usage using the method found here:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
Note that if your house has significant solar gain this method will undersize the heat.
Thanks for this tip @DCcontrarian, very helpful! I did the math based on my January 2024 usage and got around 23,000 BTU heat load. About 10% of that was the water heater (based on comparison with August usage, when the furnace heat is off). So that's more in line with a 2T heat pump capacity. However, I keep the thermostat at 68 and use DIY interior storms (similar to Indow Windows) to minimize heat loss. Renters may not do that, so their load could very well be higher.
I also spoke with an acquaintance who works in the industry. He's looking at my numbers and says he'd lean towards a 2.5T for my situation.
Finding the break even point is a little more complicated than that page would suggest. The factors that go into it are:
* The price of gas
* The price of electricity
* The efficiency of your gas furnace
* The efficiency of your heat pump
Where it gets complicated is that the efficiency of a heat pump varies with both temperature and load. Let's look at a Mitsubishi M-Series (see https://ashp.neep.org/#!/product/34583/7/25000/95/7500/0///0 ).
At 47F, the COP is 4.4 at minimum output and 3.29 at maximum output. So you have to know where you are on the output curve to figure out where you hit the break-even point.
Looking at the Energy Star list of design temperatures by county (https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/County%20Level%20Design%20Temperature%20Reference%20Guide%20-%202015-06-24.pdf ) it looks like your design temperature is 24F.
Since your winters are relatively mild I suspect you'll save very little money with the dual fuel. I know a lot of installers like them because in their heart they don't trust heat pumps. But the new generation is much better.
What is your reason for this work in the first place? Maintaining the existing system, if in good shape, is also an option, so it would be helpful to understand your motivation.
Good question! Normally I'm all for "if it ain't broke, don't fix it". My range and fridge are both 30 years old, and I only recently got rid of my 1994 Honda Civic. But all of them are showing their age and needing repairs. So I figure it's just a matter of time for the furnace. And while it still works, I don't know if it's in good shape. When I replaced my 23 y.o gas water heater, only then did I realize that it wasn't. The suddenly hotter hot water was a revelation! Another driver is that I want to spend time overseas and rent out the house, and increasingly renters want A/C (and I do too). Without whole house A/C, renters would likely install window units. That bothers me because I don't trust renters to be careful when drilling holes and such, and also the kitchen doesn't have any operable windows at present (they're stuck shut), so kitchen would still be hot. There's also the 'green' factor of moving towards electrification, and a hunch that if tariffs are enacted the cost of heat pumps will rise significantly in the future (they're already slated to increase due to the refrigerant changeover). I got quotes to just add whole house A/C, but figure if I end up needing to replace the furnace in a few years I may as well do a heat pump now. So there's my thought process :-)
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Damn, the bots are finding the new threads now.
Yeah, bummer. At least they're easy to spot.
user-171762 Like Patrick said what are your goal?
The way I see it dual fuel is a plan for someone with too much money that failed to do the math. Depending on your local fuel prices the numbers will be different but, in most places, city gas is so cheap that it will cost less to run the gas furnace unless it is over 40°. That makes it almost impossible to recover the cost of the heat pump.
For me with expensive propain and cheap winter electricty a heat pump that supplies 100% of my heat down to 6°F and electric back up for the 2% of the year when it is below that works great.
I say pick one fuel or the other.
If your old house is drafty and poorly insulated a heat pump may not be a good fit as they need to move more air per BTU of heat delivered and some of it may be blowing on your favorite chair.
If the goal is to decarbonize, your house, then get rid of the old gas appliances and cut the gas pipe at the curb.
If the goal is to be green, then keep using the existing furnace as long as possible to delay its replacements manufacture for 10 more years.
If the goal is to save money, then invest in air sealing and insulation upgrades.
Do not replace your furnace simply because of its age. Yes, it maybe old but it is a machine they do not die of old age. If a part wears out replace the part. Remember the guy selling furnaces works on commission.
Walta
Walta, all valid points! I just noted my rationale (above). I'm preparing to spend time overseas and trying to minimize chances that furnace will fail when I'm not here and I won't have the luxury of time to explore replacement options. Not to mention dealing with tenants who don't have heat. That's what happened with the water heater. I'd been thinking of getting an on-demand system, but had a renter in the house at the time so had to just go with the quickest solution when the heater failed.
I also want A/C. Cost for a new furnace + whole house A/C is about the same as for furnace + heat pump, or just heat pump alone. A bit surprised by that.
I do keep most of my appliances (and everything else!) as long as possible and do most home repairs myself, including plumbing and wiring. But when the house is rented that won't be an option.
Oh, and another reason for dual-fuel is because I suspect electric rates will go up here much faster than gas, due to increased demand (lots of new data centers here), supply lagging in the near term, and electric utilities having to pay $$$ for wildfire lawsuits and damages. We saw a significant increase earlier this year. A friend who lives in CA and uses a heat pump has seen her electric bill skyrocket in the last few years (she's started using her woodstove on colder days as a result). So I view dual-fuel as a hedge. Granted, the cost savings of gas will most likely be minimal, but the overall system cost is about the same as heat pump only, especially after the $2k tax credit.