Heat Pump With Existing Furnace as Backup
I live in Connecticut (Zone 5) and have a ~2,000 square foot home that uses fuel oil for heat (furnace, ~12 years old).
I’m looking to convert to a single zone, central heat pump system (I have central AC, so I’m assuming our existing ductwork will be generally sufficient although I’ll get it tested). But given the local climate, and the fact that the house is a bit older (1950s built) and therefore potentially less well insulated, I’ve been told by some installers that I should keep my existing furnace as backup.
Practically, how would that work, and would this be more complicated than just using aux elec resistance backup? Fuel oil is expensive up here, way more expensive than natural gas, so it’s not really an ongoing utility cost argument (I think electric resistance and fuel oil with an 80% AFUE furnace are practically not massively different in CT). It’s more a question of power outage mitigation, and also maybe a bit of upfront cost mitigation (i.e., I can size to 90% load and therefore get a slightly smaller heat pump, and also potentially avoid some weatherization upgrades I might need if going purely electric). A few specific questions for you all:
1. Would you generally recommend keeping an existing furnace as backup? Will this cause more headache in the long-run than the backup mitigation that it provides?
2. Would the heat pump and furnace share an air handler? (I assume so, but how easy / hard will it be to connect a 12 year old furnace to a new air handler?)
3. What kind of integrated controls would I need, and how ‘simple’ is the communication here? What kind of programmability would it offer? Or would I have to manually turn off one system and turn the other on?
4. Maintenance. Is this also going to be significantly more of a headache to get maintenance on two systems?
Thanks all!
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Replies
1. Would you generally recommend keeping an existing furnace as backup? Will this cause more headache in the long-run than the backup mitigation that it provides?
It’s not a bad option: furnaces are pretty low maintenance. You could avoid a potential electric service upgrade, size better for cooling, and if I understand what you’re getting at, use a smaller backup generator for power outages.
2. Would the heat pump and furnace share an air handler? (I assume so, but how easy / hard will it be to connect a 12 year old furnace to a new air handler?)
They’d share an air handler, but not in the way you’re thinking. The furnace air handler would stay and the heat pump would be added: it’d be like an AC coil, just a HP. A new handler would be more comfortable and efficient though.
3. What kind of integrated controls would I need, and how ‘simple’ is the communication here? What kind of programmability would it offer? Or would I have to manually turn off one system and turn the other on?
It wouldn’t be manual. But I’d assume a paired system would communicate better (ie new HP and new furnace, from the same manufacturer).
4. Maintenance. Is this also going to be significantly more of a headache to get maintenance on two systems?
No - the oil maintenance will be the same as before. HP maintenance would be the same as if it was AC + furnace. Basically, you’ll have to change filters.
Thanks Paul, this is really helpful
The way I see it the biggest problem with the dual fuel is the humans.
The old furnace will keep blowing the same 130° air and the heat pump air 20-25° above your set point say 68° so in the 90s. The heat pump will keep the home at the set point no problem. The humans however tend to like the feel from the 130° and are likely to push the buttons to keep the warmer air flowing.
Most of the contractors are reluctant to reuse old equipment because one touch and every problem after the touch becomes their fault. You can expect a strong up sell for all new equipment.
Remind me what is the point of buying new equipment since the operating costs are almost the same.
I say the greenest thing to do keep the existing equipment in operation as long as possible delaying the manufacture of its replacement as long as possible.
Walta
Thanks, yeah that's a good point, takes some discipline from everyone in the home. And the contractor point is a really good one too.
Part of the motivation here is also incentives - CT has $1,250 in direct rebates per ton but only through end of this year (maybe it gets renewed but who knows). So that's a $5,000 discount for a 4-ton. I also have a similarly old AC unit, so part of the calculus is if I should do it now for the rebates, so that I can get a heating & cooling system for about the same price as a new AC system.
I was doing the math on a 10.5 HSPF2 central HP, assuming backup fuel oil is used at <25 degrees, for my climate's heating degree days. I think it should draw about 6,500 KWH annually, and then I'd need about 150 gallons of fuel oil for aux. CT is about 50 / 50 nat gas / nuclear for its electricity generation, and if you apply power generation emissions to the KWH for the HP, and then calculate the CO2 emissions from 150 gallons of fuel oil, and compare that to my previous fuel oil usage, it should be a reduction of about 8-10K pounds of CO2 annually. So net-net, I think upon failure of my existing AC, the greenest move is definitely the HP switch rather than AC replacement, but the incentives and the timing of them are making me think about an upgrade.
> CT has $1,250 in direct rebates per ton but only through end of this year (maybe it gets renewed but who knows)
Incentives PER TON? Wow. It's almost like someone wanted to create a system that directly benefits the HVAC contractors and results in them installing oversized equipment...
After all, why not upsell to a larger system "just because" when it's "free"?
Agree with Patrick, don't size based on rebate, size based on heat loss. Luckily, you should have the existing oil usage to figure out the heat loss easily.
"Would I/you install a dual fuel system day one on a new build?"
That is a question worth answering, because it divorces the question at hand from its inherent sunk cost fallacy trappings. If the economics of replacing make sense, then no, don't keep the second system around. If keeping the second system around is necessary to make the economics of the new system work, then I question the new system entirely.
Arguments to kill existing system:
- More equipment, more that can break
- Someone needs to properly integrate the two
- Interior oil tank can be removed
Arguments to keep existing system:
- I have a plan to operate the oil system in an emergency
- Exterior oil tank should be removed if oil burning appliance is removed