Replacing 2nd floor A/C unit with combo A/C / heat pump system

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edronline

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A bit of a theoretical question as our SEER 16 a/c system is ok.

Big old 3 story + basement 3800 sf house. Western PA.

Heat is new Weil-McLain EVG-220 hot water system with big old rads all over the house.
2 central A/C systems - first floor system & 2nd/3rd floor system

When the 2nd floor a/c system goes (hopefully not for a long while), I wondering if it would be better efficiency-wise to replace it with a combined A/C / heat pump system and turning off the rads to the 2nd and 3rd floor.

So furnace would do basement and 1st floor and heat pump would do 2nd/3rd.

It's a bit unorthodox, what does everyone think from an efficiency point of view?

Thanks.
 

Fitter30

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Then your boiler would be extremely oversize. Radiators are quiet, even heat. Have no idea what your electric verses gas cost.
 

Dana

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Don't guess, or you'll spend a lot of money on oversized equipment delivering lower comfort & efficiency. Run a room by room, zone by zone Manual-J load calculations to see where you really are, and whether it's time to retire any equipment sooner than later. For newbies (and pros in a hurry), the BetterBuiltNW load tool (free, but you have to sign up for an account) is the best free tool out there that won't automatically oversize things when using the default U-factors.

If you have the time and inclination to learn how to get the most out of them CoolCalc and LoadCalc can also get reasonably close to the numbers an engineer using a pro tool might come up with. If you are uncomfortable with entering (appropriately) aggressive rather than conservative inputs on R-values & air leakage etc, the latter tools can deliver numbers well above reality, making it harder for those unaccustomed to running load tools to size it right.

If the existing AC is less than 1.5x oversized for the load there isn't much point to retiring it early. At 2x+ oversizing it's worth at least considering, and at 3x+ oversizing it's almost certainly worth replacing it if you intend to live there for another decade or more. (This is more about comfort and latent load handling than as-used SEER efficiency). In the current policy & energy climate going forward it's worth the upcharge to go with a heat pump (preferably right sized & modulating) for ANY replacement AC system. Recent decades trends notwithstanding, the retail costs of electricity are poised to crash (in an unprecedented fashion) within the lifecycle of the equipment, rendering even fracked-gas fossil burners more expensive to operate before 2035. (I know, predicting future energy prices is a crazy game, but the cost of solar wind & batteries combined will be cheaper than the operating costs of the current PJM mix before 2030, and will keep on getting cheaper year on year. The levelized energy cost Rubicon has already been crossed, we just haven't yet seen how far and how fast into the Terra Incognita it will go.)

Even if you heat half the house with a heat pump the thermal mass of the rads is probably going to save the already ridiculously oversized boiler from short-cycling itself into low efficiency and an early grave, but it would have been appropriate to do a more serious load calculation before installing it.
 

Fitter30

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Check with electric company see if they offer a energy audit rebates before thinking about equipment. It should include blower door test.
 

edronline

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Don't guess, or you'll spend a lot of money on oversized equipment delivering lower comfort & efficiency. Run a room by room, zone by zone Manual-J load calculations to see where you really are, and whether it's time to retire any equipment sooner than later. For newbies (and pros in a hurry), the BetterBuiltNW load tool (free, but you have to sign up for an account) is the best free tool out there that won't automatically oversize things when using the default U-factors.

If you have the time and inclination to learn how to get the most out of them CoolCalc and LoadCalc can also get reasonably close to the numbers an engineer using a pro tool might come up with. If you are uncomfortable with entering (appropriately) aggressive rather than conservative inputs on R-values & air leakage etc, the latter tools can deliver numbers well above reality, making it harder for those unaccustomed to running load tools to size it right.

If the existing AC is less than 1.5x oversized for the load there isn't much point to retiring it early. At 2x+ oversizing it's worth at least considering, and at 3x+ oversizing it's almost certainly worth replacing it if you intend to live there for another decade or more. (This is more about comfort and latent load handling than as-used SEER efficiency). In the current policy & energy climate going forward it's worth the upcharge to go with a heat pump (preferably right sized & modulating) for ANY replacement AC system. Recent decades trends notwithstanding, the retail costs of electricity are poised to crash (in an unprecedented fashion) within the lifecycle of the equipment, rendering even fracked-gas fossil burners more expensive to operate before 2035. (I know, predicting future energy prices is a crazy game, but the cost of solar wind & batteries combined will be cheaper than the operating costs of the current PJM mix before 2030, and will keep on getting cheaper year on year. The levelized energy cost Rubicon has already been crossed, we just haven't yet seen how far and how fast into the Terra Incognita it will go.)

Even if you heat half the house with a heat pump the thermal mass of the rads is probably going to save the already ridiculously oversized boiler from short-cycling itself into low efficiency and an early grave, but it would have been appropriate to do a more serious load calculation before installing it.
This is very helpful. Yes, thinking about future proofing our house away from natural gas to renewables. It's going to happen at some point anyway.

What I like about the new NG system is that it has an external and internal thermometer, so it doesn't so what the old one did- always try to get to 180, then cut off the burner for a few minutes, water cools a bit and then it comes on full blast again for a few minutes, repeat over and over...
 

edronline

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Check with electric company see if they offer a energy audit rebates before thinking about equipment. It should include blower door test.
Already done. Getting our old house well insulated by a more expensive but highly regarded company made a huge difference in overall comfort and energy bills. We did this maybe 5 years ago. Kicking myself that we didn't do it sooner. I think it'll also increase the life of our HVAC equipment because they're working less.
 

rest_in_pasta

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Already done. Getting our old house well insulated by a more expensive but highly regarded company made a huge difference in overall comfort and energy bills. We did this maybe 5 years ago. Kicking myself that we didn't do it sooner. I think it'll also increase the life of our HVAC equipment because they're working less.

Did this company air seal your home? The insulation will act as an air filter if said company didn't bother to address the air leaks.

Once air sealing is done (to the best of the contractor's ability), you can verify with another blower door test for air tightness. Your HVAC equipment will be oversized due to the reduced heat load/heat loss after proper air sealing.
 
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