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I have a standard southern California tract home that was built in 2010. It has a Carrier gas-fired furnace, Carrier 5-ton A/C, and a smart thermostat.

I'm slowly upgrading the major systems and replacing gas with electric wherever feasible.

However, since the gas furnace is already there, is it possible and/or feasible to upgrade the A/C to a Heat Pump by replacing only the outside unit, leaving the gas furnace in place as the air handler and as the secondary/emergency heat?

The system as it exists now:

  • Carrier 58STX110 gas-fired forced air furnace
  • Carrier 24ABB360W320 A/C compressor (R410a)
  • Unbranded evap coil ASFM6026A (X-reference lookup tells me Allstyle Coil Co)
  • Ecobee thermostat

My reasons for wanting to upgrade the A/C to a heat pump:

  • Gas heat as primary is stupid and wasteful in this climate
  • The current A/C compressor is going on 13 years old
  • Current A/C compressor is an 11 SEER - newer one should be a fair bit more energy-efficient all year, not just the 2 month heating season
  • I have gobs of extra solar during the winter that I'd rather use on-site than sell to to the PoCo for a fraction of its worth

My reasons to try to retain the gas furnace as backup heat:

  • Aside from the compressor being cool-only, the rest of the system is in fine working order - might as well create less waste and save material costs
  • All the indoor HVAC equipment is in the attic of a narrow 2-story. Reusing instead of replacing should also save labor
  • Only 120V/15A electric available in the attic (and it would be very difficult/destructive to route additional wire up there, so 240V strip heat for Em. heat would be costly/impractical)
  • Blower motor was recently replaced with an Evergreen IM ECM blower.
  • Ecobee thermostat has the ability to control multistage heat/cool

So... is this idea realistic? I'm going to have to involve a contractor anyway because of the refrigerant, but I don't want to be sold a full tip-to-tail system if I can realistically only swap out the parts that need to be swapped.

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    This is great in theory, works perfectly handwaving about the equivalent parts, but has a nasty habit of being problematic in practice, unfortunately...
    – Ecnerwal
    Oct 6, 2022 at 0:23
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    At a minimum, you will need to rewire the thermostat and furnace, and reprogram the thermostat. Heat pumps use separate “pump” and “reverse” wires; a combined furnace–AC has mutually exclusive “heat” and “cool” wires. Oct 6, 2022 at 11:37

3 Answers 3

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YES, basically.

Because the only real difference in the outdoor unit is a reversing valve and possibly some sensors to detect freeze-up when running in heat mode, and some code. If I were king, there'd be a retrofit kit lol.

Here is the only gotcha. Operating in heat mode requires the refrigerant to operate in a somewhat different range, obviously. That may necessitate a different refrigerant, that may operate at a different flow or pressure. That may, in turn, require replacement of the interior unit in the air handling stack and possibly hoses too. That said, I don't see a problem with R134A.

But still, R134A is a bit of an albatross. You may notice that the very high-SEER units are using other refrigerants. So I suspect this will create a practical upper limit on the SEER and COP which is possible for you to achieve.

Lastly, I somewhat question your premise. You are trying to optimize for saving the most materiél, on the presumption that things made of atoms are the scarce commodity in technical products, thus it's worth going through backflips to save that evaporator coil. I disagree. I say the knowledge to integrate complex systems successfully is the scarce commodity. The engineering skills to find the right compressor unit to match to that evaporator may well cost more than another evaporator. This is the dilemma of modern overseas robotic assembly line manufacture.

So I am sorry to say this, but you may be better off letting the old evaporator become Mountain Dew cans, and use a matching evaporator that lets you achieve the highest possible SEER/COP.

And, this will manifest as contractors being reluctant and unwilling to build the system the way you want, because, they are afraid of their valuable time being wasted by many "go-backs" to resolve problems, afraid of you misbehaving as a customer* (because the demandy ones are the worst for that), and being unable to give you a warranty on any of it. Even if you are paying for the go-backs, good contractors are so overbooked that they have no reason to touch that system with a 10 foot pole.

That leaves bad contractors.


* by "misbehaving" I mean you would presumably agree in advance to waive warranty and pay for go-backs and support, to some extent. Misbehaving would be forgetting those agreements in the heat of the system not working, and then becoming a hell customer. That alone would motivate many shops to not touch this with a 10 foot pole.

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    I agree on the need to replace the interior heat exchanger as part of the system upgrade. Adding or replacing a heat pump or A/C to an existing furnace and HVAC fan is a very common request that HVAC contractors would deal with, they'd know how to do this or go out of business. The heat exchanger is something that will clog with dust in time, or at some point develop leaks from one too many heating/expanding to cooling/contracting cycles. If the exterior unit is old enough to replace then so are all the interior parts.
    – MacGuffin
    Oct 7, 2022 at 3:03
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AC/ Heat pump compressor units ( outside) and Air handlers (inside) are made to work together an be balanced properly. Trying to Adapt a compressor to something it was not designed to work with may be achievable, however the results will most likely be unacceptable and a constant source of issues.

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The indoor unit’s plenum coil is a reasonably high quality aluminum coil. part number is incomplete, it’s been truncated leaving off details like which expansion valve it contains. There has to be a check valve bypass for the indoor coil’s expansion valve to allow the flow to be reversed in heating mode. It can be a changed if necessary, but it might not be economical, at 10y old its unlikely to fail soon, overwhelmingly likely to be the first part to fail after retrofitting. The twaddle in some answers about systems being designed with carefully matched components for maximum efficiency was uninformed. The fact is, a coil is just a heat exchanger. The right surface area depends as much on where it’s installed geographically and the building’s properties as anything else.

If the objective is high efficiency then the answer will be a system with oversized heat exchangers an inverter drive scroll compressor and electronically controlled expansion valves.

The bad news is a high efficiency system will definitely want to have new cable to the air handler space. I don’t know why that will be “destructive”. Unless the walls are murals, It’s likely to require cutting small openings in the drywall at floor & ceiling on both floors to create access where holes have to be drilled to pull cable to reach the crawl space & outdoors. The drywall openings can be closed and skim coat plastered the same day. It is necessary to wait for the plaster to cure overnight before painting. Drywall compound can be painted immediately but it shrinks as it dries and requires multiple applications, again running over to a 2nd day, and it’s as soft as snot and dents and gouges easily, so use skim coat plaster and trowel it well and get smooth hard wall patch.

“Gas heat as primary is stupid and wasteful in this climate” is an interesting assertion. The thermodynamic efficiency is irrelevant to a property owner. The total cost of ownership is the correct metric, including the equipment cost. The traditional relative prices of natural gas and electricity make adding heat pump functionality to air conditioning a modest incremental equipment cost for a moderate reduction of purchased energy cost. California electric rates are double the national average but the cost of natural gas is around 15% below the national average. The reduction in purchased energy cost is less than half what it is elsewhere, making a total cost of ownership reduction from accelerating the replacement of equipment unlikely for a building located where a narrow 2 story building needs a 5T air conditioner. All the more unlikely because the CARB certification on equipment sold in CA adds 10-20-30% to the price tag.

Don’t count on a higher SEER AC to deliver an electric energy cost reduction to make the replacement pay off. The SEER rating is biased toward rewarding increases in efficiency at moderate outdoor ambient temperatures when system operating efficiency is already very high by reducing otherwise fixed energy costs like fan speeds etc. The operating efficiency at high ambient temperature and >80% of system capacity is dominated by the refrigerant used rather than the sophistication of the compressor or other system components. (It’s right for energy policy in general because office spaces need to reject much more heat than hones during moderate weather, its just not likely to be a metric that provides good guidance for homeowners unless the house has a lot of south facing glass and you work from home and have lots of office equipment running during the day.)

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  • Could you please edit that wall of text into paragraphs? As it stands, you're not going to get many people actually willing to read through it. (If you don't have permission to edit your own answers yet, ask for help and someone will probably be willing to do it for you.)
    – keshlam
    Nov 18, 2023 at 0:16

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