Replacing AC with a Cooling-Only Heat Pump
What are the options for replacing a ducted, cooling-only AC with a heat pump?
If I’m understanding right, it’s a new unit (with reverse valves, larger coils, etc.) but my question is: can components like the refrigerant plumbing, air handler/fan, electric circuit, etc. be repurposed? Or must everything that the AC-only setup uses be ripped out?
I bought a new-to-us house that came with a 1yr old Rheem propane furnance that we’d like to downgrade to supplemental heat, and I’d like to keep a cooling function in the ducted setup. So this could be done with a reverse cycle heat pump? I’m OK with a heat pump that is only efficient for the shoulder month heating.
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While refrigeration techs are reluctant to re-use refrigerant plumbing you might be able to get some modest savings in the installed cost at fairly low risk for a system with only a few hundred hours of cooling use on it.
It's likely that the same air handler could be used, provided the heat pump's cooling capacity is about the same or slightly smaller than the current AC system, assuming you're going for a higher SEER heat pump (model names and numbers of the existing equipment could be useful for getting advice on this here.) More often than not the AC is ridiculously (and sometimes ludicrously) oversized for the 1% design load, something you can figure out by measuring the duty cycle on the compressor (not the air handler) during the cooling season on afternoons that cross the 1% design temperature. See:
https://www.greenbuildingadvisor.com/article/how-to-tell-if-your-air-conditioner-is-oversized
It's important to calculate the loads with some degree accuracy to know whether it's going to have sufficient capacity, assuming you want to mothball or remove the propane burner. If the heat pump doesn't cut it on it's own and you're using it as a "dual fuel" system only the heat pump OR the propane burner can be used at one time, and you'd have to figure out the temperature at which to cross over. If you have records of exact fill-up dates and amounts going back to November 2020 it's possible to estimate the heat load at any given temperature using the methods outline here:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
Installing smaller equipment on existing ducts usually works out well, since the speed of the air handler can usually be strapped to a lower cfm, leading to lower duct velocity and lower static pressures, quieter more even operation. If the ducts are smaller than optimal even for the existing setup that still needs to be re-assessed for the replacement equipment.
But let's start with figuring out the 1% and 99% load numbers relative to the output of the existing equipment.
Dana,
I should have known better than to post w/out more details :)
Furnance is 70k BTU 96% AFUE 2 Stage w/ ECM fan. AC is 2.5 ton(?*).
House is 1600sq ft, 2 floors, ductwork runs from basement and not in attic. I'll look up the great articles here for trying to get at ballpark loads ... it's been awhile since I did that exercise of #s.
Unit details:
Rheem R96VA-0702317MSA
ADP TG31630D175B1605AP
I'm missing the outdoor units model # so I'll add that later.
* ADP doesn't make it easy to search model / serial #s! but by reverse engineering their model code(https://www.adpnow.com/wp-content/uploads/2010/10/HE-Series-Spec-Guide-SG-HE-07.pdf), the evaporator unit should be:
HE series, Train rebranded, aluminum, piston, 30k BTU, 21" deep, 17.5" wide, 16.5" height
More to come,
Justin
>"House is 1600sq ft, 2 floors, ductwork runs from basement and not in attic. "
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>"Furnance is 70k BTU 96% AFUE 2 Stage w/ ECM fan. AC is 2.5 ton"
The equipment is (no surprise) suboptimally oversized for the likely heat load of the house. A typical reasonably tight 1600' house that is 2x4 framed insulated house with clear-glass (not low-E) double panes would have a heat load of 25,000-35,000 BTU/hr @ 0F, less than half the output of the furnace. If it's tight 2x6/R19 with low-E glass you're lookint at something in the 20-25K range.
We don't know what your actual 99% outside design temp is ), but it's probably not a whole colder than 0F- negative single digits. (Take a guess: https://higherlogicdownload.s3.amazonaws.com/ACCA/c6b38bda-2e04-4f93-bd51-7a80525ad936/UploadedImages/Outdoor-Design-Conditions-1.pdf ) I'm pretty sure it's not -50F which would be cold enough to call for a furnace that big.
But the oversizing might be a blessing in disguise, if the ducts are correctly sized for the air handler. A 2.5 ton cold climate heat pump can deliver over 25,000 BTU/hr @ 0F. The ADP TG31630D175B1605AP coil is heat-pump compatible, so it's a matter of finding the appropriate compressor/condenser unit & controls.
Rheem's RP2036BJV (3 ton) compressor is good for about 30,000 BTU/hr @ oF when married to a 3 ton Rheem coil, and would likely be easier to marry into the existing system (or not). It would require getting down into the design weeds to estimate how much you could get out of the 2.5 ton ADP coil with a 3 ton Rheem compressor. The RP2036BJV can easily be married to the R96VA1152524M furnace (115K version of what you have), and can probably be tweaked to run just fine with a 2.5 ton coil. Worst-case you could marry the RP2024BJV (2 ton), to the 70K furnace, which is good for about 16,000 BTU/hr @ 0F when using a Rheem 2 ton coil, but is probably good for over 20K with a 2.5 ton coil (TBD).
A 3 ton Rheem with a 3 ton coil typically delivers about an HSPF 11 (=11,000 BTU/kwh for a seasonal average), but you might only get an HSPF 10 out of it with a slightly mis-matched coil. eg:
https://ashp.neep.org/#!/product/29185
Normalizing to million BTU (MMBTU) that would take (1,000,000/10,000=) 100kwh/MMBTU. At Wisconsin's average residential retail price of 10.66 cents/kwh (see: https://www.eia.gov/electricity/state/ ) that costs $10.66/MMBTU.
At 96% combustion efficiency a gallon of propane delivers about 88,000 BTU/gallon as heat into the supply plenum, so normalizing to million BTU (MMBTU) it takes 11.36 gallons of propane/MMBTU, at a cost of (x $1.35=)$15.34, or about 50% higher cost than an HSPF 10 heat pump.
If you said what part of the world your house is I missed it but in most places in most years it will be less expensive to heat with a heat pump than propane. The price of propane can vary a lot.
With propane backup the problem is the resident the heat pump is going to supply air a 82-99 °over time the residents can adjust to that and it becomes normal. When the propane heat kicks in at 130° people will quickly learn to like that feeling and be very tempted change the setting to make it happen more often.
If you want a heat pump you will need to replace the indoor AC coils with heat pump coils matched to the new outdoor unit. Your furnaces fan will be the air handler. Most contractors will want to replace the copper lines if at all possible so everything in the system new and they can guarantee their work.
I have a Rheem 17 SEER heat pump with electric back up locked out when it is above 6° F. Most years we only have a few hours below 6°.
If you do this get a Rheem unit and thermostat so that they can communicate with the Rheem furnace you have.
Walta
Hi Walter,
I live in Wisconsin so zone 6a.
Propane here is $1.35 but we get a discount for being on the truck's regular top-off / fill-up route.
You make a good point about matching the heat pump maker to furnance maker. I was just thinking the other day of staying with Rheem so that any proprietary controls wouldn't become a problem.
Also, what's the typical temp for dual-fuel / supplemental furnance? So the temp when furnance takes over for heat hump. And is the temp configurable? I remember a GBA article explaning how heat pumps reach a point where their efficiency and/or capacity goes down, and that point was dependent on ambient and set temp in addition to the rated capacity of the heat pump IIRC.
Justin
>"Also, what's the typical temp for dual-fuel / supplemental furnance? So the temp when furnance takes over for heat hump. And is the temp configurable?"
I assume you mean the crossover outdoor temperature at which one changes to the fossil burner?
That is always user-programmable. The temperature can be set for capacity reasons (the heat pump isn't keeping up), or for cost reasons (the temp at which the efficiency of the heat pump is low enough that it's cheaper to run the fossil burner), and the programmable range is usually quite flexible.
BTW: Since you are on a regular fill-up schedule you should be able to analyze the wintertime fuel use to estimate the design heat load reasonably accurately:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
If you like we can run those numbers here on the forum if you'll share your ZIP code (for weather data), and the fill-up quantities & exact dates from late November through early March.
I'll take you up on that offer because I started to crunch #s and got a bit lost.
Here's #s that I put together from fill-ups and degreedays.net:
Date Gallons Days HDD (Sturgeon Bay)
01/07/21 220.33 - -
01/26/21 135.52 19 788.3
02/08/21 82.72 13 662.4
02/19/21 94.47 11 1303.2
03/12/21 66.77 21 731.6
BTW: the links to Design Temps in older GBA article looks to have gotten pulled (https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf, http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf) but I did find Energy Star's website has a list here: https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/County%20Level%20Design%20Temperature%20Reference%20Guide%20-%202015-06-24.pdf
That ^ link's closest location (Door Co, WI) has 99% of 0F. The house's zip is 54213.
P.S. I used 65F base for the HDD #s above. Also, we weren't living in the house yet so the set temp was 62F not 68F.
With a heatpump you have 2 balance points one is mechanical point and the other is economic point.
The mechanical point is the temp at which your house looses BTUs faster than your equipment can replace the BTUs. Calculations are interesting but the only real way to know is to let the system run and find out what it can do.
The economic point is a calculation with the price if both fuels and the efficiency of your equipment at different temperatures.
If you mix brands in your system you end up with 15 control wires instead of 4 and often loose features and capabilities like locking lock out back up heat based on the outdoor temp.
Yes my Rheem system allows me to lock out my backup electric heat when the outdoor temp is below a number of my choice. I would assume gas backup would have similar options.
Walta
I'll add a warning about reusing the refrigeration lines (the copper tubing): if your old system used R22, and the new system is R410A, the oil will be different. The old oil in the new system can potentially cause some big issues with the compressor motors. I had that happen once on a commerical project, and the compressor died within about 3 months. I wouldn't take that risk for the relatively small cost savings you'd get from reusing the lines, even with today's crazy copper prices. DO take your old lines out and sell them for scrap though, you can use that money to offset the cost of the new lines.
You should have no problem reusing the electric circuit. The only thing that matters here is the wire gauge and the configuration. If your old unit was 240v at 30A, and the new unit is 240v at 20A you're fine with the wire -- but you'll need to swap out the 30A breaker for a 20A breaker. If you need to change from 240v to 120v you should be able to reuse the old wire with some changes to the connections on the ends. If you need to go from 120v to 240v you MIGHT be able to reuse the old wire, depending on what the new unit needs (mainly if it needs 3 or 4 wire service). Note that you CANNOT go from a 20 amp to a 30 amp circuit using the old wire unless the old wire was oversized originally. You can change to something smaller, but you can't go bigger than what was originally there.
The air handler might be able to be reused depending on the nature of the new system. I haven't been involved with that particular part of this often enough to provide much more detail for you there.
Bill
Bill,
The nameplates here say 410A is used, but noted.
The bigger issue sounds like HVAC folks may not want to touch the heat pump unless the current AC setup is entirely ripped out, but I'll see how far I can get.
Jp