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Replace AC with Heat Pump and use existing gas furnace as backup?

wcweaver3 Member Posts: 46
edited February 2022 in THE MAIN WALL

I have a 2,300 square foot, 2 story home built in 1940. It has a Burnham natural gas fired hot water hydronic system, Model 207NSL-TEI2 (Series 2, Model "B"). It is rated at 198,000 BTU and was built in September, 2000. The boiler serves 2 zones. Each zone has a mix of the old type 1940's radiators as well as baseboard radiators for areas that have been renovated over the years. Zone 2 serves the basement, the first floor and second floor. The house has good attic insulation. Over the past year, the furnace has had extensive upgrades and repairs (costing thousands) and is in good working order.

The house is also served by a seperate Unico AC system with a 3 ton American Standard compressor outside using R22 refigerant. The unit was installed 17 years ago (2005).

While the AC system still works, given the age of it, I am inclined to replace it. I don't want to be forced into replacing it some hot August day only to be told that it takes three weeks to get a new compressor because they are on backorder.

My question is this: What would be the least expensive "ongoing cost to operate for heating" scenario?...Scenario 1: Replace the AC unit with an air-to-air heat pump (no resistance coils) and use the existing gas furnace as the backup on really cold days OR Scenario 2: Replace the exsiting AC unit with another AC unit (SEER of maybe 14?). It's my understanding that for cooling, the heat pump and a regular AC unit cost about the same to operate. I am trying to figure out, just from a cost to operate scenario, how much I could save, for heating, by going with the heat pump scenario 1. If I can fugure that out, then I can then determine the payback period for the difference between the installation of the heat pump versus the AC unit.

Some numbers:
- I live in Pittsburgh which is Zone 5A accoring to the 2018 map from the US Department of Energy.
- Total gas usage to operate the existing natural gas furnace for the entire heating season is 130.6 MCF at a cost of $4.44 per MCF which is my "cost to compare" (I assume this is the cost just for gas without the taxes, distribution charges, etc.). The heating period is November through May and I have subtracted out the base line usage for cooking, hot water, dryer, etc.
- Total electrical usage to operate the existing AC unit for the entire cooling season is 3,246.7 kWh at a cost of 8 cents per kWh which is my which is my "cost to compare" (I assume this is the cost just for electricty without the taxes, distribution charges, etc.). The cooling season is June through October and I have subtracted out the base line usage for lighting, appliances, etc.

Understood that there is a differential in cost to install and that at some point, hopefully way far away, the gas furnace is going to have to be replaced. Additionally, I understand a goethermal heat pump would cost way less to operate, but the instalallation cost is blinding so that's not an option. Neither is a mini split system.

Perhaps there are rules of thumb that if applied to this situation would clearly point to the "no brainer" option. Perhaps there are online calculators that can figure all this stuff out and if you can point me to them, I'd welcome the chance to crunch the numbers myself.

Many thanks for your help and thoughts. Really appreciate it!


  • TAG
    TAG Member Posts: 755
    edited February 2022
    You need to divide the full cost of the gas by the unit usage number to get the full cost of the fuel. Same with electric ... full cost divided by KW hours used. Most utilities have tier cost structure ... just divide the whole thing .... you are trying to get numbers in your head.

    That said .... with NG -- even with recent costs. Is going to be the cheapest way to heat a house in PA. I'm on propane at my place in PA .. so the heat pumps can get there at some temps and propane costs. My sister when I did it last fall even at 3cop the HP was 2x NG.

    A heat pump is nothing more than a regular AC working in reverse -- go online and watch a few videos. There are some great examples. It costs no more to operate a heat pump in the summer vs an AC -- they are the same.

    PA does not have the great rebates as some states .... in NJ the clean energy program makes heat-pumps cheaper to install in many situation VS just going with a straight AC. It's kind of silly as most I know end up not using the HP because NG furnaces are so prevalent.

    What is different with your setup is the Unico ... these high velocity systems are a different animal. I only had one experience with a house I bought with one already installed .... it worked well but I always thought it was a bit expensive to run relative to my neighbors .... they do a nice job on the humidity. You can use them for heat ... Typically they use an additional coil run off the boiler as this produces hotter air. The high velocity can make drafts in the winter. They do supply them with heat-pumps .... I have never lived in a house with one (mine was only AC) .... also -- since the Air coming through the system has to stay at a rather high rate ... they suffer when it comes to the ability to have multi speed compressors .... although I have read where someone used the new Bosch unit. I would contact Unico and/or your dealer and ask what is available. Get some prices and also see what if any problems they are having with getting equipment. Frankly --- if it has run for a long time w/o needing any service. I would wait until it dies or they came up with something VS that will save you some money when you do upgrade. With it not being the current refrigerent you can't just replace the outside unit -- so it's all or nothing and you would have to do the math on any possible savings in electric in the summer. My guess is the replacement cost will never add up just for replacement .....
  • pecmsg
    pecmsg Member Posts: 4,916
    1St Get an accurate Heat Load / Loss done. Manual "J".
    2nd The Unico will limit what can and cannot be done. Possibly a BOSCH heat pump with a Hot Water Coil. This is something that must be seen.

    From here 200K and 3 tons seems large especially if air sealing has been done.
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,906
    You want all marginal gas and electricity costs, which is anything that’s $/energy unit. $/month or $/bill or $/day are irrelevant for this, unless you plan to completely disconnect from gas. You’re right, the cost to compare is not the full picture, just the supply cost. The distribution costs are often significant. 

    The short answer is yes, it’ll be probably more cost effective to heat some with a heat pump, keeping the boiler as backup. A heat pump and AC will cool at the same efficiency and cost about the same to install (they’re nearly identical mechanically, and the unit itself is only a fraction of the installation cost). Doing a payback analysis is almost overkill since the upcharge is minor. When I priced it out, a heat pump was cheaper than the similar ac + furnace set up. We’re talking small amounts either way. 

    More importantly, the type of heat pump/AC you can solve comfort issues - look for a variable speed unit if you can, they’re much quieter and do a better job of matching heating output to heating needs. Same for cooling, with the added bonus of better dehumidification. 
  • motoguy128
    motoguy128 Member Posts: 393
    In colder weather, radiant heat is more comfortable.
    The UNICO will lose some heat to the attic.
    A 3 ton HP may not play nice with the Unico. 250CFM/ton wont cut it above 40F I’ve found. Maybe you can downsize to 2.5 Tons if your house has enough mass, attic insulation and shade. Does it run constantly at design… like 8-10 hours straight?
    Can you increase airflow in heating to 275 or 300 CFM/ton (900)?

    Economic Balance point will likely be around 35F. SDO keep that in mind. You make the added investment in a heat pump but it only operates less than 1/2 the total heating degree days.

    Positives is that the Unico will heat a little more evenly in mild weather compared to a cast iron boiler. However A modulating boiler with Reset would still be better.
    EBEBRATT-Ed Member Posts: 15,767
    edited February 2022
    apparently unico does have heat pump condensing units
  • TAG
    TAG Member Posts: 755

    apparently unico does have heat pump condensing units

    they do -- I remeber seeing them. I also think they have expanded the line .... one of the HV is doing air to water HP
  • SteamingatMohawk
    SteamingatMohawk Member Posts: 1,024
    Are you considering a heat pump with gas backup? I have had one in my 2900sq ft home since 1988 with only a few maintenance needs over the years, 1 capacitor for the compressor, two replacement fan motors in 33 years and some freon.

    Then you remove both the separate units and any unnecessary equipment.
  • SteamingatMohawk
    SteamingatMohawk Member Posts: 1,024
    Mine is a single zone, but I know they can be zoned these days.
  • Total
    Total Member Posts: 13
    Im planning on doing the same thing this summer , The Bosch heat pump that i'm looking at is more efficient that my old ac unit . First heat source heat pump , switches to natural gas when the temp outside reaches -5 C . And rebates are coming out also .
  • wcweaver3
    wcweaver3 Member Posts: 46
    Thank you all for the robust information. This is where I landed...

    1) Based on several bids, the cost to install a HP over a traditional AC is AT LEAST three thousand more
    2) Based on the conversation, I might save a little using the HP on half of the heating days in PA. The other half, I'd need to use the existing gas boiler.
    3) Running the AC versus a HP during the summer is probably about the same, so no cost savings there.
    4) Given that I have a high velocity Unico system, the HP is probably going to be drafty, so probably less comfortable than the radiators powered by the gas boiler.

    HP doesn't seem to be worth it as it would take a long long time to payback, if ever, from the savings in heating on half the days during the heating season.

    I'll stick with an AC replacement rather than going with the HP.