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Heating a Historic House With Heat Pumps

RRSFDV | Posted in Green Building Techniques on

Hello there-

I have a general question about HVAC systems and backup heat.   We are renovating an 1850s Colonial in zone 5b (Hudson Valley, NY) (about 4,000 sqf), and our broad goal is to eliminate as much oil/gas use as we can.  Since the house needs a full renovation (including a new HVAC system), we were hoping to move over to a fully electric heat pump system.    Because the roof and the roof deck need to be completely replaced, we’re planning to do a substantial retrofit there with insulation on inside and outside, and in addition, we’ll be insulating the basement from floor to rim with 2” spray foam (thanks to this group for great answers to this in a previous question).

We won’t be able to add much insulation between the walls, as both sides have historic details (and much of the first floor has brick between the plaster and exterior siding).

We’ll know more clearly about our options once we finish our insulation retrofits, and can have a proper Manual J done, but our HVAC contractor thinks we’ll need backup heat, and is guessing that we’ll need to install two 5 ton heat pump LP furnace systems.  (He knows we won’t actually size it until we are finished with the  insulating and air sealing).  We’d like to avoid the propane backup heaters if we could.

I’m curious what the thoughts are on being able to heat our house with just heat pumps, and more generally, what people’s experiences are with larger older houses and heat pumps when it comes to comparing expense to efficiency to comfort.

Thanks in advance for your thoughts.   -Curtis

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  1. jrpritchard | | #1

    Is there a reason why you can’t incorporate electric back up? Most conventional brands like Lennox trane carrier offer air handlers that can accept heat packs that can act as back up or supplemental heat. These common brands generally have heat pumps that are much less effective in cold temps than a brand like Mitsubishi or Fujitsu in my experience. Mitsubishi offers a nice multi position air handler that gladly accepts a KW heat pack. That being said there is nothing wrong with propane backup but if you are shooting for all electric you could skip the gas and vents with an electric rig. In Iowa, zone 5, we always incorporate back up heat somehow and most of my customers have made use of the back up from time to time. I would suggest you incorporate back up in some form or another

    1. RRSFDV | | #2

      Thanks for this information, and for your experience. I've also read that the Mitsubishi and Fujitsu are better at colder temps. We'll do some additional research and talk to our HVAC people about those options.

  2. willymo | | #3

    The only reason (IMHO,LOL) to even consider LP back up is if you are concerned about a power outage. Otherwise, just incorporate electric back-up as needed. And even there, why not just wait and see what if anything is needed. In the meantime, you can buy some cheap 1500 Watt heaters if you are worried. The better mini-splits like Mits or Fujitsu will definately be fine in your area. If you are planning for a power outage, why not spend the $$ on a good back-up generator rather than the LP- protects your fridge and freezer in addition to giving you emergency heat.

    Since you are re-doing the roof I hope you are looking into installing solar panels- in my case I went from an oil heated house to one entirely supplied by the solar panels, including the four minisplit heating/cooling units and the electric hot water heater. On an old house you may need to reinforce the roof a little, never a bad thing!


  3. charlie_sullivan | | #4

    Back up heating can be for different purposes:

    1. A heat source that can work in a power failure.

    2. A heat source that works even if something breaks in the heat pump.

    3. A heat source that can supplement of substitute for the heat pump if it can't keep up on a really cold night.

    The LP backup plan proposed sounds like it would still need power to operate, so it wouldn't do number 1. Depending on what failed, it might or might not solve problem number 2. I kind of like simply having enough electric space heaters on hand to cover that issue, because then your backup is truly independent. For number 3, there are many options, including simply making sure the design can cover it. If you plan for your design to cover almost all cases and just want some additional capability for peace of mind, adding electric heat strips is cheaper and gets you away from having to store LP gas on site.

    For power failure backup, the options are really a generator or a wood stove. One possible argument for LP is that a a tank of it could serve your generator and your heating need, and the generator could be smaller if it is only running a fan and not a heat pump.

    1. willymo | | #6

      Hi Charlie-

      Good points: some thoughts:
      1. In a power failure, you either ride it out (Can be done!) or you get a generator or solar batteries.
      2. Minisplit systems often have more than one outdoor unit, and so if one goes, the other can maintain safe heat in the house.
      3. The supplemental is the question- is it needed at all? If needed, can simple portable heaters or electric strips or radiant panels suffice- the answer is yes in almost all cases.

      A backup generator is expensive for infrequent (in most cases) use. Is it really needed? In a prolonged power outage, the two big expenses are rotten food, and if allowed, frozen pipes. Pipes can be drained in an emergency.

      1. charlie_sullivan | | #7

        Agreed on all points. I didn't mean to imply that one needs backup for all three issues--more just asking people decide what problem they want to solve before starting to pick a solution.

  4. _jt | | #5

    First, 2 x 5 ton heat pumps sounds a little off!

    Second, depending on your floor plan mini splits work well in old houses. My layout (1920 house, about the same ft2) and climate zone is similar, and I get by with about 2 x 18k mini splits on opposite sides of my first floor and a single 12k split that handles heating for the 2nd and 3rd floors.

    Third, what is your fireplace situation? You probably can just stick with plain old wood for backup heat. I retrofitted mine with a gas insert which is my backup heat source and also cuts my energy costs when the COP of the heat pumps drop.

    When I moved in - we were set up with 250,000 BTU of steam and hot water. Now we seem to be down to about 36-40k of mini splits at the 99% point - by just working on air sealing, attic insulation and rim joists.

    Also: coolcalc really works! It really matched up with my actual heat load and it was easy to set up and model changes.

    1. Expert Member
      Dana Dorsett | | #8

      >First, 2 x 5 ton heat pumps sounds a little off!

      No kidding!

      Since there is a heating history on this place, run a fuel-use based heat load calculation, which is a MEASUREMENT of the heat load in it's where-is-as-is condition, using the boiler as the measuring instrument. See:

      If there are further improvements to be made on the building envelope the fuel use derived load number can be a clear upper bound, with no upsizing necessary.

      Depending on the air tightness, shape and current insulation condition of the house it's unlikely for a 4000' house to be as much as 120,000 BTU/hr (= 10 tons) @ 0F or even -10F unless it's all single pane/no-storms, but the fuel use would show that. After air sealing + insulation (including basement walls) + low-E storms (or replacement windows) it should come down t0 6-7 tons, maybe even a bit less. Getting it to under 5 tons (for the whole house) is likely possible, but only with a substantial amount of analysis & work.

      If running a Manual-J on the "after upgrades" picture, be sure that it's performed by a qualified third party such as a professional engineer, and NOT an HVAC company. While some HVAC companies have the capability of doing it properly, the industry average isn't every good at avoiding a fat thumb on the scale.

      Since this house is in the Hudson Valley it might be in Dandelion Energy's service area ( which could make a GSHP even cheaper up front than air source heat pumps. But sizing it correctly will still be critical to making it comfortable. Sized correctly it will have a very high duty cycle during colder weather rather than the hot flash followed by the extended chill.

      Even at NY style electricity rates, sizing the heat pump for the 99% design load and using electric auxiliary heat strips to cover the shortfall during Polar Vortex disturbance cold snaps is usually a better overall solution than duel-fuel heat pump + fossil burner solutions. With the former the aux heat only needs to cover the difference needed to not lose ground- most of the heat will be coming from the GSHP at a COP of ~4, whereas with dual-fuel approaches the heat pump has to turn off, and the fossil burner carries the entire load. Propane heat is probably never going to be as cheap as a GSHP at a COP of 4, and even #2 oil is usually more expensive, even at the currently low post-pandemic oil pricing.

  5. twoodson | | #9

    Take a look at the Fujitsu Multi Position AHU's. They have electric backup/aux options. I have a 24k BTU unit ready to install in my basement that replacing a ridiculously oversized (75kbtu) 78% gas furnace. I'm adding a 5kw aux heater for the occasional polar vortex and/or outdoor equipment failure. I think the electric heat cost me 55 extra dollars... This unit services the first floor apartment of a 3000 sq foot duplex building. The upstairs unit has a 24kbtu mid static Fujitsu... and that's almost oversized.

    BTW, this is a historic brick house as well in zone 5a (115 years old!).

    PS. I concur, coolcalc works if you put in the effort!

    1. willymo | | #11

      Tom, the beauty of Dana's "fuel-use based heat load calculation" is that it's *really* easy if you have a previous year's (or even, in a pinch, a previous month or 2'S) heating bill. You can do it if you can set up a simple spread sheet.

      As an aside, I used a browser load calc (forget which one now) and had to make several guesses that I knew might affect the results. It was very encourageing when the use-based calc came back just a little bit lower. Gave me confidence that my room-based calcs were in the ballpark.


      1. twoodson | | #14

        Absolutely agree. Likely the most accurate way to do an in-situ measurements. If you have an ecobee on your current equipment, you can get data on equipment runtime, set points and outdoor temps in 5 minute intervals. I have a colorful spreadsheet with all sorts of interesting correlations from that data.

  6. RRSFDV | | #10

    Thanks to everyone here for the thoughts and great advice. It’s a frustrating world to be in, dealing with HVAC, because I imagine most consumers don’t know or care about their systems, they just want them to “work” so most HVAC contractors breeze over the details.

    William - thank you for the clear-headed way to look at the backup - we are indeed looking to install solar on our new roof (we’re actually looking to install thin-flexible adhesive panels, less efficient that the standard panels, but it’s the only option that passes the visual approval of the family on the new metal roof), so that’s all the more reason we’d like to keep things as electric as possible. I’m amazed that you are able to completely heat your home with the solar. Even if we end up with a minor to middling decrease in electric costs, I would take it—so your story is encouraging.

    Charlie and William - we are indeed intending to install a generator backup—we have one at our current home (which is about 7 minutes from this new one), and where we are, we lose power at least 3 or 4 times a season, and we’ve lost power for multiple days at least three times over the past 6 years (and seemingly only when we have a houseful of family and guests over a major holiday). In our current situation, our 11k generator backup easily keeps our heat-pump running in the new addition, and all the other systems as well. So at the new “old” house, we’ll have propane for at least that.

    And I agree, I’ve had great experience with our mini splits in our current renovation - they’ve worked great. Our ‘new’-old house has no open-concept spaces, as those old houses go, so we’d have to stick with a ducted system. Tom, thank you for the Fujitsu recommendation, that would also be a preferred option - our HVAC initial proposal was for an American Standard 4A6V8060A1000B - Platinum 18 Series 5 Ton, 18 SEER, R410a Variable Speed Heat Pump Unit which I’m finding hard to get any specific information on, including it’s low-temp ratings, etc. Bu I have read that the Fujitsus (and Mitsubishis) have better efficiencies and work at lower temps.

    Dana, thank you for your thoughts as always. Our home was a foreclosure, and it’s been vacant for about 10 years, but I did manage to get some oil usage details from back then, and I’ll check the furnace today to see if I can come up with some numbers based on your 15-minute “Out With The Old, In With The New” article, and I’ll post updates here. Where might one find a good HVAC third party qualified engineer? I’ve not had luck finding any good recommendations online.

    I didn’t really think about ground-source heat-pumps because of the general thought that they are too expensive, however, the multiple heat-pump, duel-fuel propane, or oil-burner-triple-furnace-hot-water systems (see below) being proposed are pretty much the same if not more expensive than some of the average costs I’m seeing for Ground-Source HP. I’ll bring that up, and take a look at that as an option (including Dandelion). I haven’t spent much time researching those systems, but in general, I’m assuming that there’s no need for backup heat in the case of low outside temps, correct? Any backup heat added to a ground-source HP system would just be to back up if the system itself breaks?

    Our HVAC contractor also threw out the option of installing a single LP Boiler that would feed into and backup three heat-pump furnaces in the house (two in the basement for first floor and rear wing, and one in the attic for second floor), with the idea that it would also provide our domestic hot water. Obviously NOT ideal either. Hot water is another topic which I’ll definitely need to deal with. I was hoping we could get a heat-pump hot water system, but our HVAC contractor has scared my family into thinking they’ll only give us 15 minutes of hot water…. Anyway, I might save that for another topic.

    Tom and Jay - I went and tried cool-calc last night - that you for that recommendation. I need to do a deep dive into the instructions there, as my first “quick” attempt last night revealed a lot of questions about the house I need to get answers to.

    As always, thank you all for your thoughts.


    1. willymo | | #12

      Curtis, I have to say your contractor sounds really suspect. Saying that the HPHWH will only give 15 minutes is scandalous, particularly without knowing the SIZE of the tank. I have a 50 Ga HPHWH and I can stand in the shower at least a half hour without losing heat. If you need more and can afford it you can get an 80 or maybe a 100ga heater.

      Could it be they're trying to upsell you everywhere? ;^)


      1. twoodson | | #15

        Agreed. Get a new contractor, but do some homework first. I have a 50 gallon rheem HPWH and we can take 45 minutes of hot showers without a problem. It measures usage and I think the last 2 years it has cost me a grand total of 200 dollars to run and the thing has already paid for itself after rebates.

        1. evantful | | #17

          Triple agree on this. The contractor, like a lot of HVAC contractors, is way off. You're right at this point to start doing your own calculations. I also have a Rheem 50 gallon HPHW from 2018 that sits in a basement that gets to 50 degrees in the winter and it serves up 40minute hot showers during that period on Heat Pump only mode without any trouble.

          1. RRSFDV | | #19

            Thanks for this advice. This makes sense, and is the same as what I've been seeing too.

    2. Deleted | | #13


    3. evantful | | #18


      I'm located in Ellenville, NY so I'm climate zone 6, but at the very edge. I purchased our 1,300 sqft 1950's ranch in 2017 and began renovations over the last 3 years. My house did not receive a deep energy retrofit, it still has the original r-11 or r-13 first gen Owen Corning Fiberglass batts in the walls, and r-30 in the attic that was added at some point in the last 20 years. I did a reasonable job air sealing as best I could.

      My house originally had a 30yo oil furnace that I replaced with 1x 12k Mitsubishi and 1x 9k Mitsubishi Mini Split in the spring of 2018. I have run my house solely on them for the past 3 winters.

      During the winter of 18/19 we had the 2nd and 3rd coldest lows on record for Ellenville at -13 and -14, with day time highs in the single digits. At no point did we need supplemental backup and this was during a period when we still had all the original single pane windows and much of my air sealing still had yet to be done.

      I installed myself a few wall mounted, thermostat controlled, electric heaters in the various rooms just to serve as backup so I would not be beholden to HVAC contractors emergency fees. With that being said our Mini Splits have been rock solid. During power outages I can run my house on a 7500kw Subaru Portable generator which can handle the Mini Splits, Water Heater and all of our lighting and outlet circuits along with the fridge. I will rarely pull over 3000 watts once everything is up and running in the middle of the winter.

      As Dana stated, definitely consider looking into Dandelion GSHP, given the sqft of your house that could be an extremely viable solution that would also handle your Hot Water Heating and couple that with a Propane or Natural Gas Backup Generator.

  7. Expert Member
    Dana Dorsett | | #16

    >"I didn’t really think about ground-source heat-pumps because of the general thought that they are too expensive, however, the multiple heat-pump, duel-fuel propane, or oil-burner-triple-furnace-hot-water systems (see below) being proposed are pretty much the same if not more expensive than some of the average costs I’m seeing for Ground-Source HP. "

    On a recent retrofit project I was consulting on in MA the proposals for combined ducted & ductless heat pumps were pretty pricey, but a fully ducted 4 ton modulating GSHP was still about $10K upcharge. The actual design heat load of that place (~2500' of conditioned space over a yet-to-be insulated poured concrete walled basement & garage) is easily in the range of a 3-tonner + strip heat, but being in an area with frequent power outages (semi-rural, heavily wooded), they opted for a 4 ton Waterfurnace Series 7, which has a significant turn down ratio, thus skipping the strip heat which could have pushed the limits of the pre-existing propane generator under some circumstance. Based on performance so far this season it looks like they could have done just fine with a 3 ton Serie 7 and no strip heat, but the owner is still happy. Local design temp is +6F, not too much warmer than much of the Hudson Valley.

    There have already been a few power outages since the heat pump was installed, and it all switched over flawlessly, with no startup strain for the generator.

    This house started out as a barely insulated extremely air leaky board & batten sided house with no structural sheathing. There hasn't been a post-upgrades blower door test yet, but it's almost surely up to current code (<3ACH/50), between dense-packing the walls, stripping the battens to install fully adhered housewrap behind 1/4" RainSlicker mesh and cedar shingles. The attic floor was air sealed prior to installing R50+ up top. The ceiling between the drive-under garage and the master suite got dense packed with cellulose too. The partition wall between the garage and semi-finished basement still needs to be insulated, as does most of the foundation, but it's already a pretty comfortable place. Once the foundation gets fully insulated along with the partition wall to the garage it will be even more comfortable and easier to control the summertime moisture issues.

    >" I haven’t spent much time researching those systems, but in general, I’m assuming that there’s no need for backup heat in the case of low outside temps, correct? Any backup heat added to a ground-source HP system would just be to back up if the system itself breaks?"

    Why is it assumed that a heat pump will break? A reasonably installed heat pump is about as reliable as the average propane furnace- do they require "backup" for a propane furnace too?

    1. RRSFDV | | #20

      Thanks again - yes, of course, there's no backup with those other furnaces!
      I wish we could do a retrofit similar to what you've just done, unfortunately, we haven't figured out a way to insulate the walls without removing the historic details inside or outside, which we're trying to avoid. We will do our best to get an air sealed and insulated attic, plus insulated basement walls, and do as much air sealing around the doors and windows as we can.

      All of the scenarios proposed by our contractor so far are based on a fairly large heat load. Our contractor does plan to do a manual J once we do the retrofits, but he thinks that the system will need to provide upwards of 150,000 BTU/h at design temp. Obviously, that a LOT of HVAC - if this is true, I'd need 3 of the 4 ton GSHP. I'll need to get some real numbers.

      Dana, I used your calculations from your link mentioned above—and it is VERY rough because the data I have on previous usage is by the year, and not the year I could get heating and cooling days—and came up with 99,796BTU/h with the 1.4x sizing factor and 65°F balance point assumption. This does not include a soon-to-be finished attic space.

      1. charlie_sullivan | | #21

        Note that the 1.4X upsizing factor is the maximum recommended size, not the ideal recommended size. And also, if that's based on before you insulate, you might cut dramatically. So if it's 71 without the upsizing or upgrades, and you cut it to 2/3 of that with the upgrades, that is one 4-ton GSHP.

        I have an old 2200 sq foot house in zone six that I did a "pretty good" envelope upgrade on. I heat it with a 3-ton GSHP that is turns out to be 2X oversized--1.5 ton would be fine.

        I wonder if you could do your envelope upgrades, heat it for a year with the existing system, carefully logging fuel use and run times, and then size your new heating system based on that data.

      2. Expert Member
        Dana Dorsett | | #23

        (Sorry I missed this from a few weeks ago but I'll respond anyway...)

        >" is VERY rough because the data I have on previous usage is by the year, and not the year I could get heating and cooling days—and came up with 99,796BTU/h with the 1.4x sizing factor and 65°F balance point assumption. This does not include a soon-to-be finished attic space."

        And this is ~100KBTU/hr number is 1.4x the load during the BEFORE BUILDING UPGRADES condition of the house! If the design load was (100K/1.4=) ~70K before upgrades it should be pretty straightforward to knock that back to under 60K (=5 tons) with any sort of air sealing and insulation plan. A single 5 ton GSHP with enough auxiliary heat strip to cover the coldest cold snaps should pretty much do it. Going for 10 tons of heat pump is just silly. The raw upfront cost of the second heat pump would be more than the building upgrades necessary to bring it well under 4 tons.

        Finishing out the attic won't increase the load, in fact weatherizing the attic with air sealing & insulation will likely REDUCE the total load.

        If you're not going to be replacing historical windows they can still be tightened up with a bit of weather stripping and pulley-seal rework. For a bunch of money there is a company in CT will even rout out antique sashes (leaving the muntin bars in place) to accommodate a single low-E double pane glass unit per sash, with effectively no change in appearance from the exterior, fairly minor changes on the interior. But for substantially less money low-E exterior storm windows will perform as-well or better, assuming the original windows can be tightened up. (In my area there are a number of historical buildings with low-E exterior storms that don't affect the historical aesthetics by very much.)

  8. Yeldog | | #22

    Conventional split variable speed heat pumps have no problems in colder climates and will be a comfortable as any other forced air system. Have used Carrier for the superior zoning and the 5 speed unit they make has been a great fit for my projects -- not as expensive as the full Greenspeed. One is going in to my new house -- a retrofit of a 1870 stone church. The key with any system is sizing it correctly. Sounds obvious -- so often oversized single speed units with undersized ductwork. Comfort is mostly about drafts (leaks) with an old house .... I often have exposed brick or stonework and while it's cold ... a leaky window causes more discomfort with forced air.

    As far as mini-splits .. they are a great problem solver. While they work in open big rooms w/ good insulation ... I'm not a fan in other situations for heat. The ducted/ceiling/floor models all have a place -- the wall units work the best IMO. I have used Mitsubishi hypers in three projects (multi) -- only singles with the Fujitsu's and LG. The Mitsubishi wall unit with the eye is really nice. I have used the ceiling and floor as well. I live in a town with many large Victorians -- the local Fujitsu dealer has made quite a living putting the single compressor multi branch box units it -- some with 9-10 heads. My friend did his over 10 years ago with only a couple issues .. they still prefer the NG hot water radiator system for heat .... but the HP can keep it warm in 2 degree weather.

    I'm a radiant guy -- so that changes everything.

    Propane can be more expensive vs the heat pumps .... even in the expensive electric markets. Especially the single smaller hyper units. That's really a math problem and easy to figure out.

    Would have loved to to geo -- but drilling the wells was over 60k. Even with all propane -- The payback was forever. I also looked into the air handler model that all the mini-split companies now make -- the controls were lacking. That's the problem with the hot water coils/ boiler set up .. the controls don't match a furnace/ heat-pump combination and the cost is higher.

    Good luck -- I love old buildings. Nothing matches hot water heat in an old building ... but -- it's the most expensive.

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