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Community and Q&A

HPWH with hydronic baseboards as supplementary heat method

WickedSmahtHome | Posted in Green Building Techniques on

Hi, all. I will start with the TLDR:

I want to get off oil and use a heat pump water heater for my hot water needs. I currently have a pellet stove and will be installing a ductless mini-split heat pump. Solar coming in March. Possibly a battery as well. House is in Massachusetts. We get sub-zero temps occasionally (1-2x/year). Climate Zone 5A. No natural gas available. Not interested in switching to propane.

The gap is my hot water baseboards which would be rendered useless/weak without the boiler since a heat pump water heater only heats to ~130f vs the boiler which is ~180f. Also, there is a closed-loop vs open system problem. Are there any ideas/recommendations on how to solve the closed loop and temperature problems? If the baseboards are not going to be my primary method to heat the home – does it matter if the temp is only 130f? Would they still provide enough heat to help heat the home without necessarily being capable of doing the job on their own?

Background: House built 1962. Original oil fueled boiler is still heating the house as the “primary” heat source. It’s old, parts are damn near impossible to find, and it is inefficient. Secondary heat source is a pellet stove. It is reliable, cheap to run, and effective, but no ventilation exists to send the heat from the stove to the other end of the house. Currently, we use the pellet stove as much as possible to avoid the cost of oil. So far this has worked pretty well.
Hot water is provided on demand by a built-in coil system in the boiler. No large scale storage tank.

MA has rebates between $1600 and $2400 for the mini split heat pump.

Sleeving my oil line and repairing my damaged chimney flue will cost ~$1800+. I can avoid these costs if I get off of oil, plug the flue, and remove the oil tank.

If I lose power, I will not lose heat due to the battery supporting the heat pump and pellet stove (the latter of which uses a small amount of power to operate the feeder system)

Heat pump water heater would be a cost-effective way to get hot water for washing + appliances, but not good for hydronic baseboads.

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  1. STEPHEN SHEEHY | | #1

    You don't need three sources of heat. The pellet stove and minisplit will be more than enough. I'd just remove the baseboards.

    1. WickedSmahtHome | | #4

      Unfortunately, the heat pump only works down to relatively moderate temps. On extremely cold days, I would be worried about 1. Efficient operation and 2. Non operation in the worst conditions.

      I wouldn't want to leave myself relying solely on the pellet stove which is located away from the bedrooms, though I suppose the installation of a vent (containing a fan) in the room with the pellet stove and sending it into the bedrooms might supply enough heat.

      1. Deleted | | #5


      2. Expert Member
        Dana Dorsett | | #12

        >"Unfortunately, the heat pump only works down to relatively moderate temps."

        Where would the heat from the heat pump water heater be coming from? And what is the heat rate you need? Taking it from your basement to heat the first floor will lower the temperature in the basement to where it no longer works. The output rate of the heat pump water heater is also fairly modest, even if it has an infinite heat sink from which to draw the heat. If the heat pump water heater is indoors, the best it's going to do is move heat from one part of the house to another.

        A 130F storage temp isn't a problem. At an average water temp of 125F ( 130F out, 120F return) typical fin-tube baseboard emits about 250 BTU/hr per foot. Whether it's enough to fully heat every room depends only on the heat loss of the rooms, and amount of baseboard in those rooms. The hydronic heating system at my house is designed for an average water temp of 125F with diverse radiation types (some rooms have cast iron baseboard, others cast iron rads, still others have radiant floor, and there is a hydro-air handler zone.)

        The only way to get a handle on this is to start with a room-by-room heat load and radiation analysis, but in most homes even a split system Sanden heat pump water heater won't heat very much of the house when it hit's 0F, or even +20F.

        Cold climate ducted and ductless mini-splits to just fine in MA, and deliver a COP of 2 or so even when it's dipping into negative single-digits, and a COP of about 2.70-3 at the average January temperatures. Don't let the HVAC company spec the equipment- PAY an engineer to run the load numbers first, somebody who makes their living and reputation off of the accuracy of their numbers rather than selling & supporting equipement. Figure out the equipment that fills the bill, then put it out to competitive bid.

        If you haven't stumbled across it already, this page has enough selection filters to figure out who are the gold-plated vs. economy installers in your area:

        Note, Mitsubishi owns the lion's share of the market in MA, with Fujitsu a distant second. But some of Fujitsu's ducted mini-split solutions are a better option. Carrier puts their name on some pretty good cold climate Midea mini-splits too (both ducted & ductless) and shouldn't be ignored.

        Got a ZIP code?

        >"If I lose power, I will not lose heat due to the battery supporting the heat pump and pellet stove (the latter of which uses a small amount of power to operate the feeder system)"

        It takes a HUGE amount of battery to run even a half-ton mini-spit heat pump throughout even a fairly mild winter night. Don't plan on using battery backup to run the heat pump or a heat pump water heater, but keeping the refrigerator and pellet stove and a modest amount of lighting could be realistic.

  2. mjacksonw | | #2

    I'm interested in replies to this too, and have a lot of overlap in my story (MA, 1940s oil furnace, want to go full-electric, solar going in next month, but 1920s house). We added mini-splits last year and have had "okay" experience heating – I think we have a pretty major stack effect from the leaky house, so it's not nearly as comfortable as our huge furnace and cast iron radiators (only baseboards in a couple of rooms).

    Right now, while we mull on what to do, we have the furnace fire up for a short period in the morning and evening to get the radiators going, while still relying on the minisplits for most of the work. We've even attempted to close off radiators in rooms that have a minisplit head (though some of the old/stuck valves have held up this process).

    I'd love to go the HPHW route and use that to get the radiators to take the edge off. Chiltrix / CO2 HPs offer some higher temps (~150F?) that could go a little further if necessary, but it'd be awesome if normal HPHW could do what we need.

    In our case, we haven't had a power outage in the 2.5 years we've lived in the house, so if we decided that was enough of a risk, I'd probably go for a battery over a backup heat source. I'll be happy to be rid of all the hulking apparatus for the oil furnace, and we'll be able to move our DHW heater closer to the plumbing core.

    1. vap0rtranz | | #8

      1930s old house here in Wisconsin.

      HPWH has been great for our hot water needs, but we're just 2 people + 2 furr kids. :) I've toyed with the idea of installing radiant floor heating in our coldest room (which is the kithen b/c it's farthest away from the mini-split head units.)

      The mini-splits are very good as our primary heat, with LP furnance as backup. It's mid-Jan and the LP tank has only dropped 5% after being filled in October, so the mini-splits are doing better and better over time.

      By "over time" I mean that we're air-sealing as we go. This past weekend was more work on the basement rim joist and windows. Before that was work on weatherizing doors. In the fall we had a contractor spray closed-cell foam under the roof. I run around with a caulk gun feeling for leaks, or do the smoke test, the contractor used a IR/FLIR camera, etc. It gets better over time. If I had a Money Tree, all the air-sealing (and insulation) would have been done before the mini-splits went in. With our old, drafty houses, I can't blame the mini-splits.

      Our HPHW is in a 700 sq ft unfinished basement that hovers around 50F so stays in hybrid mode, and I've thought of piping radiant from the water heater to just take the chill out of the kitchen floor. It'd only be around 150 sq ft of space (no need to heat under cabinets, etc.) so we're not trying to heat the whole house ...

  3. Trevor_Lambert | | #3

    If by "normal HPWH", you mean the non-split type, then you can't use that for space heating. The biggest problem is not the output temperature. The two main options right now are the Steibel Eltron Accelera and the Rheem Hybrid series. Neither one has adequate capacity in heat pump mode to heat much more than a garden shed (4200BTU/h for the Rheem, a little more for the Eltron). The bigger problem is where is the heat coming from? For the Rheem, you have the option of ducting to the outside. But it's not a cold climate performance unit, so that already minimal 4.2kBTU/h is going to drop to next to nothing in all but the mildest climates. The Steibel Eltron doesn't even have the option for ducting, but even if it did it would have the same limitation. If you leave it indoors, then you're trying to heat the house using heat from the house. Picture plugging a power bar plug into one of the receptacles on said power bar. Of course, with the hybrid type you could use the heater in pure electric mode in the heating season, and heat pump mode in the cooling season. I think if you run the numbers on this, it won't make any sense in the vast majority of cases.

    As for the split HPWHs, they can certainly work for space heating. As of right now, they cost a lot more than a similar performing air source mini split, so you'd have to justify the extra expense somehow.

  4. ssnellings | | #6

    Trevor Lambert raises some good questions.

    Additionally, while 130F should be good enough to prevent Legionella growth, I think the typical recommend is 140F or above in storage tanks. I don't have a good source on this, maybe someone else has one.

    My point being, your water temperature should be higher than 130F and this might change your calculations.

    Martin Holladay has a good article from 2015 that covers this kind of set-up using a tankless heater, there is a lot of relevant details regarding how to set up the circulation system. The title is "Using a Tankless Water Heater for Space Heat".

    Below that article, look for a comment from Dana Dorsett about dissolved oxygen concerns.

  5. user-6623302 | | #7

    Some of the MA water heater rebates require replacing an old electric unit or new constriction. Have you checked that.

  6. jberks | | #9

    My suggestion is to create a closed loop system off your water heater. You do this with a heat exchanger like these

    Also, read this PDF:

    I have no experience with a heat pump water heater for this application and can't comment on it's capacity.

    But on what I do know, 130F is fine and create a closed loop for the radiators to not cross contaminate.

  7. CramerSilkworth | | #10

    You need to have someone do a heat load calculation to determine how much heat the house actually needs, and how the sizing of the pellet stove and minisplit can contribute to that. The baseboards might be ok w/ 130F water from a split heat pump water heater (ie, Sanden), but its capacity is pretty low - space heating with it is only feasible in very low load (<10 MBH) homes, and unless you're doing some serious work on the enclosure, a 1960s house is definitely not there. Get a good minisplit (ie Hyperheat or equiv) and you may not need baseboards at all, using the pellet stove for the coldest days, but splits alone without a good enclosure will be costly to run.

    But without a heat load calc this all just a lot of hand-waving...

  8. walta100 | | #11

    Is your mini split a hyper heat model that would work well when it is below -10°?

    You are aware most all the HPWH contain a resistance heating element used to supplement the heat pump.

    If only need this heat occasionally when it is too cold for the heat pump to work could you add a water heating loop to the pellet stove and move some heat the bedrooms, or a electric boiler/ water heater for rare use.


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