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Community and Q&A

Heating domestic hot water

jgold723 | Posted in Mechanicals on

We’re in Southern Maine.

We just purchased a house with a 30-year-old boiler (Pensotti) and external hot water tank (SuperStor). We know they’ll need replacing shortly.

Our overall goals:

— Efficiency;
— Moving away from oil, if possible/realistic — there is no natural gas in our neighborhood
— We’re open to solar options, but we have a limited budget ($10K or so) and we’d like to see return on our investment in 10 years or less.

I know I’ve got a lot to learn.

We are focusing now on domestic hot water, specifically heat pump systems. However, one installer loves them, another says it will make our basement (which also serves as my workshop) too cold.

I’d be interested to know what people’s experiences have been with these and whether they think it’s a viable option for our situation.



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  1. GBA Editor
    Martin Holladay | | #1
  2. jgold723 | | #2

    Thank you very much -- I'll check those out.


  3. Expert Member
    Dana Dorsett | | #3

    Even before diving into water heaters you might consider looking into displacing heating oil use by heating a large zone or two with ductless mini-split heat pumps, which is likely to have a larger impact on your overall oil use and shorter payback than heat pump water heaters.

    If you keep the oil boiler, heat pump water heaters would still use oil in winter. The first law of thermodynamics states:

    " The heat sucked out of the room and into the hot water tank has to be supplied by something else, and during the winter that something else will be your heating system."

    ( Paraphrasing sufficiently loosely so as to stay on-topic. :-) )

    Better ductless air source heat pumps will run a seasonal average coefficient of performance (COP) of about 2.7 better in a southern ME climate, (nearly as good as geothermal systems at far less design risk and a small fraction of the cash), and even at 15 cents/kw is less than half the cost of heating with $4 oil in an 85% efficiency boiler. (Even at +5F they're still running COP of ~2.0, but at 40F outside temps they get ~4.0. Many are rated to run at -4F and lower, some are still doing just fine at -13F.)

    And it provides very high efficiency air conditioning to take the edge off those muggy sticky summer days.

    See this bit of RMI policy analysis that includes lifecycle cost of energy comparisons:

    And this substantial bit of in-situ testing where ductless mini-splits were retrofitted as partial to primary heating systems, monitored for efficiency in a wide range of winter climates:

    And the associated satisfaction levels of those who are living with them:

    If you're serious about getting off oil, a mini-split or two takes you miles toward that goal (or even the whole way) rather than the mere inches a heat pump water heater does. With a budget of $10K you could probably buy a 3-head multi-split AND a heat pump water heater, and your heating energy bills would be cut by half or more, FAR more than $10K of any solar technology will take you. Since the ductless heads only heat the room(s) where they're located the placement of the heads can be critical to peak-season performance, but even new code-min homes with somewhat open floor plans can be heated comfortably with just 1-2 heads of ductless these days (though high-R homes do even better.)

  4. dankolbert | | #4

    You should give Jim Godbout in Biddeford a call - probably the smartest mechanical contractor in Maine.

  5. jgold723 | | #5

    Thanks all the feedback -- and hopefully more to come. I greatly appreciate it. While I'm very excited about finding non-oil alternatives for my heating needs, the multitude of options out there present a daunting task -- but this kind of advice really helps. And yes, Jim is definitely on my list of people to call.

  6. Expert Member
    Dana Dorsett | | #6

    Be very careful when comparing performance specifications if comparing ducted air source heat pumps to ductless, since duct design & implementation can bring the efficiency of even the best compressors to their knees. As with ground source heat pumps (aka "geothermal") the air-handler power and pumping power is often fudged, and system efficiency issues like duct leakage/duct-insulation are never factored in, but are measurable in-situ. In the NEEA large scale study out in the northwest there were only 2-3 fully ducted solutions monitored for performance, and they came in at barely over half the efficiency of the ductless heat pumps. (Ecotope has a writeup on them somewhere, but I'd have to do some digging to come up with it.)

    The advantages of ducted solutions is that every room gets heated directly, but if it comes with a huge efficiency hit (either by faulty design or implementation), you'd be better off using ductless in combination with small electric resistance panel-radiators or radiant cove heaters to raise the temp of the colder rooms on an as-necessary, as-occupied basis. As long as the setpoint on the ductless is higher than that of the thermostats for the resistance heat, the ductless will still carry the bulk of the load. Even slightly overheating the common area to bring the temp of remote doored off areas up to comfort levels on the colder days is more energy-efficient than heating the remote rooms primarily with the resistance heat.

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