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Tanks or no tanks: customizing a combination of DHW and hydronic

ohioandy | Posted in Mechanicals on

I’ve spent some time in the archives here, and am grateful for the wisdom therein. It’s clear that each situation is different, and I thought perhaps I could crowdsource the best solution for mine.

I’m building myself a 900 sq.ft. house in northern Ohio. We’re going for LEED platinum, and a HERS score in the 50’s. Heating and cooling is primarily with a double-head minisplit, the best money can buy. So I hope it will handle all heating needs down to 10F outside temp. or thereabouts.

Supplemental heat will be radiant hydronic in the floor, which is cheap to install in a new build. I have no access to gas onsite, so supplemental and DHW will be electric.

QUESTION: Weighing all factors (upfront cost, lifespan cost, efficiency, elegance/simplicity, durability) what might be the best route to accomplish these two electric water heating tasks? A single unit performing both, or separate units optimized for each task?

I’m inclined against an open loop for the radiant, for health and corrosion reasons, but would still consider it.

Seems like the best would be a small tank for DHW, since my wife and I have a low but steady need for hot water. But a small tank does not have the juice to also heat a slab a couple times a year.

It’s conceivable that some years the radiant would not be used at all, so it has to be able to sit for long periods of inactivity.

Any thoughts?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Andy,
    If you are planning to heat your home with a "minisplit, the best money can buy," you don't need any supplemental heat. Mitsubishi makes ductless minisplit units that perform when the outdoor temperature is as low as
    -13°F. These models exhibit 100% of rated heating capacity at 5°F and 87% at
    -4°F outdoor temperature. Model number PUZHA36NHA Mitsubishi Mr. Slim Hyper-heat is rated at 18,000 Btuh at -13°F.

    Even if you wanted to install electric resistance backup heat -- and in Ohio, you won't have to -- it doesn't make any sense to install a hydronic system. Just use electric resistance baseboard units -- skip the water tubing.

  2. ohioandy | | #2

    Good advice; thank you Martin.

    I did have an ulterior motive for putting the water tubing in the slab, though. Possible future change in heating options (solar thermal, outside wood boiler).

  3. jinmtvt | | #3

    You could also consider electrical in-slab heating instead of baseboards ..
    but it has some more "lag".
    This is only good if you are still planning for using a mass floor.

    All that Martin said is what u really need though,
    mitsu hyper heat or Fujitsu newer models,
    and some basic backup electrical heating .
    Water system makes sense only if you are using a cheaper source than electricity to heat the water,
    because then u could've used electrical underfloor heating for much less hassle and a very similar COP .

  4. Expert Member
    Dana Dorsett | | #4

    Every good heating system design starts with a heat load calc, In a 900' high-R/low HERS house I'd be surprised if you had a heat load at +5F anywhere near the output of even the smallest 2-head multi-splits, but it might be half that. Your heating loads are probably closer to ~9000-13,000 BTU/hr, the +5F output of a 3/4 ton single head ductless. Most duals start at 1.25 tons and and higher, and would be overkill. Why do you need two heads?

    If the radiant floor is just supplemental and doesn't need to carry 100% of the design day load you can probably do just fine with a small-output electric tank combi, using a heat exchanger and 2 pumps. But is it really worth the expense for that wee-bit of extra coldest weather comfort?

    For that matter, is the expense of active solar thermal going to be worth it, compared to spending the money on photovoltaics (PV) , offsetting most or all of your power use with the mini-split? For the kind of money a decent solar thermal costs you could take the R-values up a notch and get to net-zero capable energy use levels with a PV array that fits on the roof, and wait for the cost of PV to drop to under $2/watt installed (it's about there already in Germany.)

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