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Hydronic floor heat boiler… also for domestic hot water supply?

benjamin_wooten | Posted in General Questions on

Hi everybody! I have not been on in some time, but have received a lot of helpful information here in the past two years while renovating our home.

The original house (we live in Nova Scotia, Canada) had a hydronic floor heat system installed. It runs off of a small Seisco boiler unit, model #RA18. It is probably about 20 years old at this point. We added glycol to the system, but otherwise have left it alone. The primary heat source in the house is a wood burning stove, which does great… we have only used the hydronic system when leaving the house for more than a few days during the winter – when we can’t have a fire running – to make sure nothing inside the house freezes (including, but not limited to, cats).

So, the boiler has blown some sensors, and is not functioning correctly. Nobody out here deals with Seisco anymore, and we’ve read some disappointing things about the units in general… so that we’ve taken the advice of a mechanical contractor who used to work extensively on them, and are going to replace the boiler unit.

We have a standard electrical domestic hot water heater, as well. I asked the contractor about the possibility of using a new boiler to also provide heat for our domestic hot water. He said the amount of energy required to do a fully tankless (“on demand”) system was not feasible in this climate. He did, however, mention replacing the electric hot water heater with a 40 gallon stainless steel holding tank, and that the new boiler unit could be integrated with it so that it…

…and here I stopped understanding things. I didn’t have time to dig down and really figure out what he was proposing, but am sitting down to try and do so now.

My loose understanding is that some heat from the hydronic boiler could be recaptured and used to keep water in the holding tank warm, and when hot water was called for, the holding tank water would only need to be “topped up” to a fully hot temperature as-needed.

What I don’t understand is, what actually does the “top up” heating? Does it get recycled through the boiler again – this time running on full-tilt – then distributed to the house? Is this an energy efficient system relative to our standard electronic tank? Does the floor system always have to be running 24-7/365 to keep the hot water system running? I love the idea of not having an electronic hot water tank, and the stainless steel holding tank has a lifetime warranty to the original home owner… but I dislike the idea of having to run the hydronic floor heat system all Summer so that we can do laundry or have a hot shower.

Sorry for the long roundabout question… even just having the terminology clarified would be a great place to start in helping me do my research.

Thanks to anybody who can help!

Very best,


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  1. GBA Editor
    Martin Holladay | | #1

    The type of domestic hot water system described by your contractor is called an indirect water heater. An indirect water heater is a tank connected to a boiler -- almost always a natural gas boiler, or propane boiler, or oil-fired boiler, not an electric resistance boiler.

    The indirect tank, often stainless steel, includes a heat-exchange coil near the bottom of the tank. Water from the boiler flows through the heat exchange coil, warming the water in the tank. The two types of water don't mix. The boiler water never touches the domestic hot water.

    This type of system is usually set up as a separate zone off your boiler, with a dedicated circulator controlled by an aquastat.

    This type of system is worth considering if you switch to gas or oil instead of installing a new electric-resistance boiler, but only if you will be using the boiler for most of your heat. If you plan to continue to use your wood stove for heating, the indirect tank doesn't make much sense. Nor does it make much sense if you plan to buy a new electric-resistance boiler.

    If you plan to install a new electric-resistance boiler, keep your water heater separate.

  2. benjamin_wooten | | #2

    Thanks a lot Martin, I really appreciate the response. Although many people have tanks and use oil for home heat in this province, our place never had that setup. The province is starting to see more natural gas as an option, too, but in either case (oil or gas) we'd be looking at a good bit of infrastructure to get tanks and lines put in etc.

    What we do get here is a fair bit of sun. Based on your helpful description, it sounds like our neighbor has a solar powered indirect hot water system. His solar panel feeds the indirect tank, and does most of the heating work in the spring/summer/fall, but less in the winter. He has some sort of dedicated electric resistance boiler unit to do the extra heating.

    We are hoping to put in extensive solar panels in the years to come, along with a net meter. I presume, at that point, putting in an indirect tank system might make more sense... if a dedicated solar panel fed the indirect system, and the hydronic floor system was totally separate.

    One thing I don't understand, however, is how or why our 48 gallon electric resistance hot water tank would be any more efficient than another electric resistance boiler? I assume that, in the setup this contractor mentioned (non-solar indirect system based off our hydronic setup), we would actually have TWO electric resistance boilers, yes?

    1. The existing boiler that does the hydronic floor system (that we are replacing) that sends floor water through the heat exchange coil and keeps water in the indirect tank warm, and

    2. another separate boiler (like my neighbor's) situated after the indirect holding tank and before the main domestic hot water artery for the house, that tops off the temperature of the warmed water for use in the rest of the house.

    If we left the hydronic system on a low setting during fall and winter months - but off in the summer/spring - and the above setup was in place, you are saying that it would be electrically less efficient than just running our current 48 gallon hot water tank?

  3. GBA Editor
    Martin Holladay | | #3

    All electric resistance appliances that heat water have exactly the same efficiency -- 100%. (Not counting standby losses or distribution losses, of course.)

    An electric-resistance boiler will not heat water any more efficiently than a low-cost electric resistance water heater from Home Depot or Lowes. (Or maybe I should say, your local Canadian Tire.)

    If you want a solar thermal system, you should talk to a solar thermal contractor about what type of tank to get. Don't buy a tank for a solar thermal system until you are ready to install your solar thermal collectors.

    All that said, you may not want a solar thermal system. Instead, you might just want to take the money you would have spent on a solar thermal system and instead invest the money in photovoltaic (PV) panels. For more information on this topic, see Solar Thermal Is Really, Really Dead.

    I advise you to either (a) install an ordinary electric resistance water heater, or (b) install a heat-pump water heater. Neither of these appliances should be connected to your hydronic space heating system.

  4. benjamin_wooten | | #4

    Excellent, thanks again Martin. This has been exactly the advice we needed.

    We will keep our domestic hot water situation the way it is (I just put a new tank in two years ago, so it shouldn't croak for a bit anyway).

    We are, indeed, looking at flexible PV panels as our long-term energy investment in this house.

    Thanks again!

  5. Expert Member
    Dana Dorsett | | #5

    If the primary heat for the house is a wood stove, a heat pump water heater makes sense.

    The RA18 is an 18kw water heater (not designed for space heating) which can deliver at most ~61,000 BTU/hr into the heating system, assuming you can pump water through it at a high enough rate and a big enough delta-T, but it's probably oversized by at least 1.5x, maybe even 2x. If you run a Manual-J type heat load calculation on the place you can probably find a "right sized" electric boiler that is far more rugged and more appropriate for the application, for about the same money as a brand new RA-18. As a first rough-cut, use this online whole-house load calculator (which is guaranteed to shoot to the high side of reality, even with conservative assumptions about air leakage, etc.):

    With a more formal heat load calculation done by a professional (not an HVAC contractor- an engineer) you could also consider ductless heat pump solutions. It would cost quite a bit more up front, but it would use only 1/3 the amount of electricity to deliver the heat.

  6. tommay | | #6

    If you are running your wood stove, why not use that to heat your hot water. Make a loop coming from your electric tank, out the relief valve tap, with an aquastat, and returning to the drain, and add a loop of copper tubing to the wood stove, a small pump and done.

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