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Community and Q&A

Heating bathroom and attached closet

cmag | Posted in Mechanicals on

We are beginning a long and expensive endeavor to update a 100 year-old house in the Seattle area and first on the list is updating two small bathrooms. The previous owners of the house installed an efficient Buderus boiler a few years ago, but unfortunately the radiators in these rooms have been long been removed. They are currently heated by a few wall-mounted electric space heaters which I would really like to rip out. Unfortunately, I can’t seem to get a straight answer from any heating contractors as to how to heat the space adequately and efficiently. I supposed the main options are:

Electric mat under the tile–seems to be expensive to run and possibly not very good at heating the actual room.

Install a radiator–a little more involved and more expensive

Install under-floor hydronic heat–way more expensive, but do-able.

It seems such a shame to have an efficient boiler and not use it as the heating source here, but I don’t want to overdo it if a simple under-floor mat would suffice.

I also did a quick heat-loss calculation using the Seattle DPD’s spread sheet, and it looks like the heat loss on the larger bathroom would be about 5400 btuh.

Sorry if I sound like a complete amateur, I just want to use the most elegant and efficient solution possible, and am not getting much help from the heating contractors here. . .

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  1. Expert Member
    Dana Dorsett | | #1

    Run some heat load calculations on adjacent spaces that have radiators, and come up with the approximate load to radiation ratio, then see if you can find a radiator that fits in the bathroom with a similar size to load ratio. If that works, you can simply extend the existing zone.

    If you have a bunch of big cast iron ratdiators and don't know the output of the radiators in the other rooms for estimating radiator to room-load ratios, measure 'em up and estimate using this document:

    Panel radiators capable of delivering 5400 BTU/hr at condensing water temperatures for condensing boilers tend to be about 12 square feet, but come in different length to height ratios, and some thicker-profile ones can work with somewhat lower face area. eg: A Biasi B-36.48 ECO is 36 x 48" , and can deliver about 7700 BTU/hr @ 140F average water temperature (which would be above the condensing zone, which needs return water below 125F). A 24' x 64" Biasi can get you there too, maybe even a 20" x 64".

    As a rough rule of thumb a radiator will deliver about 80% of it's 140F rating (or about 45% of it's 180F rating) with 130F water, so the B-36.48 ECO could deliver something like 5700-6100 BTU/hr @ 130F AWT, which WOULD be returning water back to the boiler in the condensing temperature range, and would deliver the full 5400 BTU/hr with 125F AWT which would be well into condensing.

    The radiator itself would set you back about $550-600 USD at internet store pricing. For (quite a bit) more money you could get a fancy Buderus panel radiator with an adjustable radiator thermostat to tweak the room-to-room temperature balance issues.

    The last thing you want to do is run the bathroom as it's own zone, since it's load is well below the minimum fire output of any Buderus, and it would short-cycle the boiler at condensing water temperatures. Extending a zone is a far preferable approach.

  2. Richard Beyer | | #2

    Corey said..."Electric mat under the tile--seems to be expensive to run and possibly not very good at heating the actual room."

    Not necessarily true. If you use a qualified contractor these systems are relatively inexpensive in comparison to running plumbing. Under tile systems have come a very long way. They are available in 240V or 120V. 240V is preferable for heating applications. I personally installed both systems indoors and out in some very high end bathrooms and exterior balconies. These systems are tremendous assets according to the clients who have them.
    One bathroom completed is 20'X20'X16 foot high ceiling, wall to wall marble and this customer uses a single 240V system with fiberglass wall and ceiling insulation in zone 5.

    The system used is made by WarmUp.

    I thought the system would be costly to operate, but my clients say they barely notice a jump in their utility bill. Regardless of the under floor heating you elect to use also take into consideration expansion and contraction issues associated with floor warming systems when tile is the preferred flooring option. See.... Schluter Ditra for the solution on crack prevention here...

  3. GBA Editor
    Martin Holladay | | #3

    You wrote, "If you use a qualified contractor these systems are relatively inexpensive in comparison to running plumbing."

    I'm not sure what you mean by "inexpensive." Using a qualified contractor won't lower the installation cost compared to a DIY job -- nor will it lower the operating cost.

    If the house has a boiler, it makes sense to use the boiler to heat every room in the house. Dana gave good advice.

  4. cmag | | #4

    Thanks so much for the answers, everyone--so much more helpful than the advice I've been getting from contractors here! Not to go off on a tangent, but how crazy would it be to consider a mini-split here? I think it would be pretty easy to service these rooms with one of the ducted units. Or would it be wiser to stick with boiler heat and then add air conditioning later if needed (and if the budget allows . . .).

  5. Expert Member
    Dana Dorsett | | #5

    At Seattle's 99% outside design temperature the smallest single-head mini-splits would deliver more than 2x (some closing in on 3x) your 5400 BTU/hr calculated heat load.

    If you had a multi-split there are 6000 BTU/hr wall-coil heads out there (that would put out maybe 7000 BTU/hr , in heating mode which wouldn't be ridiculously oversized for a 5400 BTU/hr load) , but SFAIK no mini-duct cassettes that small.

    If you were going to be adding air conditioning/heating to other rooms as well you could split the output of a mini-duct cassette with other spaces, but it would be pretty unbalanced cooling if you were delivering something like a third or half output to the bathroom, which probably has a miniscule cooling load.

    I'm thinking a mini-split solution would be pretty crazy here, unless you're heating the whole house or the whole zone with the mini-split in a room-to-room temperature balanced manner.

    If you DO end up installing air conditioning a some point, making it a heating/cooling heat pump instead of cooling-only would have fairly modest up-charge, but would give you options down the road when the wholesale price of natural gas get's off the $2/MMBTU floor and starts looking more like the $6-12/MMTU that the rest of the world is paying. At the moment gas pricing is at record low prices due to the fact that oil has been running $100/bbl, which is enough to make fracked wells with liquids profitable- they would be money losers on the natural gas alone. If oil drops below $80/bbl for any length of time the drilling rates will drop precipitously, and that natural-gas by product will go up in price. There are know coal-seam methane sources in the PNW that simply aren't being exploited right now due to the ultra-low price of natural gas, since coal seam gas has no liquid fractions that can be sold. (Full disclosure: I have a relative in the gas & oil exploration biz that has a direct economic interest in one of those PNW coal seam projects.) If your gas prices rise, high efficiency heat pumps become more economic to operate than condensing boilers, given the modest retail cost of electricity in most of the PNW.

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