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(Old-fashioned) radiators in a Pretty Good House?

415irwin | Posted in Mechanicals on

We are considering plumbing in two reclaimed 8″x12″x38″ radiators in the upper floor bedrooms of our pretty good, passive solar home near Ottawa. The house is heated by a radiant slab in the basement and will have a 2″ radiant slab on the main floor.

We are wondering if the considerable mass of old fashioned radiators, or the possibility of them having a higher designed operating temperature might make them inefficient or ineffective in our system.

We have a Stiebel Eltron Tempra 12 Plus boiler.

Any advice on mixing the old with the new would be appreciated, thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    Any heating system needs to be properly designed. The first step is a room-by-room heat loss calculation.

    Once you know the heat loss of each room at design conditions, you'll know how you can meet the BTU/h goal for each room.

    The BTU/h output of a cast-iron radiator depends on the size of the radiator and the water temperature of your hydronic system. Here is a link to a document which will help you design your system: Cast Iron Radiator Heating Capacity Guide.

  2. gusfhb | | #2

    You will need a different mixing water temperature for that loop from the floor loop, and a different thermostat. Since the house is efficient I cannot see why they would not work as long as they are large enough to out the heat required for the room at a decent water temp

  3. Expert Member
    Dana Dorsett | | #3

    The Stiebel Eltron Tempra 12 is a tankless electric hot water heater, not a boiler, and not designed for continuous operation at high flow/low delta-T. Hopefully you'll still get a reasonable lifespan out of it in this application.

    Reclaimed high-mass radiators are fine for low-temperature hydronic applications, and have a very linear output even at temps below 100F, a for more consistent low temp performance than can be achieved with fin-tube convectors, etc. But with any hydronic system you have to match the output to the room loads and water temperatures as Martin correctly points out.

    Are these 38" tall 3-4 section column radiators, or 12" high 38" wide shorties designed for going under window sills?

  4. 415irwin | | #4

    Thank you three for your responses, Martin, Keith, and Dana.

    As mentioned earlier, the bedrooms are on the upper floor of a very open plan design, with no other point source heating yet in place on the upper floor (though a couple wires are in place if needed), and there is a 2" radiant slab on the main floor below.

    Our (unfinished) house has been modeled in PHPP and as far as I know we don't have any numbers for heat loads by room, but rather have the whole house numbers.

    Annual Heat Demand: 5573 kWh / yr.

    The two upright radiators we have in mind are 38" high x 8" x 12" and the (5) fins are about 2" in thickness. I don't know their mass yet.

    And we are also somewhat concerned by the decision to use the Tempra Plus and would be interested in hearing of suggested alternatives and the rationale.

    Thank you.

  5. GBA Editor
    Martin Holladay | | #5

    1. It's odd that no one bothered to perform a room-by-room heat loss calculation. For more information on this issue, see:

    How to Perform a Heat-Loss Calculation — Part 1

    How to Perform a Heat-Loss Calculation — Part 2

    2. I don't understand the logic of making hot water with an electric resistance heater, and then designing a hydronic system to deliver the heat. If you want to heat your house with electric resistance heat, why not just install a few electric baseboard units? That way you will save considerable expense -- the cost of the hydronic circulators, the trouble of installing all that pipe and the various controls and safety features -- and you will avoid the risk of future leaks.

  6. Expert Member
    Dana Dorsett | | #6

    A 12 kw boiler is about $1K more expensive than a 12kw tankless, but it's designed to handle both the higher flow rates & continuous operation. The output temperature isn't as precise as a tankless, but that precision isn't really called for in space heating applications.

    Also, 12kw seems unusually high output for a space heating load on a PHPP high-R house, even at your -22C --25C 99% outside design temp ( . Ottawa averages about 4500C heating degree days per year, at base 18C, and your heating/cooling balance point is probably lower than that. If it's modeling a heating energy requirement of 5573kwh/year, that's only about 1.25kwh/HDD. Even if we offset the base to 15C and have maybe 3500HDD that would only be 1.6kwh/HDD-base 15C.

    Over 24 hours that's about 67 watts per degree-hour. With a balance point at 15C and a design temp of -25C you're looking at 40 heating degrees, and a heat load of about 67watts x 40= ~2700 watts. Serving a 2.7kw load with a 12kw heat source is a bit like swatting flies with a hammer. If you're worried that you won't have enough margin to manage the extreme cold snaps, even calling it 100watts/heating degree and you're still at only 4kw, and the Stiebel Eltron would then be overized by 3x.

    An ASME-rated 2.5kw or 5kw electric boiler(eg: or ) capable of outdoor reset temperature control is maybe $100-300 more than the 12kw Stiebel Eltron (but the ODR control costs extra), can run on a 120V or 240V single-phase as-desired, and would be a much more appropriate type of equipment for the application.

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