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Community and Q&A

Oversized Electric Boiler and Radiant Floor Heat

sfortier | Posted in Energy Efficiency and Durability on

Hello, my heating contractor sold me a 68K BTU/H electic boiler (Thermolec) for my radiant floors based on floor footage and the large amount of windows I have. Whatever I do, CoolCalc gives me around 25K-30K BTU/H of heating load.

I could give you all the details (insulation level, types and sizes of windows, air tightness, etc…) but my simple question is: if the boiler is oversized, what are the issues I can expect? Temperature fluctuation, shorten boiler life, etc…? Maybe one more info: the thermostat uses a in-floor heating sensor and by default it has a minimum cycle length of 15 minutes (they don’t recommend to go lower with radiant floors).


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  1. charlie_sullivan | | #1

    What model boiler did you get? Many of them can modulate and can operate just fine at lower power. If that's the case, the main problems are making sure the controls are set up right to do that, and the need for high current capacity in the electric supply which can be expensive.

  2. sfortier | | #2

    Charlie, it's a Thermolec B-20 ( The installation manual does mention modulation but requires an outside sensor.

    1. user-2890856 | | #6

      Use the outdoor sensor . It should enable the boiler to monitor supply water temps and adjust them accordingly . You may just want to invest in a small buffer tank ( 2 pipe ) to further lessen cycles and extend boiler life , relays last a finite amount of cycles .

      1. Expert Member
        NICK KEENAN | | #7

        Minimum cycle time is 15 minutes or about 15K BTU. With a 20F lift that's about 90 gallons. That's a pretty big tank.

        1. user-2890856 | | #8

          You choose the right sized tank , they come in 18 through 119 gallon . I also stated thaty a 2 pipe configuration should be used . In this arrangement , your loads are between the boiler and the tank , the system uses what it needs and lessens flow that the tank encounters keeping it more stratified . An aquastat in the tank turns the boiler on / off . Unti;l the aquastat turns on the boiler the system is able to pull stored heat from the tank until the aquastat turns the boiler on again .

    2. charlie_sullivan | | #9

      That outside sensor is a good idea anyway, and should make it modulate as you'd want it to.

      The manual shows that it uses a solid-state relay. That means it won't get worn out by short cycling. I imagine that the 15-minute run time is intended to ensure the hot water circulates throughout the system and doesn't just warm one room. With the outdoor sensor it should work pretty well, except that the curve of water temperature vs. outdoor temperature doesn't appear to be adjustable. Ideally your system designer would have planned around that curve.

      Two downsides of the larger-than needed size are:

      1. the need for a 125 A circuit, which is expensive and means you don't have as much capacity left to use for future needs like EV charging.

      2. The required high pumping speed of 7 gallons per minute, which might make it noisier than you'd like.

      1. sfortier | | #10

        Makes a lot of sense Charlie. The 15 minutes is not a boiler requirement but a default value on the thermostat (Sinopé), that value can be lowered to 5 minutes or even 15 seconds, I will contact Thermolec to make sure of the minimal value to use here. There is also the pumps that will also use the selected cycle time so must be careful there as well (the pumps are from Armstrong Pumps).

  3. dennisgray | | #3

    If radiant tubing is in concrete there is little chance of any short cycling. If tubing is in joist space you lack the thermal mass and may cycle much more. In general short cycling with a electric boiler is much less of an issue than with a fuel fired unit anyway. I would ask the boiler manufacturer for recommendations on this however.

  4. Expert Member
    NICK KEENAN | | #4

    Generally resistive heat equipment doesn't have the issues with short-cycling that motors do. That said, presumably the manufacturer put the 15-minute minimum cycle in for a reason. What you're going to see is overheating. Let's say it's a mild day and your heating load is 5K BTU/hr. That boiler can provide that in about five minutes, but with a 15-minute minimum run it's going to dump about 15K into the house. The heat will run for ten minutes after the thermostat shuts off, then things will be too hot for about three hours until it's time for the next cycle.

    This isn't efficient -- the house will lose more heat at the higher temperatures -- but more important, it isn't comfortable. And the whole reason for having a heating system, once you get past keeping the pipes from freezing, is comfort. This is especially true of a high-end hydronic system.

    How much of a problem it is depends on the heat capacity of your house -- how much the infusion of 10K extra BTU's changes the interior temperature. This isn't something that is normally modeled so it's hard to predict.

    1. sfortier | | #11

      That is exactly what I have been experiencing i.e. the temperature raises a little too much by the time the boiler stops so it is somewhat uncomfortable. I have contacted both my HVAC contractor and Thermolec, will see what they suggest the minimal cycle time to be set to. Appreciate all your inputs guys!

  5. Expert Member
    NICK KEENAN | | #5

    Just skimming the manual it looks like you can replace the heating element with a lower wattage one. That's probably the easiest way out.

  6. sfortier | | #12

    Got news from Thermolec and my HVAC contractor:
    - no need for a minimum cycle time, the boiler will be fine without it (and as Charlie pointed out, it uses solid-state relay that will not worn out over time)
    - the boiler modulates automatically based on the needs so other than having a big hunk 125A breaker, an oversized boiler is not an issue

    Thanks all for your help, I'm relieved :-)

  7. walta100 | | #13

    Seems to me the thermostat on the wall controls the pump that moves the water out of the boiler and thru the floor tubing. The pump need long run times given how slowly the large mass of the floor changes.

    The aquastat in the boiler will turn on and off the heating element as needed to keep the boiler water at it set point.

    The way I see it the oversized element is irrelevant to the cost of operation. A 30kW element that runs 80% of the time will use the same number of kilowatt hours as 60kW element that runs 40% of the time both will produce the same number of BTU at the same cost.


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