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Adding Electric Boiler to Work With Propane Boiler

PoetryQueen | Posted in Energy Efficiency and Durability on

We have radiant floor heat with a 328K BTU propane boiler and 11 zones. We just got solar and have extra electricity that I would like to use by getting an electric boiler.  The question I have is if an 138K BTU electric boiler could be hooked up to be the first go to when heat is called for and then the current propane boiler kick in if needed.

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  1. Expert Member
    Akos | | #1

    A 138k electric boiler is a big one. To run it you need at at least a 400A service. It would take a truly massive (or extremely inefficient house) to need 130k BTU boiler, never mind the 300btu propane burner you have.

    Depending on the size of your solar panels and the actual excess capacity you have, a better option would be one or a couple of 100 gal electric water tanks with 5kW elements.

    Since you are using floor heat, so you only need low temperature water, each 100 gal tank with a 160F to 100F swing can store around 0.5 therms of heat(around 1/2 gal of propane burnt) or 15kWh of excess solar production. Hot water tanks are extremely efficient at storing this excess PV energy, there is very little loss. They are also significantly cheaper than batteries.

    In my area, using typical house loads, it would take a 15kW array to have enough excess capacity to need more than 2 tanks in the winter time.

    Control would be very simple, just set your solar diversion controller to heat the tanks when there is excess sun. Set your propane boiler aquastat just bellow the low setpoint on the tanks (so 100F in my example above). This way if there is excess sun, the tanks heat up and the propane boiler won't kick in until the tanks get down bellow this minimum setpoint.

  2. PoetryQueen | | #2

    Thank you for the response. The boiler I have picked out that works for my house (we are three phase 208) runs at 110 amps which we have room in our panel to use. The house is massive at 17,000 sqf. But realistically we only use about 5000 sqf on a continual basis. Our current solar array is 57KW. Just want to figure the best way to set up an electric boiler so that a call for heat goes to our electric boiler first and then if excess zones call the propane boiler kicks in.

    I would have the electric boiler just do the specific zones but that would mean a whole new board with the copper and pumps.

  3. PoetryQueen | | #3

    Just a side note, for the solar we use it during the day and then we build up credits to get free electricity at night so no need to use the electric boiler just when we have sun.

  4. Expert Member
    Akos | | #4

    If the power or storage is not an issue, there is nothing wrong with adding in the electric boiler.

    The simplest would be to plumb it with its own pump on set of closed spaced Ts after the existing boiler and have it set to run at a temperature above the propane unit. If the electric boiler is running, or at least providing enough heat to keep the return water temperature hot enough, the propane one won't fire. Once the electric is off or the water gets too cold, the propane boiler would run as normal.

    Similar layout as shown bellow:

    The only control item to work out is how to enable your electric boiler. You only want it to run to use up your excess PV production credit, can probably do it with a bit of IFTTT programming if you have a smart power meter.

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