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Should I consider switching from a gas to electric boiler?

mdsurfrider | Posted in General Questions on

My gas boiler is coming up on 15 years and I’m looking at replacing it.  I am considering an electric option since I have a 9.86 kW PV system.  My utility does not do net metering and think switching to electric would help me capture more value out of my PV.  My house has radiant heat and is a sip panel construction, very tight and well insulated.  My mechanical room also gets quite hot which becomes an issue for the rooms upstairs from it during the summer.  Not sure if electric would also lower the ambient heat?  I haven’t had a chance to really run the numbers or look at options out there.  Would appreciate any advice or insight.

Replies

  1. GBA Editor
    Martin Holladay | | #1

    M.D. Surf Rider,
    Electric resistance heat is expensive to operate. (And you don't need a hydronic distribution system if you are heating with electric resistance heat -- all you need are electric-resistance baseboard units or space heaters.)

    If you want to switch from gas to electricity, it makes more sense to buy a few ductless minisplits than to buy an electric-resistance boiler. Ductless minisplits will use only 33% to 50% as much electricity as an electric-resistance boiler.

    For more information, see the articles listed on this page: Category page: Ductless Minisplits and Ducted Minisplits.

  2. Expert Member
    Dana Dorsett | | #2

    Why is the boiler running in summer? Is it heating domestic hot water with a tankless coil or something?

    A 15 year old gas fired cast iron boiler is still barely into it's adolescence, maybe even younger if it's been running a very low duty cycle. A 15 year old condensing boiler could be looking at less than a decade until replacement, but there's no financial rationale to swap it out earlier than necessary.

    To get a handle on the the system and house and to estimate sizing on what might replace it, run a fuel use based load calculation (winter fuel use only), as outlined here:

    https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

    Also, what is the boiler model and size? (Both it's input BTU, & DOE output BTU rates). Your 99% outside design temp is also an important consideration in this analysis.

    It's possible to run the radiant floors off a floor thermostat, using a mini-split to control the room temperatures. If the mini-split is sized to cover 100% of the heat load you can just drop the floor temperature to let the mini-split carry the lion's share of the heat load, whether the radiant is being run on a fossil burner or electric. In most locations in the US the mini-split will deliver 3x or more heat per kwh used than the radiant floor, and you would have the option to tweak the overall efficiency by how much of the heat is radiant vs. heat pump. Since the mini-split modulates with load it "plays nice" with other heat sources, as long as it isn't too oversized.

    1. mdsurfrider | | #3

      Thanks for the reply. The gas fired water boiler also heats our domestic water in addition to the hydronic system used during the winter. The boiler is a Weil-McLain Ultra 310, 207k BTU. I have a Phase III indirect fired water heater tank for domestic water. I do already have a 3 head mini-split on our south facing upper level for use in the summer months. I have not used it in the winter for heating which I will now look into. However, this only covers about a quarter of our total SF. Maybe one thing I could look at is adding an electric hot water heater for the domestic water and limiting the boiler to just the hydronic system?

  3. Jon_R | | #4

    A heat pump water heater will certainly lower the ambient heat. But overall, you need to run the numbers.

  4. Expert Member
    Dana Dorsett | | #5

    The Ultra 310 is oversized for something like 49 out of 50 houses in the US, and probably for 999 out of 1000 tight well insulated SIP houses- not by just a little bit, but a crazy oversizing factor.

    Even at it's MINIMUM output of ~58K BTU/hr it would be about 1.5x oversized for the heat load @ 0F of my sub-code 2400' 2x4 framed antique + 1600' of insulated basement. Unless your very tight well insulated SIP house is north of 10,000 square feet and in a cold/very-cold climate it's probably RIDICULOUSLY oversized for your actual heat load.

    Nevertheless, run the fuel use heat load numbers, and you'll be able to infer your actual oversizing factor from that, as well as establishing an upper bound on what would make sense for heating with something else.

    If the boiler room plumbing isn't insulated it amplifies the standby & distribution losses. Think of the plumbing as an abstract art-sculpture heating radiator. Any plumbing that runs north of 140F should get R5-R6 or higher pipe insulation. All other plumbing (including the nearest 10' of potable cold feed and temperature/pressure outflow plumbing) should get R3-R4 pipe insulation. That should reduce some of the summertime boiler room overheating.

    The model numbers of the compressor & heads of the multi-split ductless system would also be useful for figuring this out. (They're probably oversized for their heating & cooling loads too... TBD.)

    We can get a pretty good rough handle on it with some napkin-math, but before buying any actual equipment it would be worth spending some money to have a qualified engineer or RESNET rater (not an HVAC company) run an aggressive room by room Manual-J load calculation on your place, using the fuel-use heat load analysis as a sanity check. The room by room numbers would be necessary to properly size any ductless solutions.

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