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Electric vs. propane tankless water heaters

tim04983 | Posted in General Questions on

Hi all. I’ve heard a lot about this forum through the FHB podcast.

So I currently have a propane water heater that is power vented. I have this because the basement is prone to flooding. I would like to go tankless. I’m wondering about electric vs propane. We have 1 bathroom, washing machine, and dishwasher. There are 2 adults and three kids. My current propane is about $75/month, and all we do is heat water w/ it. Also, I live in a cold climate, the Western foothills of Maine near Sugarloaf Mountain in Franklin County. So I would appreciate your thought on electric vs propane. Propane is more expensive up front and their are the concerns about venting. Electric is more efficient, but not sure how much more my electric bill would be. Would appreciate your thoughts.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Tim,
    Here is a link to an article that provides guidance: "Domestic Hot Water: No Perfect Solution."

  2. Expert Member
    Dana Dorsett | | #2

    At Maine type incoming water temperatures you would probably have to upgrade the electrical service to the house to do it with an electric tankless. Even a low flow 2 gpm shower takes over 20,000 watts just for the shower, and would require a dedicated 100A/240V breaker and wiring fat enough to pull the F150 out of the ditch. There are 28-30,000 watt versions that might buy you a tiny amount of margin on a 2.5 gpm (still low-flow) shower), but tub fills would still be tedious. Large intermittent loads like that are also hard on the local power grid infrastructure, and reduce the power quality on the local grid.

    A 199,000 BTU/hr propane burner will still have lower flow than what you get out of a tank, but it'll be twice that of a 28,000 watt electric.

    Is there any place to install a tank type water heater other than in the flood prone basement?

  3. tim04983 | | #3

    Thanks for the info. I didnt realize that much extra work was involved in adding the electric version. There really isn't a place for a tank upstairs, typical 1970's ranch floor plan. During the summer we've made use of a outdoor shower I made by making a batch water heater. At some time I would like to find a way to heat water with my woodstove during the winter. But right now, I need a quicker fix, family of 5 with no hot water not good.

  4. Expert Member
    BILL WICHERS | | #4

    The electric tankless water heater is going to guzzle down power, but only during the relatively short period that you’ll be using it so your operating costs won’t be that much. Dana is correct that it’s going to need a BIG circuit to feed it, at least 50A and probably more. A 100A circuit is entirely likely, which means at least #3 copper wire (about 3/8” diameter, per wire). If you only have a 100A electric service to your house, you’ll likely need to upgrade your service too which could be a significant expense. Your savings are that you’ll have no venting issues.

    I wouldn’t be as concerned with grid impact as Dana since this is a big, but short duration load so it averages out over a large area if there are only a few users. It’s a resistive load too, so it just causes a bit of a voltage drop, not the goofy waveform issues a reactive load like a big motor would.

    Personally, I’d go with a propane tank less if you really want a tank less setup. The venting for those is usually two runs of PVC pipe so it’s not that big of a deal. Make sure your existing tank and regulator can provide enough gaseous propane during the coldest part of the year if you’re running your water heater and your furnace. The tankless water heater is a big propane load too, and you don’t want to have freezeup issues with your regulator or tank.

    If you want to stay with electric, maybe a smaller tank-type unit would work? Those are usually only 4,500-5,000 watts and will need only a 30A circuit (#10 copper wire) at 240v so you can probably handle that with your existing electric service.

    Bill

    1. Expert Member
      Dana Dorsett | | #6

      >"I wouldn’t be as concerned with grid impact as Dana since this is a big, but short duration load so it averages out over a large area if there are only a few users."

      It all depends on the capacities of the local transformers and wires whether it actually "...averages out...". Distribution grids and infrastructure in rural Maine don't much resemble those in the big city 'burbs.

      1. Expert Member
        BILL WICHERS | | #8

        True, but they’ll still be built to REA standards at a minimum. The only real limitation would be the distribution transformer serving the particular house in question, and the utility should size that adequately for whatever the service load is. Once you’re up at primary voltage, it really doesn’t matter much — even 24kw is a relatively small load there. A typical rural substation will likely be at least 5MVA. The lowest common distribution voltage is 4.8kv, 7.2kv+ is much more common these days, even in rural areas. A 24kw load ends up only being around 3-5 amps at those levels.

        The averaging doesn’t really apply at the distribution transformer level, but it does on the primary lines and the substation. Now if the entire community had electric tankless hot water heaters and everyone took showers at precisely 6am in the morning, there could be some excitement with system loading. The reality though is that demand factors come into play and the load leaks average out into a more or less smooth curve. There are some exceptions, the most famous being the tea kettle timing issue with soccer matches that the UK grid sees.

        Bill

  5. tim04983 | | #5

    Thanks. I am thinking propane will be best, especially considering I already have a gas line and vent set up. Also, I have no furnace. I heat exclusively with wood and have electric base board (70s ranch). I haven’t even turned on electric for years now.

    1. Expert Member
      Dana Dorsett | | #7

      If you have electric baseboard heat you may in fact have enough electric service to run an electric tankless if you disconnect the baseboard circuits. That still won't fix the flow limitations though.

      A cold climate mini-split or two in lieu of baseboards may be a worthwhile lifestyle upgrade at some point. They "play nice" as backup for wood heat, throttling up slowly and quietly as the embers cool.

    2. Expert Member
      BILL WICHERS | | #9

      I agree with Dana here on both points. If your service was sized to handle electric baseboards, you may have enough capacity to run a big electric tankless water heater. Check the amperage number on the handle of your main breaker (or the printed label of the fuse in the main disconnect). If you see “200” you’re good to go. If you see “100” you might not have enough capacity. 125 and 150 are less common, but are out there.

      If you already have the venting in place, propane is still probably your better option. Just make sure your tank and regulator can handle the load. Your operating cost should be fairly small since the amount of time the unit will be running is likely to be pretty small (showers, hand washing, maybe a dishwasher periodically). Remember that regardless of the size of the tankless heater, it will still only run when there is demand for hot water.

      Bill

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