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Thread: Installing an electric water heater in series with boiler

  1. #1
    DIY Junior Member paul01420's Avatar
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    Default Installing an electric water heater in series with boiler

    Hi everyone,
    I have an oil furnace that has a tank-less hot water that supplies my hot water. I am looking to install an electric hot water heater in series with my oil furnace. I have read multiple threads of people that have installed an electric water heater by hooking up the hot water return from the oil furnace to the inlet of the electric water heater. Then they would turn off the furnace during the non heating season, and during the heating season they would turn on the boiler. This allows the electric water heater to be fed with hot water from the furnace during the winter months (while the furnace is being used anyways) to help heat the hot water to save electricity. During the summer months the hot water would be heated completely with electricity so the oil furnace can be shut off thus saving the cost of oil being used just to heat the hot water.

    My question is has anybody ever installed an electric water heater by installing the return of the electric water heater to the supply of the oil furnace tank-less supply? The reason why I would like to install the water heater before the oil furnace tank-less is because even during the winter months the water that supplies the oil furnace tank-less will already be hot and therefore save oil consumption my helping to keep the burner hot. If I install the electric water heater in series after the oil furnace then the oil furnace would still turn on when I am using hot water. Before anyone states that this is a waste of money because the electricity will be more expensive then oil, let me state that I have solar panels and will never use up the amount of electricity they are creating. Thus if I install the electric water heater in series with the oil burner (before the oil furnace instead of afterwards like most post suggest) during the winter months I would have the added benefit of helping to keep the oil furnace hot. My only cost would be the purchase of an electric water heater and then I would be completely saving money.

    I would be interested in hearing people's thoughts about this. Also if you have done this and it is working for you, I would also enjoy hearing from you. Last but not least although this would not be a normal installation, does anyone know if this will pass plumbing codes? All replies will be greatly appreciated and thanks in advance.

  2. #2
    In the trades Dana's Avatar
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    If you have PV on the roof in excess of what you can use and it's net-metered, spend the real money on mini-split heat pumps and get off the oil!

    Even if you DIDN'T have the PV, mini-splits would make good short and intermediate term economic sense (unless heating oil drops back to a buck-fifty, like it's going to do any day now... after the next ice age maybe?)

    In MA you can even get 7-year/0% money for ductless mini-splits through MassSave, and the operating cost during the heating season is going to be less than half the cost of heating with the oil burner. It takes a bit of careful thinking about how many ductless heads it takes, and not all houses are ideal, but most houses can manage fine with 2-3 heads and 2-3 tons of compressor. Fujitsu & Daikin both have great very high efficiency products with fully rated outputs at -20C/-4F, and the Mitsubishi "Hyper Heating" -FExx series are good down to -25C/-13F. Even without the PV, if you spend $7-7.5K (a credible number) for a 2-3 head ductless system it's usually cash-positive in year-1, if you take the 7 year 0% financing. WITH PV it's a no-brainer- get RID of that stinking environmental hazard in the basement and move onto lower carb lower-cost state of the art heat pump technology!

    I can point you to a huge amount of online detail on ductless heat pump performance issues and help you figure out sizing issues, if it's of any interest. But for now lets keep it on your hot water question.

    Using a hot water heat as pre-heat to the coil means you have to keep the boiler hot all summer or plumb in coil-bypass to keep heat from the hot water stream from just heating up the boiler. The heat transfer in the tankless coil works both directions, after all. It's simpler to plumb it to use the boiler as pre-heat to the tank, which won't really care if it's taking in 60F summertime water or 150F wintertime water.

    If you plumbed it with the coil as pre-heat to the electric tank, you can turn off the boiler for the summer and save huge on standby loss cost and a bit on air conditioning cost.

    IIRC when using the boiler's coil as pre-heat to the electric tank, MA code still prescribes a tempering valve on the output of the coil as scald-protection.

    Heat pump water heaters are heavily subsidized by utilities in MA, but only if you're already heating hot water with electricity, but may make sense if you continue to heat your water with the boiler, since it's soaking up boiler standby as for about half of the hot water heating increasing the overall net efficiency, and behaves like a dehumidifier in the summer.

  3. #3
    DIY Junior Member paul01420's Avatar
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    Hi Dana,
    Thank you for the detailed explanation about the "elec hot water tank installed in series"... you did a great job and appreciate your response. I think I understand your explanation about having the electric hot water placed in series after the oil burner. I still don't completely understand why it would be better to install the water heater after the boiler instead of before. Is the argument that if you place it before the oil burner that the heat transfer to the oil burner will add extra cooling costs in the summer or that the electric water heater won't be as efficient due to the heat loss created while going through the tankless in the oil burner before going to the faucets? If you could explain this it would be greatly appreciated. Also by the way, I do plan on installing a couple of mini split systems as well as an electric water heater. My concern with the oil burner is that it is going to leak water when I shut it off ( it is always up to temperature because of the tankless hot water).
    If anybody else has anything else to add about this topic it would be greatly appreciated!
    Thanks again Dan for the info.

  4. #4
    Moderator & Master Plumber hj's Avatar
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    Your boiler is going to be "cold" in the summertime, and therefore, any hot water flowing through the coil will "heat" it up and thus lose its temperature. Just the opposite of how it works in the wintertime when the boiler is hot and the incoming water is cold. The more efficient the coil is the more heat loss there will be
    Licensed residential and commercial plumber

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