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Two water heater system

wij | Posted in General Questions on

The current water heater in my house is a 32 gallon Buderus boiler mate, on a zone from the oil burner. The main house foundation is 24’ x 36’, 10” concrete walls, all except the top 16” is below grade. No insulation on the walls, yet. An addition off the north 24’ wall is 18’ x 30’ built with ICF foam blocks. There is an opening between the two foundations 8’ square. 

We heat the house in winter with a remote high efficiency wood boiler (mid Nov – mid April) that is piped into the existing oil boiler to utilize existing pumps and controls. 

We have just purchased a Rheem 40 gallon electric hybrid heat pump water heater, and the plan is to use the Buderus in the winter, and the Rheem in the months of no heating requirements. The basement is dry year around, but we use a dehumidifier to keep humidity levels around 45-50% due to many pieces of woodworking equipment with cast iron work surfaces.
The question is, do we shut off and drain or leave filled whichever water heater is not in use for the season?

Thank you for your time and consideration. 


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  1. Expert Member
    NICK KEENAN | | #1

    Water heaters should be drained once a year anyway. So drain them when it's time to switch over.

  2. Expert Member
    BILL WICHERS | | #2

    If the Rheem unit can work like a "regular" tank-type water heater, it would be OK if fed with hot supply water -- it just won't try to heat it. Be sure to check first since I'm not familiar with your specific model here.

    Assuming that is OK, use the boiler mate water heater to feed the Rheem unit. As long as the boiler is running, that boiler mate's tank will be hot, and will feed hot water to the Rheem, which won't run. When you shut off the boiler in warmer weather, the Rheem unit will see cool supply water and will act to heat it. A system like this works completely automatically, and requires no intervention on your part to "switch over" between seasons. The downside is slightly more standby losses when no one is using hot water, since the Rheem unit will act to maintain it's tank temperature. If you put a bypass valve on the Rheem unit, and a shutoff valve in at least one of the lines (I would use the hot water exit line for this), you can shut off and bypass the Rheem unit when running on boiler heat to avoid the standby losses from the Rheem. During the summer, the boiler doesn't run, so there is no need to shut off or bypass the boiler mate tank the way you want to shut off and bypass the Rheem unit during the heating season.

    BTW, if you need dehumification during the heating season, I would let the Rheem unit do that and maybe save you a bit of energy. My guess is it's not a big difference between that and a regular dehumidifier running though.


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