combining hot water coil with electric hot water heater

Discussion in 'HVAC Heating & Cooling' started by Bob in Maine, Jun 30, 2008.

  1. Bob in Maine

    Bob in Maine New Member

    Based on the discussions I've seen here and on other forums, I just re-plumbed my domestic hot water system so the outlet of the hot water coil in my oil-fired boiler connects to the inlet of the electric hot water heater that had been standing unused next to it.

    The electric hot water heater outlet is now connected to the hot water line that serves the fixtures in the house.

    The boiler is nor completely off,and I hope to save money by not burning oil all summer.

    I'm thinking about re-starting the boiler in the winter months but disabliing the control that starts the boiler when hot domestic water is called for. I would leave the electric hot weater heater on and assume that the water from the boiler would arrive at the electric heater at a fairly hot temperature just from passing through the boiler in its normal heating mode. This is Maine, so I expect the boiler to be hot throught most of the winter season just from calls for heat.

    In effect, the boiler would pre-heat the water going to the electric hot water heater, resulting in the use of less electricity to bring it up to temperature., and the boiler wouldn't fire up just for hot water.

    Any thoughts on whether or not this would work and whether it would be worth while?

  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    New England
    That should you need to understand your controls to disable the thing from maintaining a set temp for the DHW coil. Sometimes it is easy, sometimes, it's not. A call to the manufacturer of the boiler may tell you. Someone else did that here, and the manufacturer told them all they had to do was remove one wire...yours may be as's just knowing which one that's hard!
  3. Bob NH

    Bob NH In the Trades

    New Hampshire
    The controller (the Aquastat) of a boiler with a tankless coil is usually set to maintain temperature of the boiler so that whenever there is a demand for hot domestic water it will be heated by running through the tankless coil. The temperature is usually fairly high to ensure adequate heat transfer rate with the small surface area of the tankless coil.

    In my situation the tankless coil delivers water hotter than the setting of the electric water heater so the electric heater doesn't use any electricity in the winter.

    In the non-heating season the boiler is turned off and the electric water heater heats all of the water as though the tankless coil did not exist.

    There is a substantial heat loss whenever the boiler is hot but there is no demand for heat.

    I have a concept for even more efficiency (less loss of heat up the stack) of the boiler/hot water system as follows:
    1. Run the boiler only when there is a demand for heat; and when the thermostats shut off continue to circulate through the baseboard units until the water in the boiler is cooled, so there is virtually no heat to be lost up the stack when there is no thermostat damand, and

    2. Circulate water through from the water heater through the tankless coil so that it could be heated even when there is no demand for hot water. This requires a bronze pump because the domestic water contains oxygen that will corrode a typical iron circulator pump.
  4. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    South of Boston, MA
  5. Thatguy

    Thatguy Homeowner

    Last edited: Feb 5, 2009
  6. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    South of Boston, MA
    That just brought me to google search results?
  7. hillpc

    hillpc New Member

    south jersey
    Well, the logic may be correct that firing up the oil boiler every time someone tuns on a faucet (or keeping the boiler at temp just for hot water) is wasteful compared to other methods, but there's something seriously wrong with the numbers at _Coil.html. 50,000 BTU to heat a gallon of hot water? Preposterous. 1 BTU/lb water/degree F is what it takes. And boilers are rated in BTU/hr, not BTU. Therein may lie the miscalculation.

    With the boiler hot and running a lot anyway in the winter, and the high price of electricity, it's not likely that electricity is going to be cheaper. All the starting and stopping of a boiler with a hot water coil (or also if the boiler is way oversized just for heating) reduces efficiency because during the non-running phase, some amount of draft still happens up the chimney. (Of course it's not like the oil burner's fan pumping huge volumes of air, though). The boiler also loses heat by heating the basement from its outer surface if it's not well insulated and is kept hot continuously by the coil aquastat. I agree that a separate zone running an indirect hot water heater is better for efficiency than a tankless coil, but that's a lot more expensive to buy and install.
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