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combi boiler with electric water heater

bob eck
bob eck Member Posts: 930
is anyone that is installing combi boiler adding a 50 gallon electric water heater?
home owner wants a domestic hot water recirculation system with their combi boiler.
was looking at installing a combi boiler and then running the domestic hot watet into the cold water inlet of the electric water heater then out to their faucets. connect the elements up to maintain water temp in the electric water heater then put the domestic hot water recirculation pump on the tank.
plus if (when) the combi boiler breaks down the home owner would still have domestic hot water.
will this work?


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,838
    unless electricity is very low cost in your area, that’s not such a good idea.  Where I’m from, the cost to run that would be very expensive 
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • motoguy128
    motoguy128 Member Posts: 393
    I had a electric with recirc without a combi.   It was expensive.  Went it indirect off a steam boiler and gas went up $30 but electric dropped by about $100/mo.  

    Could use a pump and
    aquastat to periodically circulate  through the Combi independent of the recirc.  To reduce that demand.  

    Could also set tank to just 125 and combi to
    140 and use a tempering valve.  But the check valves don’t play nice with some recirc pumps.  
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,295
    Hi, How about controlling the recirc with demand control? That will cut line loss by about 90%.
    Yours, Larry
  • bob eck
    bob eck Member Posts: 930
    Some recirculation pumps run off a sensor when you enter the bathroom and some have a button you push when you go into the bathroom. Running a recirculation pump 24/7/365 is crazy 
  • rick in Alaska
    rick in Alaska Member Posts: 1,457
    I have quite a few installations that were installed here that are set up similar to that. Basically, the tank is piped to the combi with a pump, and the supply and return to the combi are connected in to the holes where the elements go. The pump is a standard bronze taco 007, and is controlled by the thermostat in the tank. So far it works well on the ones out there.
    I am going to do something like you suggested on my mother in laws house. She has an on demand water heater that I have hooked up with a push button on a two minute timer at the kitchen sink ,but she has a hard time remembering to push the button, which makes it hard for her to do the dishes because of it being cold water. I am going to install a small 120volt electric water heater under the kitchen and just tie it in series with the on demand heater. That way, the water is always hot when she needs it, and the on demand helps to keep the tank hot.
  • hot_rod
    hot_rod Member Posts: 22,144
    How often would you need to rely on the electric tank function? maybe a 2 or 6 gallon tank is adequate. I see some of the tankless now have a small tank onboard. That is good for recirc and also helps the three way thermostatic mix valves behave.

    The Caleffi 520 mix valve is an excellent valve for combi and tankless, tankless coils also, fast responding and 100% hot shut off if cold fails.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream