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DHW limit settings DHW vs Heat

Hydronic oil fired boiler with hot water heat and DHW loop (no DHW tank)
no cold mix valve
New aquastat on system
Would it be good to run high and low limits differently in summer (DHW only)
than when heat is required
Old aquastat set HL 175 LL 135 diff 10

Got suggestions for summer set (DWH only) and winter (heat & DHW)


Diagram from Honeywell L7224U aquastat manual



  • Jamie Hall
    Jamie Hall Member Posts: 23,289
    Ideally you would have two aquastats, not just one. One in series with your space thermostat, set for whatever temperature is needed to heat your building. The other set for the domestic hot water. That way you don't have to change anything.

    That said... you should. No, in my opinion, you must have a thermostatic mixing valve on this type of system. Otherwise when the boiler is heating the space, you run a very great risk of scalding. The mixing valve temperature should be set to avoid that risk -- no more than 120 F, and perhaps 110 F would be wiser, depending on who is using the hot water.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ethicalpaul
    ethicalpaul Member Posts: 5,702
    Or better yet, get your DHW out of your boiler and have a standalone water heater (like a Rheem Marathon or a hybrid), if possible
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • Zman
    Zman Member Posts: 7,569
    You really need a mixing valve. You are sending a slug of water equal to the boiler operating temp when you first turn on the water. Irregular temps after that.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • cozmicray
    cozmicray Member Posts: 9
    There is 20 to 50 feet of copper hot water pipe between boiler and faucet, so slug of hot water is minutes away. I am the only one here and I know about the ultra hot water. The well pump varies the pressure a bit so gets hotter or colder at faucet.
    The aquastat accommodates heat and hot water by controlling circulation pumps. Circ pumps don't run in summer, no heat required.
    What is the ideal temperature at the boiler for hot water baseboard radiator heat (Hi & Low).
    Would I save some oil by setting it lower in summer with just DHW.
    I have a cold water mix valve but piping with bypass a copper nightmare over the boiler. The cold water mixing valve at the faucet seems to work well.
    I guess I may have to ask Honeywell for their take on settings, They will be very conservative but they may have answer for DHW / Heat burner duty cycle settings

  • pecmsg
    pecmsg Member Posts: 4,847
    Wrong is wrong
    Put a mixing valve on it!
    HVACNUT Member Posts: 5,835
    Keep the 10° differential for cut in 5° above the mixing valve set temperature.
    Example: Hi- 155, Lo- 135, diff- 10, mixing valve- 120.
    The high setting doesn't come in to play at all unless there's a heat demand.