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Tankless DHW heater in oil fired boiler

I'm new to this, so please bear with me. We have a Lennox oil fired boiler, probably 15 years old or so. It has a tankless DHW on it that has never been hooked up. I hooked a couple garden hoses to it this past weekend and turned on the water to see what happens. Nothing. The boiler did not start as I anticipated. I set the thermostat so that the boiler fired, and it did start to heat the water. My question is, shouldn't the boiler fire as soon as there is water flowing in the heater? The aquastat is a Honeywell L8124C-1094. Do I need to change settings on the aquastat? I didn't look to see what the current settings are. Could there be corrosion on the probe? I have tried to find out exactly how the system works, but haven't had any luck. Any help is appreciated. BTW, I am a homeowner, pretty handy, but not an HVAC professional.


  • STEVEusaPA
    STEVEusaPA Member Posts: 5,689
    edited September 2021
    Why are you trying to use this all of a sudden.
    It's important if you are using this for domestic hot water that you properly pipe it to code with a mixing valve (anti-scald) and a relief valve (which may not be required in your area but strongly recommended).

    The burner fires for domestic only when the temperature of the boiler goes below the low limit setting.
    So if no one disabled the low limit feature, sometimes they do it by removing a wire, then turning the low limit up to the 150° range will fire the boiler if it's temperature is below that setting.

    But again, the boiler can get up to 200° and you don't want that going to the fixtures in your house as someone will get 2nd degree burns instantly.
    "...Most adults will suffer third-degree burns if exposed to 150 degree water for two seconds..."
  • bburd
    bburd Member Posts: 332
    edited September 2021
    If the tankless coil was not piped to supply hot tap water when the boiler was installed, the low limit feature of the aquastat should have been disabled by the installer. There is no reason to keep the boiler hot at all times with the coil not in use. It should operate only under control of the space heating thermostat.

  • kcopp
    kcopp Member Posts: 3,990
    I also think the OP is confused as a Tankless WH will fire up w/ flow...

    He has a Tankless coil that does not operate that way.
    It maintains a set temperature if it is wired to do so.
    EBEBRATT-Ed Member Posts: 12,010
    I am out on a limb maybe he wants to use the hot water from the tankless to wash his house or a car
  • bwarren
    bwarren Member Posts: 2
    Thanks for the comments. I was thinking of plumbing it to the cold water inlet of my electric hot water tank. My thinking is, in the colder months, when we are using the boiler anyway for heat, if the water entering the water heater is already hot, the tank will be storage only. The electric elements won't need to work unless we need a lot of hot water.
    EBEBRATT-Ed Member Posts: 12,010

    There is no reason you can't use the tankless to preheat the electrical tank. It doesn't need to start the burner you only going to get heat from the tankless while the boiler is on for heat

    If you have ANY valves between the tankless coil and the water heater be sure to put a 150 psi relief valve on the tankless side of the valve