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What about constant circulation and outdoor reset for an old high mass system

bruce_21 Member Posts: 236
I have idea I'd like everyone's opinions on: I have client with a boiler with a bunch of cast iron rads, a high mass situation with big steel pipe (was maybe a gravity system originally), would it work to run the circulator continuously and change the controls to an outdoor sensor that would then vary the boiler temperature? Are there condensation or other issues if the output temp stays at something between 120 and 160? The house is small and has thin walls which likely had little insulation and leaky windows originally, now new windows and spray foam insulation are going to cut the load way down so I'm thinking comfort will go up if there are no big swings in temp from the system overshooting the thermostat setpoint because of all the water mass. Thanks in advance for your help and advice.


  • Steamhead
    Steamhead Member Posts: 14,838
    What model boiler

    is in there now?
    All Steamed Up, Inc.
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • bruce_21
    bruce_21 Member Posts: 236
    Peerless 3

    section cast iron 82% oil fired. Not sure the actual model #.
  • icesailor
    icesailor Member Posts: 7,265
    Big old pipes:

    In my experience and opinion, you have the perfect system for constant circulation/ODR (Out Door Reset)

    A really nice way to do it is with something like a Taco "I" Series 4-way mixer and make it primary/secondary through the use of the 4-way with two circulators. That way, you can keep the boiler water over the condensation point (140 degrees) and the ODR part of the valve will run the gravity part of the system at whatever temperature is needed, set by the outdoor temperature. If you have just a pumped but converted gravity system, unless you can let the circulator run to its hearts content, the closest radiators to the boiler will always be hotter that the ones farther away. When the ODR control works, it might only need 115 degree water to heat the whole house/system, and because the system is pumping 115 degree water through the entire supply, the system will go in to total balance. Which was the idea of a gravity system as designed. You can add TRV's but if you try the 4-way, it will work. And you might find that the TRV's don't do what you would expect.

    If you try to pump 115 degree water continuously through the system, the boiler will fail because of condensation. Condensation is really bad for a system.