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Fundamentals From the Dead Men

Ok. I'm hoping for a lesson from the Dead Men. Or the not dead yet, men:

Thermostat is off
Oil Boiler is firing (Weil McClain P-SGO-4 with attached water tank for DHW)
Radiators are hot (single-pipe steam system in NH in August)

Upon inspection, the water level is low (half?). There is a slow leak in the system as there is a damp spot behind the boiler under a valve. We add water to the boiler. Boiler eventually shuts off and radiators grow cooler.

The cartoon I just made up in my head and explained to my brother is that the reduction in system water volume increased the pressure in the system and siphoned heat from the boiler to the radiators. Please tear my cartoon apart and help me understand what is really going on. Probably has something to do with the passive attached water tank that takes care of DHW and the low water level in the semi-closed system, I know. But what? If possible, make me work for the answer. I have all of Dan's books but can't find this specific problem and I would love to use it as a case study for self-teaching.

Thank you all!

-Jeff Melvin

Comments

  • JUGHNE
    JUGHNE Member Posts: 10,315
    Just a WAG:

    Was the water level below the temp probe for the DHW heating.
    Probe not in the water would be cool relative to actual water temp.
    Would call for burner to run and not satisfy control.

    Would still make steam heating your rads.
    mattmia2
  • Jamie Hall
    Jamie Hall Member Posts: 20,466
    JUGHNE said:

    Just a WAG:

    Was the water level below the temp probe for the DHW heating.
    Probe not in the water would be cool relative to actual water temp.
    Would call for burner to run and not satisfy control.

    Would still make steam heating your rads.

    I'm thinking that as an explanation, too -- either that or something wonky with the DHW aquastat.

    What should happen is if the DHW aquastat senses the DHW temp is low, it should fire the burner (and the associated circulating pump? Is there one?). When the DHW temp is high enough, that aquastat should open and the boiler stop -- unless the house thermostat is also calling for heat. The temperature it should stop at should most likely be around 180 F, but could be lower -- not high enough to make any steam at all.

    Now as to the system water volume, that really doesn't have anything to do with it -- provided only that the boiler water level is high enough to reach the DHW probe. Otherwise, when the house thermostat calls for heat, the boiler will heat to steaming temperature, and steam will form and flow though the pipes to the radiators; the pressure will be, or should be, very low -- a few ounces or so, perhaps as much as a pound in a few systems. When the steam gets to a vent -- either the main vents or the radiator vents -- it will close, and the steam will condense back to water and flow back to the boiler. There is surprisingly little water involved -- only a quart per minute per 100,000 BTUh -- so if the water level in the boiler drops much when steaming, it's because of slow returns, not loss of water to steam.

    If you have a slow leak somewhere, though, you should find it -- and fix it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • mattmia2
    mattmia2 Member Posts: 7,190
    There should be 2 aquastats, one that controls the temp of the water in the indirect dhw tank and one that controls the temp of the boiler. The one that controls the temp of the boiler is probably ending up above the water line when the level is low. The one in the boiler should be keeping the boiler temp below boiling so it doesn't steam and heat the radiators.
    EBEBRATT-Ed
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 13,141
    Agree with @mattmia2

    If the tank is calling and for some reason not heating or not heating fast enough the boiler could make steam to the pressure control if there is no hot water limit on boiler.

    Maybe the indirect circ is not working