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Question about Aquastat on steam boiler block

Apologies if this is a basic question but what is the purpose of the Aquastat on my steam boiler block if there is already an Aquastat on my indirect hot water tank for domestic HW that fires the boiler? Do they work in tandem?

I understand water turns to steam at 212 degrees.


    EBEBRATT-Ed Member Posts: 15,529
    If you have a steam boiler with an aquastat and an indirect you may not need the aquastat on the boiler.

    Did you used to have a tankless heater on the boiler for DHW?

    The best way for an indirect to work on a steam boiler is for the control on the indirect to start both the boiler and the indirect circulator on a call for hot water, that does require a relay.

    Check to see if the aquastat on the boiler is still wired to the burner. They could be using the boiler aquastat to run the burner and the tank control to run the circulator.
  • neilc
    neilc Member Posts: 2,703
    edited December 2021
    pictures ?

    aquastat on boiler shuts burner off prior to steam when domestic aquastat is still calling for domestic and there is no call for steam (??)
    known to beat dead horses
  • motoguy128
    motoguy128 Member Posts: 393
    Sounds like it has a hot water loop off the steam boiler for the indirect. The aquastat on the indirect domestic water heater is to call for heat and the pump to heat the domestic water. The aquastat on the boiler is to tell the steam boiler to heat hte water, but not make steam, so they are normally set at 180F and will s usually drift up to 200f after shutdown since there’s often a lot of lag and less water in a steamer than hot water boiler.

    The indirect typically will only absorb 80-120,000 BTU of heat depending on water temps, while the steam boiler could be a 140-200+ output, so you will need to cycle the boiler so it doesn’t start steaming.

    Makes sense?

    I have a hot water loop on my steam boiler. The piping is a little tricky to keep it from flashing to steam or uncommanded “ghost” flow while its steaming.
  • pcolburn2
    pcolburn2 Member Posts: 25
    Thanks everybody. Should I think of the aquastat on the boiler block as a high limit?
    The aquastat on the indirect is set to 140 and has a mixing valve for DHW. The one on the boiler block is set to 200. It seems to me that we only need the water to be heated over 200 when making steam. I understand there is heat loss within the loop to the indirect tank.
  • pcolburn2
    pcolburn2 Member Posts: 25
    Revision: the astat on the boiler block appears to be set to 160 degrees. Does that make sense when the indirect is set to 140?