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Could someone please provide a good resource for learning about aquastats and their use in radiant heat control applications ?

Are strap on units reliable? Such as the Honeywell Hi or Low Limit and Circ Strap On aquastat, 65-200 degrees, 5-30 degree adjust differential. L6006C1018


  • Paul48
    Paul48 Member Posts: 4,470

    Why not ask....what's the best way to control temperature to a radiant system? I think there are much better approaches than strap-on aquastats.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I have one strap-on aquastat in my system.

    That aquastat is in the supply from the boiler to my radiant slab. I am not supposed to put more than 120F into the slab. And my reset curves say that. But W-M, my boiler manufacturer says if the boiler supplies higher temperature zones, I should an aquastat in the  system to protect the slab zone. Maybe they do not trust their U-control?

    But there are a few issues. That zone is controlled with +|-5F slop in the limits, so it can actually go up to 125F sometimes. I do not think that would be much of a problem, since my old boiler would go even higher than that. The other thing is that my baseboard zone can go up to 136F (143F if you include the slop for that zone). Worse, the indirect is set at 175% +|- 5F, so it could go up to 180F. Default temperature for that is 190F. So imagine inf the indirect is satisfied, and the boiler is putting out 180F water or so. The circulator to the indirect shuts down, the fire goes out in the boiler, the circulator to the slab starts up, delivering the left over 180F hot water to the slab for a few minutes until the boiler water cools down.

    So what I have done is told the controller to let the circulator to the indirect to run for two minutes after the indirect is satisfied (with the fire off), At the end of that interval, the boiler will fire only if the boiler output is less than  the reset curve demands, but after two minutes a lot of the residual heat will have been dumped into the indirect (the boiler holds only 3 quarts of water). So that pretty well solves the problem. The aquastat is normally closed and in series with other safety stuff. If the aquastat gets too hot, it opens the wire and the boiler stops firing,. The boiler and aquastat will start up again once the aquastat cools down.

    I diddled the set point on that aquastat to see that it really worked (it does), but as far as I know, it has never operated when set correctly. It is good enough for what I am using it for. In fact, I probably do not really need it. I do not think I would use it as an operating control.
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