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Thermostat that controls boiler cycles?

drtom Member Posts: 31
I am looking at replacing my thermostat. I have read a bunch of posts on here about the need for a thermostat that can control your boiler cycling frequency. This is very confusing to me. My understanding is that the boiler cycles on when heat is called for at the thermostat. It runs until either A: the desired temp is reached, or B: the steam pressure increases and hits the cut-out setting of the pressuretrol. Then it cycles back on when the pressure decreases to the cut-in pressure (assuming that the desired temperature has not yet been reached at the thermostat).

If the cycling of the boiler is controlled by the pressuretrol, why would I need to artificially control it with the thermostat?

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  • Big-Al_2
    Big-Al_2 Member Posts: 263
    edited October 2011
    Heating Calls Per Hour

    A steam boiler has about zero efficiency at the start of each heat call from the time that the burner turns on until the time the boiler actually starts steaming.  This might amount to ten minutes of each heating cycle.   (When the Pressuretrol is cycling the burner, the boiler is still steaming.)   If you can limit the number of times that the thermostat calls for heat, you can reduce the time that the burner is on and no steam is being produced. 

    By limiting the number of heat calls per hour, or by adjusting the stat to allow a wider temperature swing to accomplish the same thing, you will reduce the number of times when the burner comes on, and reduce the overall time that the burner is on and NOT producing any steam.
  • MrDvorak
    Not sure

    But using the same logic, a boiler firing twice an hour would probably begin steaming quite faster, too. I guess it all depends on the losses in the system, no? My naive understanding was that the heavy iron radiators are heating air long minutes after the steam has gone; and that the longer cycles compensate for this heating momentum. On top of that, you probably want to minimize the number of heating cycles the boiler goes through to make it live longer. I find the search for an equilibrium and optimization in the steam systems very fascinating. Time to get one of them data loggers...
  • Big-Al_2
    Big-Al_2 Member Posts: 263

    You do have a point.  The shorter the time between heat calls, the shorter the time the boiler cools off, and the shorter the time it should take to make steam. 

    The hotter the boiler, the faster it will lose heat though.  I'd wager that a lot of the cooling down takes place quickly after the burner stops, and that more frequent heat calls would lead to overall more dead time before steaming. . . . but then again as the boiler cools off, it does lose heat into the conditioned space, so not all of that heat is lost to begin with , , , oh my brain hurts!
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