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Thermostat for radiant heat (gas fired boiler)

Dave_4 Member Posts: 1,405
the 'stat.

The problem is in "overshoot." The 'stat calls for heat. The boiler and circulator come on. When the 'stat is satisfied, the hot water in the slab and the slab's delivery from the tubing trapped in the pour to its surface (where you finally feel it) takes a while. Despite the boiler and circ. being "off" - the migration from the radiant tube, down 2+ inches from where it will ultimately end up - at the surface where your feet are - takes a while. What you experience is the delayed and continuing heat rise that occurs because of the simple but confusing phenom of the so-called "flywheel" effect.

The solution? Drop the water temperature to the slab to a point where the "overshoot" is minimized. The circulator will run more, but the boiler will run less. This, bodes well for a more even heating experience - which seems to be exactly what your issue is about.

Again, it isn't the stat, it's the flywheel effect at work. Would outdoor reset, floor sensors, or other strategies be helpful? Perhaps. All might do what you may already have access to, namely a device that will lower water temperature.

Got a bypass valve anywhere? Play with it! If that doesn't work, perhaps constant circulation would be a possibility?

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  • Tank Merdock
    Tank Merdock Member Posts: 1
    Thermostat for in floor heat (gas fired boiler)

    I have a very basic in floor radiant heat system with no outdoor reset or slab temp sensor.

    Right now I am running a T87 set at 1 CPH mounted 5ft from the floor on an interior wall. My temp seems overshoot by 4 degrees and I would like to fix that.

    Can anybody recommend an inexpensive two wire thermostat that works pretty well for radiant? Any other ideas?
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