Temp Stays Higher Than Thermostat -- Radiant Hydronic Heating

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I just bought a house with in-floor hydronic radiant heat, gas boiler. It's my first experience with this type of system.

There are 5 thermostats (Honeywell T8400B1018) throughout the house. 3 are in bedrooms we don't normally use. I set those thermostats to 55 degrees and close the bedroom doors. In one bedroom, the temperature drops to 55 -- good.

But the temperature in the other 2 rooms stays around 65-70 degrees, and the floor is obviously warm. So it seems like heat is still circulating. I tried swapping thermostats between the 55 degree room and one of the 70 degree rooms, but no change. I tried turning off the thermostats in the 70 degree rooms, but no change. So I don't think the thermostats are the problem.

Any thoughts on what would cause this behavior?

Thanks for your time!

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Using this type of heat is a great way of keeping the house comfortable. However its adjustments can be a bit tricky to make it work as it should.

There are two things besides the thermostat control that maintains the temperature in the system.  But seeing that you swapped out the thermostat this rules out the heat anticipator within the thermostat itself.

The other is the water temp control at the boiler.  On most radiant systems there is a manifold which distributes the water to each zone.  This manifold should have on it a mixing valve. This valve controls the output water temperature going out the the zone in question.  If the water is too hot it will overheat the room quite quickly causing it to run much warmer then the setting on the thermostat control.  It causes large temp swings as a result. 

Also if the boiler temperature is set to high this will also cause all sorts of issues.  Depending on design the outlet water to the zone should be somewhere between 80 and 125 degrees max.  This temp is determined by floor type, type and amount of pipes in the zone.  But if the boiler is running at 180 degrees which is a very common temp to run at, the controller that turns the water temp down is faulty and should be looked at.

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