Funny thermostat problem

Frank Cheng

I encountered 3 Honeywell thermostats that had the same funny problem. The thermostats would heat themselves up and cause turn-off in about 1 - 2 minutes after turn-on. It seems that the heat was caused by the mercury contact in the bulb and the heat cause the temperature sensing coil to expand like a rise in the room temperature. They are all round simple type of thermostat of 1-3 years old. I don't know how to fix them. I replaced them all with regutangular ones and got rid of the problem.

Does anyone have the same problem and know what is going on with this products?

Steamhead

Were the anticipators

in the round stats set to match the thermostat current of the relay or valve being operated?

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JimGPE_3

Heat anticipators?

Honeywell T87 thermostats have heat anticipators. These are small resistance heaters located behind the sensing element that begin to heat up when the stat calls for heat. They artificially heat the sensing element so that the thermostat loses its call before the room heats all the way up to set point. That way, the residual heat in the boiler/furnace will heat the space just up to the setpoint, not drive it past setpoint which would happen if the stat lost its call when the room hit setpoint.

Okay so far?

Well the anticipator is settable. You are supposed to set it for the amp draw of the control circuit. Set it too high and the room overshoots the set temperature. Set it too low and it undershoots.

I bet the heat anticipator was set too low.

Frank Cheng

I will check the current.

Thanks for the input.

Frank

Frank Cheng

Thanks

I think that must be the answer. Most of my heating is forced water system with outdoor reset control which almost eliminates overheating totally. So, I am going to simply set every one of them to the maximum or near that neighborhood.

Frank