I have to disagree with the comments that have been made so far that your old thermostat "didn't run on anything" and was STRICTLY mechancial. That's simply not correct. Your old round Honeywell was PRIMARILY mechanical, but also used the 24 VAC power that it switched on and off in a tiny electric heater called a "heat anticipator heater" which was positioned next to the bi-metallic spring.

Basically, the old Honeywell consisted of a bimetallic spring that held a mercury switch on the end. That bimetallic spring would wind up and unwind depending on the indoor temperature, thereby making and breaking the circuit between the 120/24 VAC transformer in the boiler/furnace and the boiler furnace control to turn the boiler/furnace on or off.

But, wait, there's more...

In that old round Honeywell, there was an small electric heater that operated off of 24 VAC power that the thermostat switched on or off called a "heat anticipator heater".

Since every heating system is different, every thermostat in every house requires a different heat anticipator setting. Basically, it's the heat anticipator's job to heat the bi-metallic spring, thereby fooling the thermostat into thinking the room is warmer than it really is, and that results in the thermostat shutting the furnace or boiler off a little bit earlier, or a lot earlier, than it otherwise would, and I'll explain why.

Essentially, the heat anticipator is little more than a rheostat resistor whereby you can vary the length of wire the 24VAC power goes through, thereby changing the resistance of the resistor and thereby changing the amount of heat it generates. That resistor is physically located close to the bi-metallic spring so that the heat it generates warms the bimetallic spring causing the heating system to shut off earlier than it otherwise would.

That's because if that thermostat is being used on electric heat, then once the thermostat feels that the correct temperature in the room has been reached, then since the thermostat is centrally located in the room and the electric baseboard heaters are all located around the perimeter of the house, the AVERAGE temperature in the house has actually overshot the temperature setting on the thermostat. So, it woulda been better to shut the heating system off a bit earlier so that the AVERAGE temperature in the house is at the thermostat setting, not above it.

Similarily, if the thermostat is being used on a hot water heating system, then once those radiators get filled with 180 degree water, they're going to be convecting heat into the room for a long time after the boiler shuts off, so in that case you want a much higher heat anticipator setting on the thermostat.

And finally, in the case of a forced air heating system, when the thermostat detects the correct temperature in the room, the heat exchanger in the furnace is still hot, and it would make sense to keep the fan blowing until you get the heat from that heat exchanger into the occupied space, so again you want to shut the furnace off a bit early. The furnace has it's own switch that will keep the fan running until the heat exchanger in the furnace cools down.

Now, mechanical thermostats use a 24VAC heater to accomplish heat anticipation for different kinds of heating systems, but electronic ones do all of that electronically. It could be that the heat anticipator setting in your thermostat is way high to simulate the situation where the thermostat heats up well before the room is warm. In that case, your thermostat is shutting the heating system off way too early, and you just need to change that heat anticipator setting parameter.

So, try replacing your batteries.


If that doesn't solve the problem with your thermostat, then download the original documentation that came with that thermostat off the manufacturer's web site and see what it says about "Heat Anticipation" in the thermostat's set-up instructions. Or, do what most people do, which is to just tinker with the heat anticipator setting until the thermostat works well.