I don't know about furnaces, but for reverse cycle A/C, the thermostat measures the temp where it is located, then communicates with the compressor wherever that is located to tell it when to run and when to shut off.

I'm not sure why this would be different for a furnace--you care about the temp where you are, not where the machinery is.

Thanks. I'd like to know this because my furnace is running more than usual. The only thing that has changed is the basement door (the new door has a slight draft). It doesn't affect the living area, but it does affect the furnace area. If the thermometer is located near the furnace, that would explain why it's running more.

Thermostats generally measure temperature at their actual locations. There are somewhat newer thermostats that have the capability of measuring temperature in a different location. That control temperature is measured at a location representative of the spaces being conditioned - that would not be the furnace's location!

If the new basement door has a "slight draft" that certainly could effect the temperature in the upper living spaces.

Fix the "slight draft!"

As salti said, unless the basement is totally isolated from the rest of the house, then the drafty basement door is definitely affecting the indoor temperature.

furnace = hot air; boiler = steam or hot water.

A steam system is different from a hot water system. And steam systems come in single pipe and double pipe configurations.

A hot water system will have an air bleed at the *top* of each cast iron radiator (opened with a type of hex key). A steam system will have a steam/condensate valve toward the *bottom* of each cast iron radiator.

Newer radiators (the kind w/ alum. fins) don't have these "bleeds".

This has absolutely nothing to do with the OP's question.

SaltiDawg - perhaps you have never had a house with an two-pipe cast iron hot water radiators. The OP is apparently concerned about where heat is measured and how it controls the boiler.

If radiators in one part of the house have not been bled you may get very uneven heat. If the radiators that work well are near the thermostat, it will heat that room up, and the thermostat will cycle off, but the heat will never reach rooms with air-locked radiators. This may lead someone to believe the thermostat is not working correctly.

Bruce

This also has nothing to do with the OP's question.

" I have a programmable thermostat upstairs, but I am trying to find out where it measures the temperature."