On Thursday, April 19, 2018 at 4:53:53 PM UTC-4, Robert11 wrote:
Hello,

Have a typical Honeywell Thermostat that controls our home's gas forced hot water
heating system.

Can't find the manual for it, or a model number on it.
Model is probably on the back, and I really don't want to take it off of the wall.

It's a rectangular unit, about 2 years old.
Has a digital display showing the Set Temp., and what it senses as the actual temp.
Runs off of two AAA batteries. Nice green backlighting when the button that sets the Set Temp is pushed.

Would like to have a better understanding of how it works.

a. Question: Do these thermostats, when turning on, merely provide, in effect, a contact
closure for the control box mounted on the furnace ?

Yes.




And that there is a 24 transformer inside this control box, as well as a relay to control the 110V Circ..

Yes. The transformer may not be in a box though, it's more likely just
mounted inside the furnace, separate from a control board.





When turning on, therefore, the thermostat just closes its contacts allowing
the voltage from a transformer in this furnace mounted box to then
provide 24 volts to a relay, via the thermostat, also in this control box ?

It could be that simple on an old boiler. Newer ones typically have
electronic controls, with the board connected to the thermostat which
still acts like a switch. The board has the relay or drives the relay
that turns on the circulating pump, fires the boiler, etc.




Is this the way it probably works ?
With only two 1.5V AAA batteries, I cannot see how the thermostat could provide any 24 V.

It doesn't. Some thermostats, eg Nest, actually do the opposite.
In systems that have no wire at the thermostat for power, they steal
power from the 24 volts when there is no call for heat and use it
to keep the rechargable batteries charged.




So, it only closes a set of contacts in its (the thermostat's) case when it is calling for heat ?

b. If the batteries go dead, will the thermostat still function ?
I realize the display won't light up anymore.


That depends on the thermostat. Some have an emergency mechanical thermal
switch that will close if the temp goes below 40F or so, but I think most
don't, so if the batteries go, you're kaput.