Home Heating Thermostat Question

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 ?
And that there is a 24 transformer inside this control box, as well as a relay to control the 110V Circ..
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 ?
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.
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.
Thanks, Bob
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On Thu, 19 Apr 2018 13:53:48 -0700 (PDT), Robert11

It just pops out of the wall bracket - maybe.
I just had to replace my 8 year old thermostat - - the touch-screen was failing to adjust certain settings - including the set-temperarture - it was stuck on 64 F. degrees. I discovered the 3 AAA batteries had corrosion - no doubt after being neglected for 8 years ! The other little 3 volt battery that was well-hidden - seemed fine. I replaced it in an attempt to reset / revive the unit - to no avail. I'm not positive that the cause was the batteries but I can't think of anything else to blame ..
https://yourhome.honeywell.com/en/products/thermostat/7-day-programmable-thermostat-rth8500d
John T.
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On Thu, 19 Apr 2018 17:35:32 -0400, snipped-for-privacy@ccanoemail.ca wrote:

Just pull the faceplate straight off. It is held on by pins going into a socket. Note indents at top and bottom to put your fingers in. Once you have the model, the manuals are available on the Honeywell website.
Don. www.donwiss.com (e-mail link at home page bottom).
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Any smokers in the house??? Will kill the pressure contacts between the screen and the board for sure.
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On Thursday, April 19, 2018 at 4:53:53 PM UTC-4, Robert11 wrote:

Yes.

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

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.

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.

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.
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On Thu, 19 Apr 2018 13:53:48 -0700 (PDT), Robert11

Not sure how YOURS works, but mine runs off the 24 volt transformer - the battery just acts as a backup for the clock and timer memory.
Youare right in what it does otherwize - not physical contacts on mine but solid state switches
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