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

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


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/pr...ostat-rth8500d

John T.

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.

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

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
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

On Thu, 19 Apr 2018 17:35:32 -0400, wrote:

On Thu, 19 Apr 2018 13:53:48 -0700 (PDT), Robert11 wrote:
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 just pops out of the wall bracket - maybe.

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).

On 4/19/2018 2:35 PM, wrote:
On Thu, 19 Apr 2018 13:53:48 -0700 (PDT), 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 ?
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


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/pr...ostat-rth8500d

John T.



The touch screen contacts may be failing. You could try pushing and
holding the buttons, and see if they start working after a few seconds.
Or take it apart and see if some part between the screen and the switch
it operates is broken.

On Friday, April 20, 2018 at 12:48:48 PM UTC-4, Bob F wrote:
On 4/19/2018 2:35 PM, wrote:
On Thu, 19 Apr 2018 13:53:48 -0700 (PDT), 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 ?
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


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/pr...ostat-rth8500d

John T.



The touch screen contacts may be failing. You could try pushing and
holding the buttons, and see if they start working after a few seconds.
Or take it apart and see if some part between the screen and the switch
it operates is broken.

Say what? The OP said nothing about any problems with it, they only asked
how it worked.

On Fri, 20 Apr 2018 09:45:08 -0700, Bob F wrote:

On 4/19/2018 2:35 PM, wrote:
On Thu, 19 Apr 2018 13:53:48 -0700 (PDT), 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 ?
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


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/pr...ostat-rth8500d

John T.



The touch screen contacts may be failing. You could try pushing and
holding the buttons, and see if they start working after a few seconds.
Or take it apart and see if some part between the screen and the switch
it operates is broken.
Any smokers in the house??? Will kill the pressure contacts between
the screen and the board for sure.

On 4/20/2018 11:13 AM, trader_4 wrote:
On Friday, April 20, 2018 at 12:48:48 PM UTC-4, Bob F wrote:
On 4/19/2018 2:35 PM, wrote:
On Thu, 19 Apr 2018 13:53:48 -0700 (PDT), 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 ?
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


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/pr...ostat-rth8500d

John T.



The touch screen contacts may be failing. You could try pushing and
holding the buttons, and see if they start working after a few seconds.
Or take it apart and see if some part between the screen and the switch
it operates is broken.

Say what? The OP said nothing about any problems with it, they only asked
how it worked.

I did not reply to the OP.