Does this thermostat/furnace wiring make sense?

[jwmarsha]

Hello,

Hoping I am positing this in the appropriate forum - please see the image below:

https://imgur.com/upWbb6g

Thermostat is a Honeywell TH4110D and furnace is a late 1980's installed Lennox G8D2.  What is confusing to me is why the "R" and "Rc" terminals are bridged at the thermostat, and if they should be bridged in this configuration.  I ask because I am interested in upgrading to a smart thermostat ("C" is available through an unused wire in the bundle).

https://imgur.com/SNkmxpK

https://imgur.com/a/MHlaNdQ

Any help is appreciated, thanks.

[slo-115]

Make sure you test the high limit with the thermostat set to "fan on" It can sometimes backfeed 24v through the y and energize the Rh. Its best to have the two 24volt feeds separate......or a whole new heating system.

[Admin]

I found this Lennox schematic that seems to show what you're describing.

I agree it's time to replace that furnace. It was common over 10 years ago to find a crack in that heat exchanger. 

Here's an older G8 schematic that shows how to connect a Nest.  The fan center you have should work the same way.

[jwmarsha]

I opened it up again and did some further tracing - there appears to be a second transformer in the lower furnace assembly, a Honeywell AT20B10072:

https://imgur.com/aNbyZ0m

https://imgur.com/a/mnpPED6

Does this make more sense now? Still don't know why they'd be bridged at the thermostat.

[slo-115]

That is quite scary. That little black transformer should be wired into the combination high limit control. Use that 24volt source as your RH.  The 24 volts in the fan centee relay sholkd be used as your Rc. Even if the fan centre relay is also being powered through the high limit, it can backfeed power through the 'G" termial when the fan is energized. Most likely the fan center relay is wired direct to power and you may not have any high limit protection. I would start by separating Rc and Rh. Call for heat and test the high limit

[jwmarsha]

Thanks - what is the procedure for testing the high limit?

[walker]

Thanks - what is the procedure for testing the high limit?

Pull the neutral from the blower off. Call for heat.  Watch the fan limit when it gets passed it set point, the flames should go out.  If it doesn't you effectively have no safety limit switch in operation. 

[slo-115]

Or with something that old you may be able to remove the fan belt

[Frosty]

Lennox G8 furnace was never a belt drive!

[NoDIY]

why are you still using a 1980's low efficiency furnace?   

[walker]

why are you still using a 1980's low efficiency furnace?   

Must be a hipster.