Wiring diagram for furnace relay & thermostat common wire

Users who are viewing this thread

Theodore

Member
Messages
127
Reaction score
4
Points
18
Location
New York
Hi,
I'm adding a modern thermostat to my home's furnace (currently has mercury tstat). The thermostat and furnace is for heat only, so I wired a 7 conductor cable (more than enough) to it. I bought the 24V transformer and the relay. I know I have to create and power a common wire. I've been trying to follow this video:
because my furnace's two "T" wire connections are the same as here, but the connections are a little confusing. Is there such a thing as a "typical" wiring diagram for adding a new relay, so I can study it up close?
Many thanks
Theodore?
 

Reach4

Well-Known Member
Messages
38,749
Reaction score
4,400
Points
113
Location
IL
Strange, I can't seem to see a diagram on that page. It suggests there's one, but no image. -If- you have time, is it possible for your to upload diagram here?
Try a different browser.
 

Theodore

Member
Messages
127
Reaction score
4
Points
18
Location
New York
Hmm, I tried Chrome, IE, and even my iphone. no images shown where it says "You'll wire it up something like this: "
How strange.

Capture.JPG
 

Jadnashua

Retired Defense Industry Engineer xxx
Messages
32,771
Reaction score
1,191
Points
113
Location
New England
Somewhere in the furnace is very likely a 24vac transformer...right now, one lead has the power coming from it to your thermostat...when the thermostat closes, it applies that voltage to the internal controls on the furnace (often a relay) and on the other side of the relay coil is the other side of the 24vac transformer.

So, all you need to do is find that relay or the 24vac transformer in the furnace, and attach one of the leads to your new cable to it, and now you'll have power to energize your new thermostat. So, the 'red' would have one side of the 24vac transformer in the furnace, the new lead would have the other side of the transformer on it providing power to your device, and when it calls for heat, it would apply that power on the red terminal as well back to the furnace to turn it on.

This should work on most systems unless they use line-voltage thermostats or millivolt control (not absolutely positive on that, though). If yours uses 24vac for control...you do not need any extra transformer unless the new thermostat has a very significant load that would overload the furnace's transformer (that would be rare, and if required, you could just swap that transformer with a higher capacity one). Most furnaces have enough extra capacity on their 24vac control circuits to run things like an a/c unit and maybe a humidifier, so a thermostat is a piece of cake.

The fewer parts you add, the simpler things are, and the less chances for complications later on when trying to troubleshoot it.
 

Theodore

Member
Messages
127
Reaction score
4
Points
18
Location
New York
Hi Jim,
I thought the same, and was advised by the guy who cleans the furance and my HVAC-savvy neighbor that I cannot run anything else off of the existing transformer and I must add an isolation relay and new transformer to power a modern thermostat. It's def an older model, almost exactly the same look as in the video link above. And since that video also explains about the need for an isolation relay, here I am.

To others: thanks for link to photo and for your patience. I figured out it was my lousy corporate firewall that suppressed the image, thinking that I was looking at HVAC pictures or something. Works fine from home.
 

Jadnashua

Retired Defense Industry Engineer xxx
Messages
32,771
Reaction score
1,191
Points
113
Location
New England
If you have the manual, in the parts list, see if it has the specs for the transformer. Also, see if you can find out how much power the thermostat requires. I'd be very surprised if there was not excess power available on the transformer. If not, it should be pretty simple to swap the transformer in the furnace for a higher output device. I know I had to do that to power a new electronic doorbell - took all of about 10-minutes. The builder used the smallest one he could find, and it didn't have enough oomph to power the new one, but that was easily solved. In a simple furnace, it is probably only powering maybe two relays or contactors: one for the burner control circuits, and maybe one to turn the fan on. Neither one is likely much power. The thermostat won't take much.

FWIW, my air handler is powering it's internal controls, an electronic (Nest) thermostat, a humidifier, and the a/c compressor unit's controls - way more than a simple thermostat would require. Not saying yours can't but I'd be surprised if it couldn't.

But, assuming you do use an isolation relay, move the red and white wires from the old thermostat to the normally open contacts (common and NO) of the isolation relay (opposite sides of the switching contacts). Then, from the new 24vac transformer's secondary, run the hot and return to the power contacts of the new thermostat. Run the return lead also to the coil of the isolation relay, and run a wire from the white contact on the thermostat to the other side of the coil of the isolation relay. Essentially, powering the thermostat from the new transformer, and using it's switching to control the coil of the isolation relay, while the relay contacts replace those in the thermostat for controlling the furnace..
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks