HVAC Thermostat help

Factual Questions
1

Heard the smartest people in the world reside here. I suppose if someone can solve this problem that would be worth a subscription to the service.

My roommate just hooked up a digital thermostat. His heart was in the right place but apparently his wiring skills are lacking. Unfortunately I was not there when he did this so I cannot confirm the way the wires were hooked up on the old thermostat.

Symptoms: Fan will not come on in auto mode but it sounds like the compressor may be coming on. I looked in the outside unit last night with it in auto mode and saw something glowing inside (actually something’s.) There is a main switch to the outside unit that has some sort of breaker/fuse that completes the circuit to power the unit. It is a gray plastic square cap looking thing with flat copper terminals that are longer on one end and shorter on the other end. You pull it out and either flip the short ends up and plug it back in (which I assume is off?) or the long ends up (on I assume?). It has a wire on it that is used to pull it out of the slots it is plugged into (a handle I suppose) and thus terminating the power. I say this because if I have the thermostat fan in auto mode, I can hear what I assume is the compressor running (or perhaps trying to) but the fan/blower does not come on. If I switch the thermostat fan control to ON it does not come on unless I remove the fuse/breaker (whatever it’s called) and plug it back in (not flipping it just unplug and plug it back in). I can hear the thermostat click if I set the temperature to lower then the room temperature, but the outside unit blower/fan does not kick in unless I have the thermostat fan setting in the ON position.

I suspect it has to do with the way my roommate wired the new thermostat. Something is either backwards or not connected. There were several options in the manual for hooking up the wiring. Options for 2, 3, 4 & 5 wires, and what terminals they needed to be connected to. My roommate has it wired for the 4 wire option. I suspect this is the problem and I will explain why.

It is wired as such:

Wire: | red | blue | brown | green |
________________________
Terminal: | rc* | rh | w | y | g | * rc jumped from rh
Here is what I suspect the problem is: On the old thermostat there is a yellow wire connected to the Y terminal and then riveted to the back of the circuit board/thermostat (i.e. the wire does not come out of the wall with the other wires that were connected to the old thermostat). So, if it should be connected using the 5 wire schema then the RC-RH should not be jumped. But there is no contact on the back of the new thermostat to hook up the Y terminal. Perhaps I should remove the yellow wire and connect it to the Y terminal on the new thermostat and somehow ground it?? Then what do I do with the wire he hooked up to the Y terminal? It is hard for me to imagine that there were 2 wires hooked up to the Y terminal on the old thermostat. Especially considering that the yellow wire still attached does not look as if the screw has been loosened (the other terminals he disconnected wires from are still loose).

Anyone?

2

Here is a link to a thermostat wiring guide which may answer some of your questions.

3

I see my formating was destroyed on submit.

Here is the wiring:

red wire to ----> RH terminal RC jumped from RH
blue wire to —> W terminal
brown wire to–> Y terminal
Green wire to–> G terminal

4

You’ll need to find the other end of the thermostat cable at the furnace and see how things are at that end - ie: blue wire is connected to W, Y, etc? and so forth. Then, you ought to be able to set things right at the thermostat end.

I say this from experience - just because there’s a common usage of color-coding in thermostat wiring, there’s no guarantee that the installer actually followed it however many years ago.

5

I am an HVAC Contractor in Houston, but I will say I second gotpasswords’ suggestion and check the bus at the furnace to see what the wires are there.

If you cannot find the bus on the furnace try this:

Red —> RC—RH
Blue ----> Y
Brown —> W
Green —> G

Normal installation techniques put the Blue wire on Y, unless you have a heat pump. Red almost always goes to RC. I just assume that brown goes to W.

Is the brown wire a dirty white wire?

6

Thanks for the replies. I did exactly as suggested and found in the box that Red does go to RH/R Green to G, White to W (looked brown in the house but it is just a very dirty wire) and Blue to Y. There was a yellow wire connected to the blue outside with a big tag on it that said connect the yellow wire on the thermostat to Y. Actually it’s blue on the thermostat side.

So the good part is now it works. But it is working to good. It will not shut off unless I go outside and remove or turn the breaker to off. Even with the thermostat in the off position it stays on.

I think I forgot to put the jumper wire back on. It jumps the RH to the RC.

Could this cause the new problem I am having? I’m at work now and just realized that I forgot to put the jumper back on.

7

That could cause the problem. Try putting the jumper back into place, and let me know if it works.

8

My mistake. The jumper wire was connected. Still can’t get the unit to shut off. I found out last night from my roommate that this was also the reason he replaced the thermostat in the first place. He drives a truck and is out of town a lot so I had no idea about the problem.

Any other thoughts about why it will not shut off after hitting the target temp?

Thanks.

9

It might be a stuck contactor. At this point, I would get a proffessional out to take a look at it.

Only so much I can do with words, eh?

10

Coming from a professional such as yourself, Rob, I have to agree. Oh well, we’re only renting so I suppose the landlord can pick up the tab.

11

Also, I would keep the system off until someone comes and looks at it. Especially if the contactor is welded shut.