Okay, electricians and handymen, explain this. [edit]
by Papa November (2012-12-29 18:41:08)
Edited on 2012-12-29 18:51:17
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I installed a programmable thermostat this morning. Simple enough, even for me: pull out the wires from their connections, and place into the same respective connections on the new thermostat.

Except the furnace hasn't turned on all day. When we pull the batteries, the thermostat display doesn't come up, indicating it's not getting power from the wiring. So we tried hot wiring and bypassing the thermostat by just direct connecting the red power wire to the white furnace wire. Nothing.

The fuse box has been checked repeatedly. Nothing.

A voltmeter shows power at the furnace switch in the basement... but mysteriously, only when the switch is in the off position. That one's a real head scratcher. Especially since the switch had been in the on position with the furnace running just fine, prior to swapping the thermostat.

So, we have power to the furnace when the switch is in the off position, but no power to the thermostat upstairs under any condition. I give up. Thank god for gas fireplaces.

EDIT:

I just realized something. When I went to turn off power to the thermostat at the fuse box, I turned off power to the hallway the thermostat is in... which would have done nothing. I didn't kill power to the furnace itself, from which I assume the thermostat derives its power. Which means I was playing with live wires. Sweet. Is it possible I blew out a fuse in the furnace?


Check for a short
by CndDomer  (2012-12-30 08:35:51)     Delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post

Check there isn't a short in the "R" cable between the jumper and the line. I had what sounds like this very same problem. The jumper was contacting the R line in two places, shorting out the control. With the thermostat set to where it should come on, I just jiggled the wires to find the short and then rewired it.


I cannot speak electrician, but I had the same problem.
by dulac89  (2012-12-29 23:46:55)     Delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post

The issue was the wiring on my system, not the thermostat. After trying as much troubleshooting as I could myself, a professional electrician came in and essentially rewired my air handler to account for the new thermostat.


Is there a power switch (like a light switch) on the furnace
by bigjinx74  (2012-12-29 22:50:20)     cannot delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post

that may have been switched off by accident?


Already replaced it in case that was the problem. *
by Papa November  (2012-12-29 23:17:31)     Delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post


Do you have 24 volt control power at the thermostat?
by Lifetime Irish In TN  (2012-12-29 20:52:26)     cannot delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post

I assume you have a split system, outdoor condensing unit with indoor furnace...there will be a 24 volt control transformer, typically located in the condensing unit wiring compartment, but could be located within the furnace.

The 24 volt control transformer will have protection in the form of fuses or sometimes an integral circuit breaker with built-in reset switch. The first thing I would check is that there is 24 volt control power present. You can check this right at the secondary of the transformer or between the 'hot' and 'common' wires, typically the red and blue wires at the thermostat. Sounds like you have lost 24 volt control power from the transformer, if so, reset the breaker built into the transformer or replace the fuse just ahead of the transformer, whichever you have.

Just a thought.


I do have one wired next to the switch on the furnace.
by Papa November  (2012-12-29 23:18:34)     Delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post

I will check this along with the fuses. Thank you.


It is possible you blew a fuse. I have a heat pump. Didn't
by 730104  (2012-12-29 20:10:38)     Delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post

turn off the power when I installed a new thermostat. After the install, the heat pump didn't work. Turns out, I did blow a fuse in the air handler unit.


I've replaced several of these
by bmef  (2012-12-29 20:00:51)     cannot delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post

If I'm not mistaken, the thermostat is 24 volts. I've never turned the power off to the furnace when changing them. The battery only runs the electronic controls in the unit and doesn't get power from the wiring.

I think it has something to do with the wiring in the new thermostat. I would go back and check the wiring instructions. Thermostats are either 3, 4 or 5 wire. Some require that you either remove a "jumper" from one of the posts to another. Some require the installation of a "jumper." Some even require different colors going to different posts on the back of the thermostat. Check the instructions again and confirm it is hooked up correctly.


I'll take a look.
by Papa November  (2012-12-29 20:09:49)     Delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post

I have a red, white, yellow, green, and blue wire. They were all connected to R, W, Y, G, and C terminals, respectively. The new thermostat had those same terminals. The only difference is that instead of a single R like on the original thermostat, the new one has an Rc and Rh, with a jumper connecting the two. The instructions indicated that if you only have a single red wire from the house, which is the case, that you leave the jumper in. That would make sense in order to supply power to both the furnace and the A/C.

I have the single red house wire going into Rh, with the jumper still connected to Rc.


A couple things
by yooper  (2012-12-29 19:54:35)     cannot delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post

The two most likely things are that you hooked it up wrong or the thermostat is bad. Have you tried putting the old thermostat back to see if it still works? That could at least eliminate any potential problems outside of the thermostat itself.

As far as I can tell most of your troubleshooting isn't really showing much. When you pull the batteries, the thermostat display probably should go dead, as the batteries are what actually powers the thermostat in most cases (not the wires from the furnace). Also, if you're checking voltage across a switch, it makes perfect sense to have voltage across an open switch and no voltage across a closed switch. And those live wires you're playing with are most likely low voltage and not really all that dangerous. But don't put your tongue on them or anything.


Yes, we tried the old thermostat. And a 3rd to be safe.
by Papa November  (2012-12-29 20:06:08)     cannot delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post

So that's not the problem. And by pulling the batteries, I meant to imply that's the only reason it looked like it was ever getting power. The original thermostat did not have batteries (although it had available battery slots), so it was obviously deriving power from the wiring. I think that's what the blue wire (labeled "constant") was for?

Do you think the voltage of those live wires would be enough to blow a fuse?


Voltage is irrelevant, it's current that blows a fuse.
by Mr Wednesday  (2012-12-29 21:15:52)     Delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post

Your car is only a 12 V power supply, but it has lots of fuses and an electrical system that can handle some pretty serious current.


The issue is
by LondonDomer  (2012-12-29 19:10:15)     cannot delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post

In your first line..."I installed..."

You screwed it up.


Same thing I thought as soon as I read it. *
by LocalSubAlum  (2012-12-29 21:59:16)     cannot delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post


I thought the same thing before attempting it. *
by Papa November  (2012-12-30 00:29:04)     Delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post


Well, obviously.
by Papa November  (2012-12-29 19:17:24)     cannot delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post

But how do I FIX it?


Take it back, play stupid and say it doesn't work and...
by NicevilleFlIrish  (2012-12-30 00:43:57)     Delete  |  Edit  |  Return to Board  |  Ignore Poster   |   Highlight Poster  |   Reply to Post

have someone who knows what their doing install it.


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