Problem getting power to my thermostat


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Old 02-16-19, 06:52 PM
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Problem getting power to my thermostat

Just installed a new Honeywell WiFi Thermostat - my third - first two went perfectly. This one, not so well.

First, I had no C-wire so I used the blue one twisted around the sheath and did the same at the PCB in my Carrier gas furnace. I'm not getting power to the Thermostat. Switched to the spare brown wire at both sides and still no joy. Not sure where to go. Shouldn't the thermostat power up now?

FWIW, I switched the thermo with a known working one on a different furnace (but not the mounting wallplate) and it worked fine, so I don't think it's the thermo.

Here's a pic of the PCB.
 
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Old 02-16-19, 07:35 PM
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Welcome to the forums.

Did you turn the power off when changing the wiring ?
In your picture..... towards the bottom is an ATC-3 fuse. It's probably blown.
Replace it with a 3A fuse only.
 
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Old 02-16-19, 09:05 PM
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Is the light lit on the board? Is there 24V power at the board?
 
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Old 02-16-19, 09:24 PM
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You say this is the third thermostat? If in the same house you likely have a zone controller board between the thermostat and the furnace. You need to look at attaching a C wire there.
 
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Old 02-17-19, 03:57 PM
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@PJmax - Thanks for the welcome! I checked the fuse and it looked fine, but I replaced it anyway. Did not fix. Thanks!

@roughneck77 - When I depress the switch that the cover closes, a light does come on. As to 24V power - I presume so. Old non-C-wire thermostat works fine. Hope I understand you. Thanks!

@ Astuff - No, there is no zone controller. There are upstairs and downstairs thermostats and moms house. The problem one is the downstairs one, which, BTW, has no AC. Upstairs does and works like a charm. Both furnaces are Carrier, but upstairs one is much smaller.

@All - Put the RiteTemp thermo back and all works fine, except for it needs batteries to work, again pointing to the c-wire. It's always run on batteries, so nothing seems different.

Could both the Blue and Brown wires be bad?
 
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Old 02-17-19, 05:31 PM
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There will be no voltages at all with the blower door open.

Very doubtful both wires are broken. If you have a voltmeter. Check for 24vAC between R and C.
If you have it there.....check at the other end.
 
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Old 02-17-19, 06:45 PM
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Hi, do you have 24 VAC across Common 24 and R on the PCB ?
Geo
 
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Old 02-18-19, 08:03 AM
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Thanks both. Not sure I know how to use a voltmeter. Do I leave all power on? Blower door open? Wait til furnace is running? Set voltmeter to 200VAC? Voltmeter wires are red and black - does it matter which I use to measure?

Feel a bit sheepish, but appreciate the help.
 
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Old 02-18-19, 09:12 AM
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Hi, if there is a door switch you can put a piece of tape over it to keep the furnace running, set the meter to the 200VAC, if that is the lowest setting over 24VAC , place 1 lead on Com 24 volts and the other on R ,color makes no difference, you should see 24 VAC +/-.
Geo
 
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Old 02-18-19, 09:39 AM
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Thanks, Geo.

When I did this at the Thermo, I got 1.1V which ticked down by .1V every second. Wen I did it at the Furnace, it said 26.5V and was steady.

Does this mean my both wires are indeed bad?

I'm going to switch back to the Blue wires right now.
 
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Old 02-18-19, 09:47 AM
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Hi,where are you testing at the Tstat? Which Honeywell Stat do you have?
Geo
 
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Old 02-18-19, 09:59 AM
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I'm testing at the Rh and C terminal lugs on the tstat.

Right now, I have the original RiteTemp installed. It runs on batteries, but has a C-wire. If I pull the batteries the display dims immediately.

I changed to the Blue wire - same thing.

Now I have both Blue and Brown wires connected at the Furnace but only Blue at the RiteTemp.

I'm wondering, since I have no AC, can I try using the Yellow wire for C?

Or... like @Astuff brought up before, I wonder if there might be something between the thermo and the furnace PCB?

Could both wires really be bad?
 
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Old 02-18-19, 10:30 AM
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You could try using Y as the C wire. Pretty strange the other ones are not working.
That would indicate a possible splice in the thermostat cable.
 
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Old 02-18-19, 11:06 AM
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Hi, try this, take the blue and brown wires off the PCB and remove them at the Tstat, wirenut them together at on end, with your meter set to Ohms , place one lead on the Blue and one lead on the Brown, you should have continuity between them, if not there is a break or open somewhere.
Geo
 
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Old 02-18-19, 11:19 AM
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Brilliant. I will...

... no continuity. Must be bad wires - probably both.

It's not likely that I could pull those wires, Is it? They're probably stapled to a 2x4 somewhere.
 
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Old 02-18-19, 11:27 AM
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Those wires are most likely in cables, can you trace them and see where they run? You could use the yellow to test with and see if you get 24VAC at the Tstat, Connect it to Com24VAC and test between C & R
Geo
 
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Old 02-18-19, 01:33 PM
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Uncle. I guess I'm stuck without a C wire. I'll return the Honeywell. Alas!
 
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Old 02-18-19, 01:49 PM
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Likely there is a splice that you haven't found yet.

Since you don't have a/c with this thermostat then you should be able to use the yellow wire as a C. This presumes there is nothing else in the middle other than a splice.
 
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Old 02-21-19, 06:09 PM
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Well, I now have a third working thermostat. The Yellow wire did the trick. Still surprised that the Brown and Blue wires were a fail. If there is a splice, it's behind drywall, and I'm not up to that task.

One odd thing, that wire was taped to the side of the furnace with foil tape (which is no longer adhesive and is falling off). Like an RF protection thingy. Has anyone seen that before?

Thank you @Astuff, @PJmax, @roughneck77, and @Geochurchi.
 
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Old 02-21-19, 06:28 PM
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The thermostat cable was taped to the side of the furnace ?
Yes.... that's pretty common. The foil tape usually lasts a long time.
 
 

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