Hi,

I was trying to replace my old stat with a new programmble one. I had turned off the furnace before installing new one. But the new one didn't work and so, I put the old one back on. But the old one does not power now and display on the old stat does not come up. Can any one help me here what to look for? And why isn't getting power?

The old stat (Weathermaster) does not have batteries and has 5 lines
Red wire -> R
Green wire -> G
White wire -> W
Yellow wire -> Y
Black Wire -> C

Any help would be reallya appreciated.

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. It may be wired to have the control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Apparently yours uses a black wire as the common in place of the blue.

You should have 24 volts AC between the red and black wires. If you have no other way to check it, a small regular bulb may at least give a dim glow. Touching the red and green wires together should kick on the fan. If you don't seem to have power at the thermostat, look in the furnace for the transformer. Check voltage there, and any fuses. If none at the small red and black wires, check the larger black and white wires connected to the incoming power. If you don't have power on the red and black, but do on the black and white, the transformer is bad. I think Radio Shack has lower prices on them than a heating and cooling place.

If this doesn't help you find your problem, post back with what you found.

Hi,

Thanks for your suggestion. The actual problem was that the fuse on the control board in the furnace had blown and that was preventing 24VAC power supply to the thermostat. When I replaced the fuse, everything worked fine. In fact, I removed the old one and installed the new one ("rite temp") and is working great. Although, I had turned off the furnace before installing new one, I think I had some how shorted wires that might have caused the fuse to blow.

I'm having a problem where the 24v transformer keeps blowing the fuse. What can cause that? Bad thermostat? It's a 5amp fuse.

Still in the heating season? The gas valve or blower relay could be drawing too much current. Check their resistance if you can. 14 ohms would be a typical value. Try running the furnace on fan only if you have that setting. If it blows, the problem is not in the gas valve or A/C relay. Check the primary on the transformer. It should be about 10-15 ohms. Look over the wiring. I lost a couple of unfused transformers after chipmunks got in and chewed the insulation off my thermostat wires.

Here is where I am now. I tried another thermostat from home depot fuse still blows. I'm in Florida, no heating needed here. It worked for a few minutes then fuse blew. This whole transformer thing is on a board inside the blower. Can the board be swaped or bought somewhere? What causing it to blow the fuse? Also when I Ohm out the therom I get a short on C & Grn, witch is the fan relay. Is this Right?

The coil in the relay should have enough resistance not to look like a short, maybe 10 ohms. Disconnect it, and if it still shows a short, you have found the problem. If you can't find a new relay anywhere else, Radio Shack should have one that works. Check the coil voltage and the contact rating.

I haven't checked yet, but the wire I'm chasing is leading outside to the comprssor area. Is this where the fan relay is located. When I lifted both green wired from the 24V transformer it did not blow the fuse. The meter still showed a short with 1.6 ohms between C and Green. Am I on the right path?

Update: Sun came up, I went outside and checked the wiring. The Green wire at the compressor is just wire nutted. It doesn't connect to anything. So there was some grass and other stuff in there so I blew it out. Now the reading on the ohm meter is 1.9K. So I cut the fan on and it worked. Now I'm going to put the cover back on the compressor and see if it holds.

I haven't checked yet, but the wire I'm chasing is leading outside to the comprssor area. Is this where the fan relay is located. When I lifted both green wired from the 24V transformer it did not blow the fuse. The meter still showed a short with 1.6 ohms between C and Green. Am I on the right path?
... the math: 24V/1.6Ohms = 15A and that the fuse stops blowing, it looks as though you are onto something. ;)

Labman can add the details and expertise, but chances are these wires will lead you to the fault.

Well Air has been running for hours. Seems like it's holding. Could have been a short in the condenser area, where a blow out grass and stuff. Thanks everyone for your input.

Hello...

I have a similar problem to the one that started this discussion. I purchased a Robertshaw programmable thermostat, model 9851i, and have attempted to hook it up twice to no avail. I have a 4 wire system and have hooked up the wires as follows:

White --> W1
Yellow --> Y1
Red --> R
Green --> G

I was replacing a Honeywell MagicStat/32 that is 12 years old. The wires were connected in the same fashion (white to white, yellow to yellow, green to green), but the red was fixed to an Rc screw that had a jumper to an R screw.

I've looked at the unit in the attic and confirmed that the colors connected at the unit match what was connected to both the new and old stats. However, the display on the Robertshaw unit when connected and powered up is blank. If I disconnect the new stat and reconnect the old one, all is OK (old stat works fine).

Any suggestions, comments?

Keith

P.S. - A .pdf file showing the install guide can be found here: 9851i Installation Manual (http://www.icca.invensys.com/manuals/Robertshaw/111-340.pdf)

Refer to some of my earlier posts for background info. The thermostat must be connected to both wires from the transformer to draw power for itself. In some cases, one side comes from jumpering the R and RC. You must run a fifth wire from the other side to the C terminal on the stat. Check your furnace for a C or B terminal. Another way to identify the terminal you need is to look at the A/C control wires. One will be connected to the Y terminal, and the other to the terminal you must connect to the C terminal on the stat.

Hi,
Please help me. My upstairs thermostat has no power. I have a Honeywell thermostat with 5 wires: W=white, Y=yellow, G=green, R=red, C=blue. I purchased a new Honeywell thermostat, but still not have power. I then unhooked the old one and wired it to the one downstairs and it worked fine. I use the thermostat from downstairs to plug into the upstairs and still, no power. I concluded that it's not the thermostat. Please advised me what look for in this case. Thanks

Some of the digital thermostats need to have both the red and blue wire connected to the control transformer. If the upper one doesn't power, it is not connected to the transformer. It could also run off a second transformer that isn't working. You just need to trace the red and blue wires to it, back to the furnace, making sure everything is connected. If you have a meter, you can check for 24 volts AC between the red and blue.

The thermostat is connected to switch the power from the red to the white, yellow, and green wires as needed with the blue completing the circuit...



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Thanks labman, I can't believe a 6 years old answer resolved my issue too. It was the fuse.

Sunday night, October 30th, 2011. I had the same issues as Charyaskme under almost the same circumstances. It was (in my case) a 3 volt automobile type fuse on the circuit board in the Frigidaire heater. All I had for a replacement was a 5 volt, but as it's going to get into the 20s tonight I decided to take the chance of a 2 volt difference and use it. Breaker off, thermostat off and installed the fuse. My wife turned on the breaker and I saw the red status light illuminate on the circuit board. She then turned the thermostat on and set the temperature higher than the inside temp (which was down to 58 at this point). The blower came on immediately, followed a few seconds later by the automatic relight of the pilot light. A few moments of heating the tubes and the manifold opened to let the hot air into the distribution system.

I will change the 5 volt fuse out tomorrow for a 3 volt, but we'll sleep warm tonight. Thanks for this forum. It saved me a cold night tonight and a service call tomorrow.

And by the way, please change 5 volt and 3 volt to 5 amp and 3 amp respectively. And it's still working...

I have a new Wifi thermostat - It seems like there is no power to the thermostat. There is on 13.5V in the "C" wire and 27V in the Red and White wires. Can you please help?

I have a new Wifi thermostat - It seems like there is no power to the thermostat. There is on 13.5V in the "C" wire and 27V in the Red and White wires. Can you please help?