An odd HVAC problem. Help!

willness33

willness33

Here for the memes
Joined
May 3, 2005
Location
Alexis,NC
Alright, this is a stumper. I installed a new programmable t-stat last night. It was wired correctly, but the furnace was acting odd on shutdown at the end of the heat cycle. Then it started blowing fuses. So what did I do? I put in a bigger fuse. Yeah, I know. So, because of that I blew out the low voltage transformer. New one is in and the old thermostat is back on. The furnace heats fine, but still with the odd shutdown. It keeps kicking that blower fan back on a few times after the heat exchanger is sufficiently cooled. It's throwing a code for me as well. It's saying "limit circuit open or external load on 'W'". Ok, I checked the limit switches while it was doing it odd thing while the code was flashing. THis is on the cooldown cycle. All of the switches had good continuity. That means they are not open and that's not the problem. So, what I checked next was the voltages on the different leads, while the furnace is not running. 27 on red, 0 on green or yellow, and here's the odd part, 5V on the white wire. I thought the white wire was supposed to show 0V to ground when the furnace isn't running. Someone please confirm or deny my knowledge and set me straight. IMO, the 5V on the white would possibly be the external load on the W? Any info would be very helpful as it's going to be another cold night tonight!
 
what leads are you checking? there are a ton of wires in there and if you are checking inside the unit, the 5V could be for anything...
 
I'm checking the thermostat leads inside the furnace. The R has 25V (normal). The W has 5V. Is that normal, or should it be 0V to ground when the furnace is not running? Like I said, I'm getting an code from the board that says limit circuit open or external load on W. I checked the limit switches while the code was flashing and the limit switches all have good continuity. That leads me to believe the other issue "external load on W" is what's causing the code.
 
do you have an outside thermostat? what unit do you have? packaged or what? how old is it?

wiring for the "W" wire is somewhat different for different units.
 
Split system. No outside t-stat, inside only. Gas furnace inside, a/c condensor outside. Tappan. 4 years old. It's one of those that uses a PVC pipe for intake and exhaust. The W wire only goes to the t-stat and activates the control board to fire the burner, turn on the exhaust fan and turn on the blower fan when the heat exchanger it the right temp. If I jump the R to the W, it starts the whole heat cycle. I'm just not sure why I have an error code other than the W wire having 5V. I'm under the assumption that it should read 0V to ground when not running and 24V to ground when running. The R wire is the hot for the t'stat. When the t-stat kicks on, it closes a switch form the R to the W, powering up the board for a heat cycle. Is this making any sense? It does in my head.
 
No brown. The only wires are red, green, yellow, and white for the heat on the control board. There's another yellow and a black for the A/C.
 
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