A/C dead, no 24V power

nam11b · 09-13-2010, 06:17 PM

Alright, a friend of mine works HVAC, but is tied up until at least tomorrow. I am trying to troubleshoot this myself and looking for any help you guys can provide.

I have a 5 ton Carrier that is installed in the attic. When I got up this morning my thermostat was completely dead and the unit is also. I have 2 overflow switches, one at the drain pan and one in the drain tube. Both are dry and not cutting electricity. There is a fuse on the board and I verified continuity with a meter. I have 120 VAC going to the main board, but no 24 volts (DC?) present anywhere on the unit. My red status LED is not blinking and nothing else is working. I have a transformer mounted on a rafter right next to the furnace and I do not show any power coming off of it. I assume that is my 24 volt source. Can anyone verify that makes sense? I am assuming my transformer is shot, but this is amateur troubleshooting at this point. Any advice is greatly appreciated, thanks!

yuri · 09-13-2010, 06:22 PM

The 24 volt AC transformer should be on the mounting plate near the circuit board or nearby. Would not be mounted outside the unit. Post the model and serial # of the airhandler or furnace.

HVAC_NW · 09-13-2010, 07:40 PM

You mentioned DC.

The 24v control is AC. If you have the meter in DC mode, it would not register, even if there is voltage.

**beenthere** · 09-13-2010, 07:53 PM

Quote:

Originally Posted by HVAC_NW

You mentioned DC.

The 24v control is AC. If you have the meter in DC mode, it would not register, even if there is voltage.

What he said, and what Yuri said.

HVAC_NW · 09-13-2010, 08:08 PM

If you touch it, it should be warm to touch. Check for 24v in AC mode. If it isn't present, see if there is fuse.
After checking the fuse and you still find no voltage, de-energize the unit, isolate 24v transformer and measure resistance across 24v side. If it's open circuit, its burned out.

Transformer is quite reliable, so at this point, you need to make a service call. The transformer didn't likely fail. Something killed it most likely and could be something as simple as chaffed thermostat wire shorting out 24v.