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HVAC - not running no 24v power

720 Views 5 Replies 4 Participants Last post by  WyrTwister
I have a package unit on my roof. It stopped running. I have checked all the electrical. 120 (x2) out of 30 amp breaker. 120 (x2) power to disconnect and out of disconnect and slow fuses are good per DMM. 120 power on line, both legs of contactor. Disconnected and bypassed thermostat at board. Nothing. Contactor is new and checks fine with DMM. 120 power through each leg of contactor (load) when depressed. Pushing in contactor does not start unit. 120 to circuit board. 120 to capacitor. 120 to 24v transformer. NO 24v coming out of transformer. There are no manual reset switches and no door switch. There is no status light or fuse on the board. Neither condenser fan or blower will start. Suggestions on what I can check next? Thanks in advance!!
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First, stop checking to ground. Check across the circuit to verify you have the proper line voltage.
If you have the proper primary power to the transformer, but no secondary then the transformer has likely failed. DO NOT just slap a new transformer in there as something has likely shorted and caused the failure. Install an inline fuse on the secondary as protection.
Thanks. I believe I also checked it not going to ground. What size in-line fuse would you suggest for the new transformer?

This unit has a ghost. Every six months or so it seems to stop working I've gone up there, moved the wires around and then after about 5 to 8 minutes it resets and starts and runs fine for another six months. This is the second or third contactor I have put in. I've never found the voltage drop or broken wires.
Fuses are typically 3 or 5 amp.
It seems like your going to ground while checking power. At least that's what I got from your description of 120 X 2.
In this case , you should check across both hots , Should be around 240 VAC ( or 208 VAC or 480 VAC on many commercial buildings ) .

Yes , replace the transformer with one of the same voltage and Volt Ampere rating .

Yes , install an inline fuse on the 24 VAC line that connects to the Red thermostat wire . Buy some spare fuses .

Now , start looking for what ever caused the transformer to burn out .

God bless
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