problem with an older AAON unit form 1992
The original service call was for no heat... after checking out the unit the transformer was tripping out when the unit was calling for heat. I narrowed the problem down the a shorted 24v coil on the 2nd contactor for the strip heaters. I pulled the 24volt side of the coil and the unit started up when the power was brought back on with only the 1st and 3rd heaters energized.
Ok so i thought that was the problem... went to the supply house bought a new contactor and came back and installed it. Started the unit and all three contactors for the electric heat pulled in ( delay on the third) and the unit was running fine. Checked my amp draws for the heaters and called it a day.
Two days later the customer called me saying that the unit wasn't working again... so went back to check out the unit and the same problem. The coil had shorted again causes the transformer to trip. Im not sure how long the unit ran before failing again could of been 10 minutes or a day....
So let me give you some of the details of the unit... the transformer has been replaced a few years ago ( i checked to make sure that the VA was the same as the original, checked voltage on the secondary side of the transformer had 24.6 volts but when checking voltage at the coil i only had 23.5 volts... this should be fine right?) The wiring for the heat goes from w1 through 2 limit switchs (in series) then to a to a time delay relay (enables w2 and the third contactor for the electric heat) and the 24 volt coils for the first and second contactors (wired in parallel) with the way they are wired it doesn't make sense to me that only one 24 volt coil would fail??
There has been a problem in the past with contactors failing according to the customer so im not a 100 percent that the problem has ever been fixed. Im looking for things to check for that could be causing the problem... any help would be appreciated thanks