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Lennox HVAC 80MGF4/5K-100A-1

3 posts in this topic

S/N 6395L37886 Natural gas. Prod date 11/95. For the last four years, on a yearly basis, this unit has smoked the aforementioned transformer. First time was my fault; installed a new electronic setback t/stat and connected the blue wire. This was in spring/early summer. A/C worked fine. Come to need heat, system no workee.

Called the smell good plumber here in SoCal--he sent out a kid that barely speaky Engrish and all he could say was $600.00. I was at work (30 yr auto tech, not HVAC) and had talked to guys who know HVAC so by then I realized what I'd done. Told the wife to pay the kid for his service call and tell him to go away. Got a transformer for $28.00, fixed the wiring screw up and all was well. The next year my two dragonslayers, Joey & Shadow (55 & 70 lbs, no fat) were chasing lizards around the condenser unit and rubbed the wiring bare, shorting it against the condenser housing. More magic smoke gone. Fixed the wiring, put it in conduit. Replaced the transformer again, adding a 3 amp atc fuse inline.

Oh, just remembered. This incident also took out the circuit board in the thermostat.

Replaced the t/stat, heat & A/C is working.

OK.2009. New T/stat, protected transformer & wiring. NO HEAT! Fuse popped. I unplugged the t/stat and shut off the circuit breaker for the system but it pops the fuse as soon as I plug it in. The led is not lit. Really getting tired of this. Help?


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There is a few ways to go about this:

Pull all of the t-stat wires off of the control board at the air handler.  Put a wire back on one at a time starting with Common...restore the power and see if the fuse blows.  If not, then go to Red and put power back on.  If all is good, then go with green, restore power and then turn the fan on at the t-stat.  Put Yellow on, restore power, turn on AC.  Go through the process up to the point you loose a fuse...then you know the circuit that is defective.  This is the non-technical way.

The technical way is to grab your meter, pull the control wires off of the control board (disconnecting the t-stat from the unit) at the air handler, restore power, measure the incoming HV power, make sure your transformer is tapped for the correct house voltage (230 VAC most likely) and then verify 24-27 VAC on the secondary side of the transformer.  While you are looking at the transformer, check to make certain it has enough VA rating to run all your equipment.  If you put one in with insufficient VA you will smoke the fuse every time.

With the power on and no control wires to the board, use a wire jumper out the unit directly from the control board - jump Red to Fan (green usually) and let the fan spin up.  Then Red to Cooling (yellow usually) and make sure the condenser spins up - the indoor fan may not run-some units must have the Fan terminal energized separately.   Then jump Red to Heat (usually white) and check operation.

If all that works without a hitch and does not smoke the fuse on the transformer then you have isolated the problem to the t-stat and the wiring from it.  Take the head off of the t-stat and remove all the wires from the sub-base and separate them out so none are touching anything or each other.  At the air handler set your meter to continuity and with one lead on a chasis ground touch the other lead to each one of the disconnected t-stat wires - checking for shorts to ground.  If none are found then check for wire to wire shorts between all the combinations of R,G,Y,W,Ground.

Do these things and let us know back at what point you let out the magic smoke or observed a ground/short and we will go from there.  Best of luck.

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Short somewhere. Thermostat make and model number will help. :)

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