Fried transformer on Weil McLain boiler


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Old 10-21-06, 10:43 PM
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Fried transformer on Weil McLain boiler

This one has me stumped. I can't believe these two problems are coincidental. Mod, if this looks like a better fit for electrical that's fine with me.

My neighbors were out of town and their kid (staying at home) came to me for help. The second-floor toilet had clogged and the fill valve stuck open for quite a few hours, so I helped him clean up and shovel away the fallen plaster.

There was water dripping out of a light fixture so I went down to the breaker panel (1960s vintage Square D 100A) and saw that breakers 1&3 were marked "LIGHTING" on the placard and "MAIN DISCONNECT" next to the breakers and they had a handle tie. I flipped 1&3 off and in the OFF position I could see that they were 50A breakers. Huh?

Well I left them off, made sure there was no power at the fixture with my inductive probe and disconnected the light fixture, then turned the breakers back on. Did more cleanup and told the kid to turn up the thermostat to help dry things out.

So the parents come home today and they've got no heat. I check out the boiler, a Weil-McLain about two years old, which I had never touched in my life, and find no lit LEDs on the control board. Completely dead.

They've got 120VAC from the disconnect to the control board and 120VAC from the board to the 24VAC transformer primary, but no 24 VAC coming from the transfomer. I checked the stat, which opened and closed normally, and the stat wire was not shorted or open.

I pull out the transformer and find resistance on the secondary winding seems OK. I don't recall the figure. But the primary is open. I pry off the cover (24VAC, 40VA Honeywell) and see that the primary windings are hanging loose, apparently fried with melting of the tape wrapping and charring on the inside of the cover.

This cannot be a coincidence. Why did the transformer fry?
 
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Old 10-22-06, 05:04 PM
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Fried Transformer

With the screwy stuff you found in the panel, I'd almost bet there is some other screwy wiring such as shared neutrals. It could be a coincidence but I doubt it. I will ask one of the electrial mods to take a look at this thread in hopes he has some input.
 
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Old 10-22-06, 07:14 PM
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Looks like I wasn't as thorough as I should have been troubleshooting. My neighbor had the boiler guy over this morning and he found the thermostat wire was shorting. He said it was OK while the stat was calling for heat but when it stopped calling for heat it was shorted and overloaded the transformer.

I don't really understand that, and the wire did show "OL" on the DMM when I switched the stat to OFF and connected the DMM to the disconnected stat wires above the boiler. It's possible the probes weren't making good contact. I should have tested twice.

But, I have forced air and have never done any troubleshooting on a boiler, so maybe they're different. At any rate it was related to the water leak since the old stat wire was cloth and probably 1920s vintage.
 
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Old 10-22-06, 07:21 PM
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Cloth covered wire

That stuff can cause all sorts of troubles. Glad they got it fixed.
 
 

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