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I installed a CGi5 Weil Mclain boiler a few weeks ago in a small museum.

Upon completion, it fired and went through the normal sequence of operation.

1) call for heat 2) draft inducer fan starts then proving switch closes 3) igniter sequence to light intermittent pilot 4) flame rod proves pilot flame and gas valve opens. Text book.

Next day the boiler is out and all lights are off on the control module. I meter the power wires at the module - 127 volts. I order a module and replace it. Still no lights. I finish what I should have finished the day before and meter the power wires and the output of the transformer- the transformer is bad.

Replace transformer and all is good. Next day transformer is bad again. Buy a new transformer and park myself in front of boiler with an inductive ammeter on transformer output. Normal operation for 4 hours at .7 amps. Then, during a start sequence the module lights go out and the transformer starts drawing 10 amps. I quickly move the ammeter from wire to wire looking for the high current draw and managed to eliminate the gas valve, the high limit switch, the temp sensor and the flame roll-out sensor before the smoke leaked out of the transformer. That is the sum of the 24v devices.

I installed a new transformer with a 3 amp inline fuse on the 24v side. The boiler runs for hours and hours and then pops the fuse.

I wired the high limit switch in series with the tstat at first and then I moved it and ran it in series with the flame roll out switch. No change. I called the factory and we went over each wire, one by one. Everything is correct except this:

According to the factory schematic, the circulator plug is wired backwards

at the module. I'm going to swap the wire tomorrow and try it but I've been racking my brain on how the 120v device could be waxing the 24v power supply. For you rocket scientists here is the question:

If the circulator L1 and L2 are reversed at the module and the module controls what it thinks is the L1 but it's really the L2 would the motor field collapse at disconnect be enough current to do harm?

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By 127 did you actually mean to say 120 or perhaps 27?

The relay that switches the 120 volts to the boiler circulator will draw much more amps than normal from the 24 volt transformer secondary if the relay contacts fail to close for any reason, assuming it's a mechanical electromagnet type relay. If that relay is a solid state device then I'd look to a failure of that device. Such a failure could result in an unintended connection between power and control circuitry...between the power section and the low-voltage section of the module. In either case, the Rx might be module replacement.

Boiler circulator connects to power section which in the middle of the power module. Lamps on left, low-voltage section on right. Connection of the meter leads to one terminal in the power section and the other to the low-voltage section might yield such an odd value. It would not have any significance.

If I'm wrong don't blame me. I've never seen a boiler with my own eyes before. I just took a look at a Weil-Mclain schematic and ladder diagram.

Too much time on my hands maybe.

Marc

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Thanks for the thoughts, Marc.

I sat and watched this cycle for over three hours today and it finally failed. I was able to eliminate everything, one item at a time until only the thermostat connection was hooked up.

The thermostat wires at the module are hot w/ 24 vac. To make a long arduous diagnosis story as short as possible, I'll skip the next two hours. One of the thermostat wires between the boiler and the thermostat was grounded to the plumbing and thus the chassis of the boiler. Some dilhole ran the t-stat cabling through the baseboard radiators and it shorted to a fin.

So, every once in a while when conditions were right, when the tstat called for heat it provided a dead short to the 24vac transformer.

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tn_2013111322222_tstat.jpg

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Your fortunate it only took 3 hours to find that.

Read the tale agin. Sounds like two days and two smoked trannys to me.

Dillhole? [:)]

A little embarrassed to admit I went through three transformers before

installing the fuse. I purposely said the entire word "transformers" because "smoking three trannys" sounds pretty perverse..

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