Boiler Problem


Old 02-17-16, 08:45 AM
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Boiler Problem

OK, I did this to myself, inadvertently.

Boiler is a Vaillant single zone, baseboard and domestic. Circulator is a Taco. All worked fine until the following happened.

In removing an old water tank above boiler, it broke loose and came down first on the stove pipe entering chimney then dropped onto the MC wire between cut off switch and aquastat. I saw a spark as it hit the MC and knew what happened. The MC cut the wire, shorted and threw the breaker, as I would expect. I dented the stove pipe so replaced those 90's and went to temporarily repair MC and wire nut the wires together. I plan to reroute this anyway as it is poorly run and hinders access.

Turned breaker on and boiler fired up fine, but noticed immediately that the circulator was not working, based on pipe temps, but boiler continued to run until pressure valve tripped and spilled out some water. Turned boiler off at cutoff switch.

Tested voltages and I get normal 120 nominal across L1 & L2, but see zero volts between C1 & C2 which goes to pump, explaining why it is not running. House is plenty cold and thermostat is set high enough to be calling.

Questions are how did what happened possibly cause this. Only wire hit was the main line voltage, and nothing else. So why no voltage out from aquastat to pump? Why would boiler not turn off, as it seems to be run until I turn it off.

I can't fine a breaker or anything in aquastat housing, so not sure what else I can check. Boiler was not running when this happened, so nothing should have been live but the main line voltage to aquastat. Can I bypass aquastat by bread-boarding to check that it circulates? I suspect it would as it was not directly involved in this.

Any ideas, or things I can check??

Thanks, Keith
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Old 02-17-16, 11:38 AM
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Welcome to the forums.

Depending on how your boiler is set up it may heat the water without a call for heat from the thermostat. Post the model number for the aquastat. It should be an Lxxxxxxxx number.

Inside the aquastat are two low voltage terminals marked TT. You can short them together to see if the relay is working.

Whether or not the circulator runs.... the boiler should not heat to the point of the expansion valve opening. The tank you removed.... was that the expansion tank.... and did you replace it ?

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