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low voltage circuit

I would like to know what is happening to a series of 24 volt controls that have either stopped working or have actually fried on a gas boiler that I have been tring to repair. the first time I found there was no 24 volts coming from the transformer that is wired into a junction box under the house. I replaced this transformer and everything worked for a couple of days. Then I found one of the Taco power heads was no good. that is typical, I replaced it. it was ok again. then I found the aquastat/relay was not working. I replaced that. as I was checking out the boiler...it was working... I saw a flash and then smoke from the new aquastat/relay. I shut off the power and gas and has been off since. I do not want to fry an more parts tring to find the problem. I tested the high voltage that is ok. the new transformer is ok. anyone have a clue as to what may be causing this. one thing I do not understand is...why is there another transformer as part of the aquastat/relay? what does it power?
the boiler is an old Utica. the controls are Honeywell t-87 t-stats, Taco 3 wire zone valves,Honeywell L8148E aquastat/relay, Honeywell V 810 gas valve. all typical items. the system is 2 zones,standing pilot,no vent damper.
Help if you can. Thank you.


  • Dale
    Dale Member Posts: 1,317
    2 transformers

    Usually 2 transformers are used if the zones valve like Taco are pulling too much power for the transformer that came with the boiler. This is not uncommon. The problem comes when these transformers are installed out of phase or into the end switch instead of the heater at the zone valves. You may have done everything correctly when you got there but got unlucky making the connections, sometimes on the tfmr since some of the old ones weren't identified R and C or the R and C were wrong. What taco and the boiler maufactures say to do is to cycle ALL, one at a time, your 3 wire zone valves BEFORE hooking up the tt wires to the control. If you have any voltage, ( like 48 or 24, should be 0 ) at the tt wires when ANY of the valves are calling, you have the polarity wrong on one of the tfmrs or connection at one of the valves. If I had a transformer away from the boiler I would reinstall it near the other one on the boiler or very near it. The other route is to replace the original one with a Nema rated 40VA which should be enough for 3 tacos. If the taco instructions say that isn't enough buy a higher rated va tfmr. It was common for the old oil boilers to only have a 20 va tfmr since that would work.I preach this too much I know but get in the habit of ALWAYS clipping in a 3 amp circuit breaker, fuse or watched Amp-check meter whenever you power a tfmr you just replaced, saves alot of parts. Good luck and please let us know what you find. Posting a question here means you're willing to ask questions and learn (probably more from the other posts you'll get)
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