Help with electrical mystery

Brant Beach

first no heat call found 24 volt transformer not working.
replaced transforner with proper unit. worked fine for a few days.
second no heat call found TACO power head not working. replaced it, worked fine.
third no heat call found relay on L8148E not working.
replaced with L8148E 1257.
boiler came on was working fine. as I was waiting for high limit to shut burner I saw smoke come from new relay.
I turned off power.
I tested line voltage, that is ok. low voltage is also ok.
any idea of what could be killing off these controls?
FYI: boiler is Utica gas model 100ACB 20+ years old.
2 Taco 500 series zone valves with Honeywell T87 t-stats.
Honeywell v810 gas valve.

Dale

Yes, dead short

You have a load that's shorting to ground, maybe you're lucky and it's the relay coil although this is rare. Whenever you change a transformer for any reason put a resetting 3 amp circuit breaker in line on the tfmr. Then cycle the system through all it's functions. If the breaker trips disconnect the loads one by one until it doesn't. Also disconnect the thermostat as the tstat wiring is often shorted against a sheet metal edge. We usually find the problem in the gas valve coils. If it's an intermittent problem look for something like a humidifier that uses the 24V or wiring that touching a sharp edge. Disconnect one side of the air conditioner 24v wiring. On a warm air furnace wait for the blower to start, another possible circuit and sometimes the expanding duct work makes the short.The key is to connect loads one by one until the problem appears. Besides the little circuit breaker I use a little 0 - 1.1 amp ac meter and watch it as the components cycle. When the thing slams over I shut the power.

Empire_2

I would use a clamp on amprobe which should tell you if your circuit is shorting out. I feel that you are probably drawing about 4 amps on your control circuit which will cause a slow burnout of various controls. Shorts are easy to find if you know how to isolate your control circuit.

JimGPE_3

Okay, I'm confused.

By definition, a short is a "zero" resistance path from hot to ground. This, again by definition, means the relatively high resistance/impedance loads like relay and gas valve holding coils get no amps.

How does a short increase the current to a coil and burn it out?

Tell me you let the magic smoke out of the transformer with a short and I'll understand, but how does a short burn out a gas valve or a relay coil?

Dale

Shorted coil

Good for you for asking by the way. When you reach the point where you truely understand ohms, volts and amps you will be in the top 1/10 or 1% of techs. The shorted coil may be a portion of the coil wire itself or a connection wire inside the valve or whatever is providing a path to ground before the load can limit current. You started saying you changed a tfmr, why, I will say that's usually because it can supply about 1.7 amps out of the 24 volt side without overheating and burning itself out and it was supplying about 10 amps for time and then died. (I always ohm out "bad" transformers and almost always find the primary (fine wire)winding open which tells me it was probably overloaded.)I have a cheap transformer on my bench I dead short and check amps for my new techs. They put out about 10 amps for a few seconds until the 3 amp circuit breaker trips. Except for the heatanticipator or small solid state loads in stats loads are not in series. So, if you see a relay smoking, and melting it's usually that the contacts inside are bad and acting like a little heater,120 load being fed through them. The people who make the 24 volt loads like relay coils make sure the current needed is well within the capacity of the tfmr so although a relay coil shorted at the beginning of the coil is possible although it is rare.