Hi,

I have a hot water boiler (Weil Mclain) that recently lost one of the two switching relays that power it. I replaced the relay (taco sr501) with the same model although they have clearly updated the part as the enclosure is different and the circuitry inside has small differences. Once I hooked it back up, the boiler would only fire if I turned on the zone that still uses the old relay. When I turn on the zone with the new relay, the circulating pump runs but the boiler does not fire.

For troubleshooting purposes, I went to trying to run a single zone off the boiler at a time. When I wire up the zone using the old relay, it works fine. When I wire up the zone using the new relay, the boiler does not fire. I have checked the wiring and both are done identically. I have also checked the voltages with both relays hooked up and they appear to be the same. When the relay is activated, I see 0VAC across the wire running to the boiler control. I expected to see 24VAC across the thermostat wires when the relay was not activated but I see 0VAC. Am I correct that the transformer on the boiler should supply this voltage? Thinking that I got a bad relay, I tried another one and it gives the same results. I even tried hooking a thermostat directly to the boiler and it doesn't work that way which it should.

Any suggestions on what to test or what the problem might be would be appreciated. I seem to have 1 magic switching relay that works and nothing else does. Luckily the temps here are not too cold right now.

Thanks.

 

You're using the terminals 5 (COM) and 6 (NO) to run to the boiler control, correct?

And these two wires are in parallel with the same two wires from the other relay?

Yes, the voltage you see at the above terminals comes from the boiler transformer.

24VAC should be seen on those two wires when there is NO heat call, and 0 VAC when there IS a heat call.

Sounds to me as if you may have a broken wire inside the insulation.

 

Clear well lighted pictures MIGHT help.....................

 

Hi NJ Trooper,

Thank you for the response.

Yes, I am using terminals 5 (COM) and 6(NO) for the connections to the boiler. There were also 2 wires coming from the other relay going to the same terminals putting the two relays in parallel. I have disconnected that wire and have been taking the wire that comes straight from the boiler and putting it on each relay for troubleshooting purposes.

I measured 0 VAC at the transformer on the boiler with the wires disconnected which is what makes me think that something is wrong with this transformer (it did have 110VAC power).

One important piece of information that I totally forgot to include in my initial post is that I actually replaced the control board inside the boiler which includes the transformer earlier today as I had come to the conclusion that it was bad and your reply suggests to me that it was a valid conclusion.

What is really weird to me is how the old SR501 still controls the boiler. To be honest, I am wondering if all of my problems didn't originate there and it has been destroying the other components. I am going to measure what the voltage is on it when the boiler is on and make sure that it isn't supplying the 24VAC.

I will try to get some pictures at some point soon but everything is wired exactly to the wiring diagram from Taco.

Thanks.

 

One new piece of information. I noticed that the motor on the damper makes a terrible buzzing noise when operating (with the old taco sr501 hooked up) and it never did that before. I shut it down before thinking to check the voltage on it last night. It still does not run using the new relay and I still can't measure any voltage on the output of the transformer on the boiler. After work today I will take another look at it.

 

Just curious,what is this damper motor, isn't this a hot water boiler?
Geo

 

I believe referring to the flue damper?..................

 

Yes, I was referring to the flue damper.

Well everything is back together and working well. I ended up needing a new transfer/relay unit in the boiler and 2 new switching relays. I appreciate the help. Thanks.