Weil McLain CG 4 SPDN fires on zone 1, but not zone 2


Old 10-13-14, 12:43 PM
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Weil McLain CG 4 SPDN fires on zone 1, but not zone 2

I have a Weil. McLain CG 4 SPDN boiler with 2 zones controllers by a Taco SR 502 2 zone switching relay and 2 Taco 007 cartridge circulators. These zones feed heat exchangers in 2 air handlers (1 for each zone).

I turned on the heat today, and found that when I raise the temperature on zone 1, the boiler fires as expected and the air handler fan runs. Zone 2, however, makes the same call, the red led lights on the relay, but the boiler does not fire.

I've swapped the relays, but this does not resolve the issue.

Any ideas where to go from here?

Thanks for your help!
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Old 10-13-14, 06:41 PM
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Yeah, it sounds like something is up with the switching relay. Here is a link to the manual:


Are the indicator lights on zone 1 behaving the same way as they are for zone 2? When you switch the zone 2 thermostat to "on", does the circ pump for that zone turn on? This would help isolate if the issue is with the switching relay, or if it's another problem.

On another note, be careful working with this switching relay as it switches the 110 volts to run your circulation pumps.
Old 10-13-14, 07:01 PM
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I've swapped the relays, but this does not resolve the issue.
I suspect that the relay which turns the pump on is separate from the relay that switches the 'endswitch' on...
Old 10-14-14, 05:03 PM
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Some of the newer SR502's have a endswitch relay.
If I recall correctly, the relays are DPDT, and one set of contacts switch the 120 VAC load and the other switch the TT output.

You may have a defect in the actual PC board of the SR502.

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