My boiler (Weil Mclain CGA Gold ~2003 install) failed the other day. After some help from a plumber and reading the manual, I replaced a few parts (more below) to get it running again. Now one zone is always hot, about 10 degrees hotter then set on the thermostat. There's no advice in the manaul for troubleshooting this type of issue (all the led's on the boiler are normal). The boiler shuts off periodically, only to start up a few minutes later. What are my next steps?

My initial guess is to checkout the thermostat. how do I do this? Do you remove it and cap the white (heat) line, keeping it from contacting the red?

What parts of the boiler do I need to look into and what order to track down the cause?

I have a (different) plumber coming tomorrow to check it out, but I'd like to try to solve it on my own if I can.


Tuesday night I woke up to no heat, and the boiler was off with the 'damper' led flashing. While trying to wake up/figure what was going on, the transformer that powers the two thermostates (for two zones) died in a puff of smoke.

Not really knowing what I was doing, I bought a new transformer and a new damper, installed them and the boiler still sat idle.

My plumber inspected the wiring with a multimeter and concluded the main board was dead. So I replaced that (he was busy at another site, and I was happy to save a few bucks doing it myself). After that, still no luck, so I decided to read the manual.

Following the troubleshooting flowchart, I found that the transformer driving the damper was dead, so I replaced that and everything started fine.

It was late the next day by then, so I called it a night. The following day I found my basement 10 degrees hotter then the thermostat was set.

I've tried cutting the power, letting the boiler start up again. Both thermostats "click" as they've always done when set the temperature from above to below the current temp, which I take to mean they are turning off the heat.

Any advice is much appreciated.

  • Do you have a zone controller, and if so what is the make and model?
    – Tester101
    Commented Feb 6, 2015 at 22:29
  • @tester101 if I flipped the lines from the thermostat transformer and one thermostat is old school and has no need for C line would that explain it?
    – Yann
    Commented Feb 6, 2015 at 22:33
  • How are the thermostats wired, that allow two thermostats to control a single boiler? Typically a zone controller is installed between the thermostats and the boiler, and it's the zone controller that actually tells the boiler to turn on.
    – Tester101
    Commented Feb 6, 2015 at 22:38
  • @tester101 there is no zone controller, both thermostates are wired to the boiler, but both are wired to independant zone values. I think my ultimate solution is to install a zone controller.
    – Yann
    Commented Feb 11, 2015 at 17:28
  • So what it sounds like, is that when either thermostat calls for heat, the boiler turns on. Then each thermostat activates its own zone valve, to direct the heat to the correct zone. If this is the case, it could be that the one zone valve is stuck open. Or that one of the thermostats is always calling for heat, whether due to a bad thermostat or faulty wiring.
    – Tester101
    Commented Feb 11, 2015 at 17:40

1 Answer 1


If you had a DMM or VOM and know how to use it, that would make diagnosis easier.

However, the approach to take is to divide and conquer. Identify what component(s) are causing the signal for heat. Is it the boiler controller, the thermostat, or a defect in the wiring?

I would begin by inspecting the areas where anything has changed since it last worked okay. Look for anything amiss: stray wires, debris, bird nests, junk.

Next, I would disconnect the thermostat from the wiring to the boiler. This should be just a couple screws to loosen and then pull the wires out from under the terminal. If they are not naturally stable, wrap each individually with tape—masking, electrical, scotch, duct: whatever you have handy. If the system continues to heat, the defect is probably1 not the thermostat.

At the end where the wires leading to the thermostat connect to the boiler controller, disconnect them, being sure to record which goes where—use a camera, labeling the wires, or writing on paper. If the system continues to call for heat, the fault must be in the boiler controller. Otherwise the wire itself has a flaw. If there are unused wires in the bundle, you could try swapping unused wires for the needed one.

If you get here, it is likely the new controller is either defective, or more likely, not correctly connected after its replacement. Double, triple, quadruple check the connections and everything else which could go wrong. Reread the installation instructions to see if maybe there is a configuration option which might relate to the problem.

1 There could be several problems at once, so repeating tests of all items might be needed after the first round of repairs.

  • I have an analog VOM, would you mind explaining how that might make it easier.
    – Yann
    Commented Feb 6, 2015 at 20:10

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