24 volt transformer and the relay.
First you need to test to see if the transformer is putting out 24 Volts. Some older transformers only have 2 terminals on them so it is easy to test, just set your electric meter to DC volts and touch the 2 terminals with the two leads on the tester. On the transformer above the 2 terminals on the top is where you test for 24 volts you may find you have 26 or 27 volts, this is normal. If you do not have 24 volts at the transformer you need to be sure you have 120 volts to the boiler. You can no longer assume at this point. If you can not figure or do not want to mess with it call a pro. If you are sure you have 120 volts to the boiler all of your switches are in the on position and you do not have 24 volt, chances are the transformer is defective and needs to be replaced. Please have a pro double check your conclusion.
If you do have 24 volts at the transformer the transformer is not the problem. Many boilers have more then one transformer, the one on the boiler with the relay controls the components on the boiler like the gas valve, the vent damper, the relay and the power goes through all of the safety features on the boiler. in addition sometimes the zone control valves are also controlled by the boiler 24 volt transformer. But many boiler installers use a separate 24 volt transformer to control the zone valves. If the separate transformer is not working this can cause the boiler not to fire because the zone valves are not opening. So look for another transformer someplace around the boiler, god only knows where a person will put the other transformer. Any way once you find it test it to see if it has 24 volts, if not it may need to be replaced. if it does have 24 volts check to see if your zone valves are open.
If you do not have any zone valves and only have one thermostat, test the wires that go to the terminal that is marked "G" and the terminal that is marked "C" touch the 2 wires together, this should cause the relay to pull in you should hear a click. Then if you keep them tied together you will hear the vent damper open (if you have one) then the boiler should fire. If not check your spill switch and your roll out switch and your low water cut off.
You may not have any of the items mentioned. if you have a control box like the one to the left jump the T T terminals to see if the relay pulls in. If the relay on the control box does not pull in have a pro come out to be sure the control is bad before you replace it.
Now on the transformer relay top right also known as a fan center if you did not hear a click when you tied the G and C wires together separate the wires and remove any wires connected to the wires connected to either G or C did you hear the click now? if so the relay was already pulled in and you just caused it to pull out put the wire you removed back, you should hear a click again. this means that the thermostat is working fine and the relay is pulling in.
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