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Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. First I’ll need some more info so I have a more complete picture of you situation.
What is the model number of your boiler?
Do you have electronic ignition or a standing (lit all the time) pilot?
Is the motorized damper open?
The damper is in line in the vent (see attached pic). When it's open the flats on the motor shaft will be vertical.
Looks like somebody removed the damper. Try pressing the reset button on the blocked vent switch on the side of the bell shaped draft diverter on top of the boiler. If you push the small button between the 2 wires and it clicks you have reset it and the boiler will fire up with any luck. I'm assuming the pilot is lit.
Dampers are a waste of money. You'll never get back in fuel cost saving what it cost for a replacement. It's only there to help hold the heat in the boiler when it isn't firing.
Was the pilot lit? If so then the boiler has to be down in order to troubleshoot this. It's an old boiler. The gas valve could be failing or maybe you have a bad connection.
You can't unless the boiler isn't working. When it doesn't fire up and it should (call for heat boiler & is well below high limit setting) you need to check for 24 volts AC at the gas valve. If you have 24 vac a lit pilot and no burner igition then you have a bad gas valve.
Given that it's starts back up own its own then yes a bad transformer is likely. R & G terminals are thermostat, R & C are 24 v constant. Besides the vent switch the only other things in the circuit are the aquastat/limit and the rollout flame sensor down by the burners. Rollout is 1 time use. If it's open it has to be replaced. Here's the schematic (with damper in place)http://www.weil-mclain.com/sites/default/files/field-file/cg-119_1.pdf
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