Normal sequence of operation
- Thermostat calls for heat.
- Circulator is activated.
- Water temperature is monitored to determine if thermostat can be satisfied, without firing the burner.
If the boiler must fire.
- Damper opens.
- Blocked vent safety switch is closed.
- Gas valve opens.
- Burners ignite.
Safety switches and limits
High limit sensor
If the boiler overheats, this switch will open and the burner will shut down. The burner will remain off until the temperature drops below the high limit setting, the circulator will continue to run until the thermostat is satisfied.
Blocked vent safety switch
If this switch is open (vent is closed) the system will shut down, and should not be run until the cause is fixed.
Flame rollout switch
If the flame or exhaust gas backs up, this switch will shut the system down. As with the blocked vent switch, the system should not be run until the cause is fixed.
From your description, it sounds like the damper motor is not functioning properly. This causes the blocked vent safety switch to stay open, which does not allow the furnace to fire. This could be caused by a bad motor, a stuck damper, a malfunctioning switch/sensor elsewhere in the system, or even a bad control board. You'll have to physically diagnose the system (which can't be done over the internet), or call an HVAC technician to service the unit.