gas fired steam boiler overriding thermostat at night

Carol_7

We have a 12-yr. old Lennox PSB-6D gas fired steam boiler with intermittent ignition, auto vent damper device, and programmable water feeder. It has been overriding the thermostat ONLY on the night time setting, heating our bedroom about 8-9 degrees above the temp. called for, between the hours of 11:00 p.m. and 6:00 a.m. Tried turning OFF the heat, changing the thermostat, and even removing the thermostat from the wall, and the heat kept coming! Have had the electrician and local utility (PSE &G) look at it and we are all totally baffled. Problem started on very cold nights (outside temp. below freezing), but is now happening nightly. No problems whatsover with other time settings (morning, day, evening) - only the night. Thermostat is on an inside wall, but there did seem to be a big connection between low outside temp. and the start of the problem. Is there a part on the heater that is mistaking a call for a drop in temp. for an increase?

Scram Bulleggs

Why would you have an electrician and local utility company look over your steam heat instead of a local heating professional?

Norm Harvey

When you changed the thermostaat, what kind did you put on, and did you set the heat anticipator for steam heat.

When the thermostat is above its setpoint is the boiler off, or still firing?

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Carol_7

boiler overriding thermostat

My electrician also installs and repairs hvac sytems and Ithought it might be a wiring problem. In addition, I have a service contract on the boiler with PSE&G. Have also had my plumber look at it. (He repairs boilers,too.)

I don't know if the furnace is 'firing'. Wouldn't if have to if the temp. is 8-9 degrees above normal? How would I check this?
Thermostat is White-Rodgers digital 1F90W-71. Have tried several, makes no difference.
Thank you for all suggestions and keep them coming, please!
Dread seeing our bill this month...

Unknown

Check that vent damper

and see if it is sticking and keeping the end switches in the damper in the closed position and holding the boiler on. This can also happen with power stealing thermostats and the cure then is an isolating relay or resistor across the thermostat contacts.