In my house I have a one-pipe steam heating system with a gas fired boiler. It is power vented. The problem I'm having is that when it's really cold overnight (say maybe 30s and below), the boiler will not come on. When I wake up I find that the house is cold below the thermostat temp, the thermostat is on, the power venter is blowing, but the boiler is not firing. The hot water heater is also attached to the power venter and if I run hot water, the heater will eventually fire. The thermostat was recently replaced by an HVAC guy as an attempt to fix the problem, but no dice.
Last year I did have a problem with the pressure tube on the power venter not being clear or loose; this was causing similar symptoms of the venter blowing but the furnace not firing. He came out and blew in the tube and tightened it and the furnace came on. However, the problem has reappeared, and other techs have looked at that pressure tube on the venter and think it's OK.
Does anyone have any ideas what might be going wrong here? I'll note also that when the day heats up, like after the sun comes out, the furnace will start working.
Thanks for any help.