What is the model # of the furnace? I worked for Carrier/Bryant dealers for almost 15years and never saw a true 'power vent' model. What kind of 'power draft' are you talking about? B-vent is NOT allowed for positive pressure venting so unless the 'power vent' is right at the termination and pulling through the entire length of b-vent it is illegal and potentially dangerous! Canadian code for b-vent termination is minimum 2' from the roof penetration (on the high side if a sloped roof) AND 2' above anything in a 10' radius.
gasguy, combustion blower is in the furnace. This, I assumed, created a positive pressure up the B vent.
I have no model #. When it is 150 degrees in the attic, heat problems can wait!
Its probably tripping the rollout as said by others. And the venting needs corrected.
Being so close to the roof, wind can cause a down draft.
Since it is a Carrier furnace, the limit control that is tripping is likely the vent temperature limit.
With an improperly terminated vent system, the vent temperature limit tends to trip when wind conditions are interfering with the drafting of the vent system.
I still have alot to learn. Is bvent under positive pressure?
NO. The draft inducer fan is only meant to overcome the resistance of the HX. It's called induced draft as it pulls the gasses through the HX from the exhaust end as opposed to a power burner (think oil burner) which pushes in from the burner end. I agree that you need to lengthen the b-vent and you may need to install a high wind style cap.
Code up here stipulates 6" above expected snow level so we go 24" minimum off the roof. It sounds like a high wind cap is a good idea.
Do you have adequate combustion air supply in the room? Should be 5" is your inducer fan working properly? Those inducer fans on the carrier mids like the pav/wav and tua tend to get tight bearings over time.... Check your draft at the pressure switch when the furnace is running.
Given that your vent is so short, is it possible the vent is undersized?
I had a crown boiler with a bad spill switch right out of the box, is it spill or rollout? I don't remember if I read which one it was.