Re: heater vacuum control
Well the air blowing in will be from no vacuum and possibly the flap vacuum control also being broken and not able to close, just flip it to closed to put fan on re-circ and stick a screw through the plastic, or a small wooden wedge to hold it from flipping back again to solve the issue quickly.
I would just block all the pipes off, then one by one apply vacuum pressure till everything works fine inside, then work out the rest later when more info is on hand :0)
I did put something here, but it vanshied, I think
"So long and thanks for all the fish!"