A friend recently purchased a home with a Carrier Weathermaker 8000 residential furnace (forced air heating). It's finally getting cold and they tried to fire up the heater but the system did not work. I took a look at things, and the system passes the self test sequence, and even goes so far as to fire up OK. The problem is that after 1-2 min, the flame goes out. The problem appears to be in the ID manifold and is confirmed by the pressure switch, which closes (as expected) when the ID blower starts up, indicating sufficient vacuum, but then opens after things heat up, indicating a loss of vacuum. Once things cool down, the system restarts, but then fails again due to the same problem.

Has anyone else come across this problem or have any ideas what would cause this?