The flexible tubing that is shipped with these furnaces must be stretched taunt to allow condensate to drain properly into the trap on the outside of the furnace. If the tubing is connected and ran across the width of the furnace and rests on the middle divider it will not drain and cause the pressure switch to open after 20-30 minutes of run time.
SOLUTION:
Cut tubing shorter than the supplied length so it is stretched and slopes towards trap.
