I just purchased a Coleman/ Evcon gas furnace. The little pressure switch with two hoses has to be tapped to engage the firing sequence. After ignition and cycling it doesn't re fire the control box is blinking two times. It will, however, run perfectly at times? I've been a week and a half with my installer battling this problem. We've checked the flue and to the advice of his distributor, he calls them " gurus" They claim it's a roof jack issue. My installer has never changed them out before, so he's baffled? Your help would be greatly appreciated, thanks.

The draft pressure should be checked with an incline manometer or similar instrument. If it meets the design criteria you may simply have a faulty pressure switch.

If this is in a mobile home the roof jack has to be replaced to meet the requirements of the new unit. Most of the roof jacks are different in design for different models and years of manufacturer.. The distributer is probably correct on the roof jack situation and just because the installer has not had to replace them before means nothing.