TNHVAC

I have been working for myself for 2+ years, mostly on residential units, I recently got a contract with a factory & have completed several jobs with no problems. NOW I'M in over my head,this darn thing will run between 5 & 6 minutes then stop & lock out. (Gas Heat) I have robbed a 2nd unit that has at some point had a internal explosion & no longer in use. Changed out both boards, etc. still same thing. crack in heat exchanger maybe? This is the 1st unit I have worked on with a negative gas pressure & combustion blower motor instead of induction motor. Thanks for any advice.

az-guy

check the flame sensor, is there a flash code for self troubleshoting

HeatPro

Perhaps you can find a make and model. A negative gas pressure is usually done with an induction blower. A power burner usually feeds a negative chimney draft and chamber.

briandurussel

Can you go into test mode by leaving your jumpers on teminals 1 and 2 and pulling them off when the unit goes into its heating stage? How long will it run then? You can single out a control problem that way.

Make sure all of your safeties are made and auxillaries are staying made. You may need to take an hour or so to check them but you have to make sure.

Check the board('s) look for any shorts or water damage.

If it is a "power" burner make sure your screen is clean and your tubes do not have any cracks at the bottoms (heat exchangers).

Definitly check your pos-neg pressure tubes to make sure they are not filthy.

Had a unit do the same thing and found an Auxillary faulty, it was located behind the main 3 pole 480 volt main breaker on the unit.

TNHVAC

Thanks for the advice,found the problem today the unit was heating some new offices with relocated then removed & replaced T/stat drywall guys ran a screw into the wiring D--- thing acted crazy as -ell would call the lady in the office to turn on & off up/down work for awhile lose call for heat, then start up again. Thanks again