Probally something in the safety circut. (Thermostat, high limits, fuse link, pressure switch...) The most common will be the pressure switch not closing. This will be caused by low pressure from a dirty filter or baskets. Also a plugged impeller will do this. If you find the pressure switch and using a jump wire, jump past it. If it fires it is probally one of these things. Next check the fuse link or roll out switch. This will tell you if the flame is rolling out of the front of the fire box. If you jump past this you have a problem with venting, a plugged heat exchanger or rat nest's on top of the burner tray. Past these you should have a repairman out to check it. All of these are safety circut itims and are only simptom's. The real problem will be what caused these to fail and that is what you need to fix before you run the heater. Good luck.
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Probally something in the safety circut. (Thermostat, high limits, fuse link, pressure switch...) The most common will be the pressure switch not closing. This will be caused by low pressure from a dirty filter or baskets. Also a plugged impeller will do this. If you find the pressure switch and using a jump wire, jump past it. If it fires it is probally one of these things. Next check the fuse link or roll out switch. This will tell you if the flame is rolling out of the front of the fire box. If you jump past this you have a problem with venting, a plugged heat exchanger or rat nest's on top of the burner tray. Past these you should have a repairman out to check it. All of these are safety circut itims and are only simptom's. The real problem will be what caused these to fail and that is what you need to fix before you run the heater. Good luck.