RV Repair: furnace problem, flame sense, board tester
Questioni have a hydroflame furnace model 8520-11.
the fan comes on then the furnace lights but goes out after 4 sec.
do you think it is the limit switch or the sail switch?any help would be great.
AnswerBlaine,
First make sure that you have a good strong 12 volts at the furnace with the fan running and that there is gas in the LP bottles and that they are on. You would be surprised how many times I have people bring there units in and the only problem is they forgot to turn the gas on. I doubt that it is the sail switch since it is closing and allowing the circuit board to attempt to light the furnace. It is also unlikely that it is the limit switch for the same reason. Check for voltage at the top wire on the circuit board after it attempts to light and goes out. If there is still 12 volts there then power is going through the sail switch and limit switch. If the other gas appliances are functioning properly then it probably isn't a gas pressure problem. I would first do a visual inspection in the exhaust tube on the lower left corner to insure that there isn't any blockage. If there isn't then the problem is most likely in the flame sense circuit with the fault being in either the electrode, the wire to the electrode, the circuit board or the quality of the flame. Inspect the electrode making sure it is clean and unbroken. You can check the wire for continuity with an Ohm meter and inspect it for cracks or cuts where it may be shorting to ground. If these both look okay then you can remove the circuit board and find an RV parts or service department that has a fenwal board tester and they can check the circuit board to see if its the fault. If these all check out then the flame may be weak. This could be caused by a dirty burner, a plugged orifice or incorrect gas pressure. As you can see there are a multitude of possibilities that could cause your problem. Just eliminate the easiest first and you should be able to trace it down. Good luck.
Darren
