Originally Posted by Bruce Wray
We've been on the road a week. The first morning when I switched the heater thermostat on, the ignitor cycled through a three-attempts try at lighting and shut down without firing off, I toggled the thermostat into a new lighting cycle and the heater finally lit. This went on for 5 days and suddenly yesterday morning the heater started operating normally, lighting after the ignitor sparked for a second or so. Any ideas of what's going on? I'm tempted to let this sleeping dog lie, at least for the time being.
We had a similar problem on about our 3rd or 4th night camping, but that was with a new trailer/new furnace, so I'm not at all sure it's the same issue.
Ours was caused by black flakes which were inside the burn chamber and one of them was large enough that it was clogging the burner inlet. You can remove the cover to the burn chamber by removing a couple of screws. If you see any debris inside, remove it. If the furnace won't fire properly afterwards, it's probably the sail switch, as others have mentioned.
If you don't know exactly what that is, it's a switch with a flattened arm attached to it. As the fan causes air to flow, the air pushes against the sail switch arm, which closes the switch. That completes the circuit for ignition to take place. The purpose of the sail switch is to insure adequate air flow. If there is gunk or build up in the chamber, this can reduce air flow, which means the sail switch won't "sail", the switch won't close, and the furnace won't light. This doesn't mean the sail switch isn't working - in fact, it means it is working: it didn't sense adequate air flow (enough to push the switch closed), so the circuit was not completed.
But, sail switches can also have build up on the switch arm itself, making it too heavy to move and close the circuit. They can also have bad contacts, so that even if they close, there is no electrical signal sent that closes the circuit.
The first thing I'd do is take the cover off the burn chamber and look for build up or an obstruction on the inlet. After cleaning it out, try lighting the furnace again. If that doesn't work, and you think the sail switch may be defective, the good news is that sail switches are pretty inexpensive, and easy to replace.
There is one other possibility, as Glenn alluded to - the piezo for the igniter may not have the right gap to produce a good spark for ignition. Sometimes you can correct this by gently bending it a bit closer with a pair of needle nose pliers.