Mystery with Suburban furnace

RLS7201 wrote:

dougrainer wrote:

RLS7201 wrote:
My SF 35 does the same as your furnace. It's not the thermostat. It's not the fuse and wiring. What I find is that when the furnace runs for an extended period of time it shuts down and will not restart. Like you I have to pull the fuse and reinsert to get the furnace working again. This would indicate the high limit switch is shutting the system down. I have added duct work so I'm convinced that I have plenty of air flow. Time permitting, I will find and replace the high limit switch.
Wiring schematic.

Richard

Pulling the fuse or resetting the Tstat will NOT reset a Hi Limit switch. They will sometimes reset on their own or jostling the furnace mechanically may reset the Hi limit. But just disconnecting power will not reset it. It is a bi metal switch. Once it cools down will normally reset. Yes, GOOD air flow will prevent intermitant tripping of the Hi limit. To the OP, replace the control board. It has defective logic. Doug

It was not my intentions to indicate the high limit switch was resetable. Note I said "shut the system down". Hopefully you don't consider the high limit switch the system. I know it's nothing more than a bimetal device. But if the high limit switch does trip, won't that cause the circuit board to lock out?

Richard

No, it will not cause lock out. Because, the power must go THRU the Limit switch to get to the Circuit Board to start an Ignition sequence. Lock is caused by the 3 attempts at ignition and 3 failures. The Circuit Board needs to be replaced. Doug