Furnace 3D wiring model
Suburban 12 V board
A furnace is a chain of switches feeding a control board. The board only lights the burner when every switch in the chain is in the right state, in the right order.
Parts in this model
- Thermostat: Closes when the room calls for heat. If voltage stops here, there is no call.
- Valve switch: Normally closed. Drawn in this spot on Suburban's wiring ladder.
- Sail switch: Starts open. The blower's airflow pushes it closed. If the blower is slow (low voltage), it may never close.
- Limit switch: Normally closed. Opens if the furnace overheats.
- Blower motor
- Gas valve
- Spark + sense
The sequence
- Waiting. The thermostat contacts are open, so nothing is happening. The board keeps checking that it has at least 9.5 V.
- Thermostat calls for heat. The thermostat closes and sends 12 V to the board. The board checks that the sail switch is open, then turns the blower on.
- Sail switch closes. As the blower gets up to speed, the air pushes the sail closed and the board has to see that. If the sail stays open 30 seconds after the blower starts, the board goes into lockout and shuts the blower off.
- 15 second purge. The board checks that the gas valve relay contacts are open, then runs a pre-purge of about 15 seconds to clear the combustion chamber.
- Trial for ignition: 7 seconds. The board opens the gas valve and sends high voltage to the electrode for seven seconds.
- Flame sensed. The board senses flame through the spark wire and electrode, which is why the electrode has to sit in the flame. Sparking stops. The valve and blower stay on while the board watches flame, sail, limit and thermostat.
- Heat satisfied. The thermostat opens. The valve closes and the flame goes out. The blower runs a 90 second post-purge, then stops.
Faults you can find with the meter
- Sail switch won't close
- Low battery (8.5 V)
- Thermostat won't close
- Limit switch opens
- No flame sensed
- On-off switch / breaker open
- Blower motor dead
Numbers
- :
- :
- :
- :
- :
Keep going on furnaces
Learn it
Prove it
- Furnace blows cold
- Furnace sparks but never lights
- Furnace keeps shutting off, fan keeps blowing
- Thermostat clicks, furnace does nothing
- Comeback: furnace quits again