One of the best ways to prepare for troubleshooting any piece of equipment is to learn it’s normal operating sequence. If you understand what is supposed to happen and the order in which nit is supposed to occur, you can narrow the probable causes by observing the unit operation. By carefully observing the operation of a gas furnace, you can often narrow the possible causes of trouble to just a few items based on how far into the sequence the furnace gets. Most furnaces today use many similar components. This includes hot surface igniters to light the burners, induced draft fans to pull the combustion gasses through the heat exchanger, and draft proving switches to insure the draft.
The first step is to energize the induced draft fan. This is the little fan on the front of the furnace, not the larger indoor blower. The induced draft fan runs for a minute or so to purge the combustion chamber. The draft safety switch should close after the induced draft blower starts. If it does not, the process stops there – often with the daft blower continuing to run indefinitely. If the draft switch senses a proper draft, it closes and the sequence continues.
Next, the hot surface igniter is energized. If it is working, you will see a bright glow.
After a warm up period of 30 seconds to a minute, the gas valve is energized. The gas should ignite when it flows over the hot surface igniter.
A flame rod must sense the presence of flames within a few seconds or the system will close the gas valve and start the process again. After several aborted attempts, the system will lock out and quit trying to ignite.
After the flames are ignited, the control turns off the hot surface igniter and waits for the furnace to warm up before operating the indoor blower.
When faced with a furnace that does not operate properly, pay attention to how far into the sequence it gets before the process stops. That can help you decide where to start your troubleshooting.