This video clearly shows how a Gas (in this case an LP Propane model) furnace starts up. The same startup and timing sequence applies to a natural gas furnace. For reference purposes only: This is a Rheem "90 Plus" model which is about 15 years old.
1. The process begins with the PVC vent stack exhaust blower coming on to push out any leftover gas in the vent stack. In a high efficiency furnace model, the vertical vent stack is made of PVC plastic and not metal.
2. After about 15 seconds, the electric gas ignitor (glow plug) will turn on as seen by its bright red glow. This is extremely hot (over 1000 degrees) and could not even be measured by a laser temperature gun.
3. After about 15 more seconds, the gas valve (which also contains the pilot switch and pressure regulator) "clicks" on and allows the gas to flow to the main gas burner "jets." The gas arrives at the rightmost burner first because it is closest to the source (main gas valve). Carefully watch the gas flame as it ignites and quickly travels from right to left. Each burner jet ignites its neighboring burner.
4. Moments after ALL the gas burners are evenly lit, the main air blower that moves the heated air will come on to move the heated air into the house through the ductwork. The startup sequence is complete.
Troubleshooting:
1A. At times, especially at the begining of the heating season, the furnace may not light and follow this sequence reliably. One of the principal reasons for this is dirty gas "jets." The jets build up a very fine layer of residue from the gas or propane passing through them. They are usually made of brass with a small hole the size of a paperclip in them.
2A. Another common problem is with a small "tripped" saftey switch that actually "smells" the the gas chamber and plenum area. Look for a small (normally round) button with one or two electrical wires leading to it. If you see a little "stem" protruding from it, press it and see if you hear a little "click."
NOTE: After any changes or repairs are made to the furnace system or heatpump, it is often necessary to turn the furnace switch OFF and then back ON again.
HVAC Gas Furnace Startup Sequence Timing of Events Troubleshooting
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