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HVAC School - For Techs, By Techs

Dec 28, 2021

In this short podcast episode, Bryan goes over the basic gas furnace sequence of operations. There are a few variations, but the sequence of operations tends to stay consistent across most furnace types.

First, a W call from the thermostat calls for heat. The circuit board will then assess the safeties to see if it can bring on the heat without causing flame rollout or other dangerous conditions. If the safety switches are all closed, then the furnace can bring on the heat. However, in some cases, the induced draft motor may come on first in some 80% furnaces. There needs to be a small negative pressure in the induced draft motor housing. To confirm that we have that pressure, a pressure switch will close under the right conditions.

Then, ignition begins. In most cases, we use intermittent-spark ignition (ISI) or hot-surface ignition (HSI). It takes some time for these methods to light the pilot, which then lights the main burner. After that, the gas valve opens to fuel the burner. On an ISI system, that’s about it until the blower comes on. However, once the main burner opens on an HSI system, a flame sensing rod can verify if you have a flame on that burner. There is a blower delay that prevents the blower from coming on and blowing a bunch of air that hasn’t yet been heated.

After that, the blower delay ends, and the blower comes on. Then, the furnace shuts off when the W call ends. Then the blower continues running for a little bit before turning off.


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