Primary & Secondary Air in Combustion - Short #187

HVAC School - For Techs, By Techs

In this short podcast, Bryan dives into a gas heating topic: primary & secondary air in combustion.

Primary air is the air and oxygen content that enters the furnace BEFORE combustion. In older furnaces, prior to induced combustion, air was drawn in through the front. These older furnaces had adjustable shutters that we could modify to bring in more or less primary air based on our combustion analysis readings. We could also use flame color to get an idea of the CO content (yellow tips on the flames indicate higher carbon monoxide content).

In systems like those, air is drawn in via Bernoulli's principle; there are areas of low pressure around areas of high velocity. There is pressure associated with the natural gas, which draws air into the burner. Nowadays, we have induced draft systems (not to be confused with power-vented systems) to draw air in at a fixed rate for more consistent combustion. These inducer fan blowers are necessary for the more complicated heat exchangers we see in more recent furnaces. Secondary air is the air after combustion.

We only want to adjust primary air if we have the correct gas pressure, so we will want to make sure we perform combustion analysis. We can also clock the meter (if applicable), but you will not always have a meter, and you will want it to be informed by combustion analysis.

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