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HVAC School - For Techs, By Techs

Dec 19, 2016

Benoit Mongeau comes on the podcast to talk about high-efficiency gas furnace basics and maintenance practices. 

When maintaining a high-efficiency gas furnace, you typically don’t have to worry about cleanliness in the same way you’d worry about an oil furnace. However, condensate drains need regular cleaning. You’ll want to look for cracked heat exchangers and pay attention to your manifold gas pressure.

We also have to check the temperature rise, also sometimes called delta T. Bypass humidifiers will often affect your temperature split, so that's something you need to account for in cold, dry climates; hot air will go from the supply plenum to the return. (However, corrosion typically isn't a concern for bypass humidifiers.)

Water is a product of combustion, so drainage is very important to gas furnaces. Manufacturers tend to recommend that most high-efficiency furnaces be built slightly pitched to aid condensate removal. Since the combustion air contains moisture, the condensate ends up being quite acidic; that condensate is really a mix of condensed combustion products.

Combustion analysis is also an important part of maintenance. If your gas burns incompletely, the furnace will end up making deadly carbon monoxide, which is the main value we measure in combustion analysis. Typically, the carbon monoxide levels should stay below 100 PPM. You can do your combustion analysis anywhere in the flue pipe as long as you're past the inducer motor. (Note: the exhaust pipe should never be above the air intake.)

A liquid petroleum (LP or propane) furnace differs from a gas furnace because it has different pressure needs. You may also have to add a restriction to the burners.

Benoit and Bryan also discuss:

  • Air intake drains
  • “Condensing” gas furnaces
  • Inducer motor types
  • High vs mid-efficiency furnaces
  • Excess air
  • Low-fire vs. high-fire gas pressure and potential effects on combustion

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