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HVAC School - For Techs, By Techs

Nov 21, 2019

Jim Bergmann is back on the podcast. This time, he talks about common faults with high-efficiency 90+ condensing gas furnaces and their installation.

Like A/C units, 90+ furnaces often suffer from clogged drain lines. Other common problems stem from issues with inputs, temperature rise across the appliance, trapping, and venting. On high-efficiency gas furnaces, procedures like clocking the gas meter are much more important than on an 80% gas furnace; you must clock the gas meter to get the proper inputs. To get the furnaces to condense properly, you need to make sure you control excess air and get the temperature rise in the correct range.

During the adjustment process, combustion analysis remains important as ever on 90+ gas furnaces. CO poisoning is always a deadly possibility on any sort of gas appliance work, and too many things can go wrong. You must use a combustion analyzer every step of the way.

In high-efficiency gas furnaces, you essentially condense water out of the fuel-air mixture. (Think about water dripping out of your car's exhaust pipe in the winter.) Many furnaces counterflow, meaning that the flue gas gets pulled down instead of wandering upward. We need cold return air to meet with cool flue gases for optimal condensate production.

Two-stage 90+ furnaces also use two-speed induced draft fans, which normally require an exhaust accelerator. Issues pop up in retrofit systems when we don't update the venting system to prevent the recirculation of flue gases. Two-stage furnaces tend to be very efficient, but they may not be as comfortable as single-stage furnaces.

Jim and Bryan also discuss:

  • Chemical causes of premature failure
  • Orifices, fuel pressure, and impingement
  • Heat exchangers
  • Order of operations for checking condensate drainage
  • CO poisoning
  • Byproducts of combustion
  • Energy savings of 90+ furnaces over 80% furnaces
  • Interlocked systems
  • Filtration

Learn more about Refrigeration Technologies HERE.

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