Commercial buildings like the one pictured here have been in use for more than 80 years and have had little to no impact on venting or condensation. So why are they the target of such concern today?

Based on the current GAMA venting tables, the flue shown below is not considered acceptable. Well, without knowing its exact size and the Btus of the equipment venting into it, this would be more of an opinion than a fact.

Jim Davis, NCI
Jim Davis, Senior Instructor, National Comfort Institute

So why do some believe this flue is unacceptable? Because it is on an outside wall, has four sides exposed to the weather above the roof, and can’t warm up properly. However, it is close to 80-years-old and has been working fine.

After reading the original ASHRAE engineering manuals, the flue size and height were typical for large buildings, especially those with flat roofs. The purpose of the extended height was to minimize wind effects that could cause flue gases to recirculate back into the building.

If you drive around most towns and keep your eyes open, you will see many old buildings with massive flues, some even bigger than this one, that never had a problem.

  • So why are officials now blaming flues for equipment not operating or venting correctly?
  • Why do Code Officials and manufacturers say these flues are too big?
  • Why are Code Officials making contractors install liners and making flues smaller?
Don't blame a big flue for condensation or tight building issues.
Popular belief is that this commercial flue is not acceptable. But is that a fact or more of an opinion?

Condensation Issues on a Big Flue?

One of the reasons mentioned for adding liners and making flues smaller is that higher efficiency equipment temperatures are lower today and don’t heat flues adequately enough. This causes condensation.

Having measured thousands of flue temperatures on residential and commercial gas-fired equipment during the course of my career, there is little difference today if the equipment is operating correctly. The exception would be condensing equipment that does not vent into standard flues.

Tight Buildings?

Another reason stated for modifying flues like this is buildings are much tighter today and are removing too much air.

I am not even sure how tight buildings have anything to do with flues. Tight buildings are a combustion air and ventilation problem, not a flue problem. If the equipment isn’t venting properly (assuming the flue is open), the problem is combustion air.

A flue is an inanimate object that does nothing but provide a path for hot combustion gases to flow from the inlet to the outlet. Much like the return duct on a furnace or the return piping on a boiler, you must apply an external force to create this flow.

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