Plenum Problems


What do many frozen DX coils, tripped furnace limits, failed heat exchangers, and failed compressors have in common?

Answer: Probably poor plenums.

Before you say to yourself; “Self, I don’t need to read this; I know how to size ductwork” do yourself a favor and read on.


Most installers know how to size supply and return ducting, and usually do it properly.

But, too many duct jobs neglect the supply and return plenums.

Who cares about plenums?


You should.

Every cubic foot of air in the system flows through the supply and return plenums before it travels down the ducting.

Unfortunately, air normally doesn’t ‘flow’ through plenums...it’s ‘shoved’ through.

Picture the air moving through the duct work like a car taking a turn. If a car can make the turn at high speed, so can the air. Want an example?

Picture driving toward a cement wall at fifty miles an hour and turning the steering wheel hard right five feet before you hit the wall.

Will the wall help “ease” you around the turn?

Not hardly.

The car is going to smash into the wall with parts flying in all directions (not a pretty sight).

Slamming air into a sheet metal wall (the plenum) has the same effect; the air hits the wall, and stops dead in its tracks.

There’s no “flowing” taking place, only “smashing”.


You can size the mains and branches perfectly but flush the whole job down the toilet with a crappy (pun intended) plenum.


Question: Who cares if the air flows nicely as long as it goes where it’s supposed to go?

Answer: You do.


Remember the frosted coils, high limit trips, and slew of other issues we mentioned?

Turbulent airflow creates “dead” areas in the plenum where there is reduced or no air movement. The dead areas can cause evaporator coil to freeze, furnace limits to trip, and heat exchangers to fail.


Question: How can you tell if a plenum is restrictive?

Answer: Look at it.

Hard, 90 degree turns and “bull-heading” are dead giveaways to poor plenum configurations.


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