Do Your Duct Plenums Blow? (Some Pun Intended)

stock-photo-duct-work-of-air-conditioner-311328491 (1)What do frozen DX coils, tripped furnace limits, failed ECM motors, failed heat exchangers, and failed compressors all 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 duct-work”

do us both a favor and read on.

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

They take out their handy Duct-Calculator, select the cfm, align it with .15’wc, and select the duct size from the chart. Easy, right?

Not so fast.

First, .15”wc pressure drop is based on 100 feet of straight duct, what does that have to do with the 25 foot run you’re installing?

Not much.

But sizing straight ducting is a topic for future article. Today, we’re all about plenums.

Who cares about plenums?

You should.

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

Unfortunately, air normally doesn’t ‘flow’ through the plenums…it’s ‘shoved’ through the plenums like a runaway tractor trailer.

Picture the air moving through the ductwork like a car; if a car can make the turn at high speed, so can the air. Want an example?

Picture driving a car toward a cement wall at fifty miles an hour and turning the steering wheel 5 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 (not a pretty sight)

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

There’s no flowing taking place, only smashing.

You can size the mains and branches perfectly and flush the 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 the slew of other issues we mentioned?

One of the primary causes is restricted or uneven airflow.

Turbulent airflow creates “dead” areas in the plenum where there is no air movement. The dead areas cause coil freezing, furnace limits to trips, and heat exchanger failures.

What about the ECM motor failures; how does restricted or uneven airflow affect those?

I’m happy you asked, Grasshopper.

The excessive static pressure causes the motor to work harder to overcome the resistance to flow, working the motor into an early grave.

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/duct configurations.

The second way is taking static pressure reading at various points of the duct.  Wherever there is a drastic change in static pressure there’s resistance to flow.

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Patrick is Zen HVAC’s diagnostic and training guy. Patrick started in the trade the day he left technical school and never looked back. He's served in various technical and training roles in the HVAC industry but specializes in system troubleshooting and diagnostics, retro commissioning, and technical training. His moto: If I can understand it, anybody can. Patrick uses the Zen common sense approach to teach Patrick’s Likes- His Wife, kids and dog. Old pickup trucks. Hiking. The industrial Revolution. Patrick’s Dislikes- Taking work too seriously. Anything unintuitive. Emoticons :( Patrick’s Favorite famous person- Theodore Roosevelt “I am only an average man, But I work harder at it than the average man" Famous Patrick Quote- “Well, that was stupid of me”

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