Wednesday, February 12, 2014


Engineering News
By Dave Jones



 Well, here we go with the second issue of our brand new idea: a regular monthly newsletter.  Woo Hoo!  I hope you took a few minutes to read about electric wall heaters last month.  We will probably stir-up a hornet’s nest of discussion with this month’s subject:  Flex Duct.  If any of you have ever attended one of my code seminars, you've heard me say: “Flex duct is evil.  It’s necessary, but it’s evil.”

Why do I say that?  Well, it’s because flexible duct is the most abused item in the whole universe of HVAC.  It gets squished, twisted, warped, compressed, drooped, sagged, mashed, punctured, etc., etc., etc.  You know what I’m talking about, don’t you?  You've seen it too. It only works like it’s supposed to if it’s pulled tight and straight.

 There’s a reason flex duct comes in boxes of 25’.  But, we still see jobs where some jake-leg contractor has strung two or three boxes of flex together.  Those folks just can’t take the hint.  When I see that kind of nonsense, it’s usually because I got a call to come troubleshoot a ‘bad project’.  And, that ‘bad project’ has caused someone to complain that their spaces are too hot, too cold, energy bills are really high, or some other easily avoided problem.

Getting the air from the furnace (or rooftop unit) to the occupied rooms is the reason we have ductwork.  We size the duct work to allow the proper amount of air flow from the equipment to the room.  When we size the duct, and pick the fans, we allow for the Pressure Drop of the air flowing through the ductwork.  That is to say the amount of “Friction” that the air will experience as it travels down the duct. 

Twists and turns, rough surfaces, squishes and mashed ducts all create air friction in the ductwork.  More friction means more pressure drop.  More pressure drop means it’s harder for the air to get from the equipment to the rooms.  The amount of air that can get to a room is inversely related to the duct air pressure drop.  In other words, the more friction in the duct, the less air to the room. 

Now, we all use flex duct in order to make ceiling diffusers easy to install, or to soak-up some vibration or expansion / contraction.  That’s the intent of flex duct.  But, when people use flex duct instead of rigid sheetmetal duct, there are always problems.  Why is that?  Because it’s not smooth inside!  The wire helix, the convolutions and the ‘lack of stretch’ all make it rough.  The roughness causes air friction.  And that, my friends, reduces air flow.

Keep in mind that the friction (pressure drop) of the air through flex duct is at least FOUR TIMES what it is through a smooth metal duct.  ASHRAE  (American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.) is the authority in HVAC design; in 2004, they published a study on airflow through flexible duct. The authors (Abushakra et al) found real world pressure drops to be four to ten times manufacturers’ published values.

At issue is the degree of “compression” of the duct: They found the only way to replicate the manufacturers’ data was to pull the ends of test specimens farther apart than is realistically possible in the field (the specimens were “fully extended”); when real world configurations were tested (we say there was some degree of “compression”), resistance multiplied.

Summer is coming, and with it, the inevitable "insufficient cooling" calls. That's right insufficient cooling, not to be confused with "no cooling" calls. These calls almost always sound the same; "most of the rooms are fine, but the (fill in the blank) rooms just don't seem to cool down.

The technician checks the obvious first; filter clean? Check. Coil clean? Check... Registers open? Check. Then they leave after telling the customer- "give it a couple hours, the temperature should pull down by then". 

Most of the time, the customer calls back the next day with the same complaint. What the technician missed was the biggest stealer of air flow:  the duct work, or more specifically, the flex-duct. Before you get your undies- in-a-bundle, I'm not bashing flex-duct, I'm bashing the way it restricts air flow when it’s not sized correctly, and when it’s not installed properly.

Flex duct is great, when it’s installed correctly.   Installed wrong, it's a nightmare. You're thinking, "how can you install flex-duct wrong?" It’s easier than you realize. First and foremost, the flex-duct needs to be stretched out so the internal spirals are not "accordion".  The more stretched-out the flex duct is, the smoother it will be inside.

Second, direction changes and 90-degree bends need to be smooth and supported so the duct doesn't kink at the bend.

Third, the duct should be inspected during maintenance for crushing; no, the duct will not crush itself, but you wouldn't believe what home owners and other tradesmen do to the stuff when it’s in their way. So, that's about it; keep it straight, tight, kink and crush free, and you'll be in good shape.
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