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Building Science

The Uniform Mechanical Code Looks to Limit Flex Duct

A current proposal recommends limiting length to 5 feet and requiring rigid elbows

Long, sagging flex duct like this would not be allowed if the Uniform Mechanical Code adopts a new proposal to limit flex duct.
Image Credit: Energy Vanguard

Some people love flex duct. Some people hate it. Some of us are OK with it if it’s done right.

As I’ve documented here numerous times, many flex duct installations leave a lot to be desired. They sag. They’re kinked. They’re twisted around pipes.

If there’s something bad that can be done with flex duct, someone has done it. And the result of all those mangled flex duct installs is poor air flow, which creates comfort problems, uses more energy, and is one reason systems get oversized.

A proposal to limit flex duct

Now, some professionals in the industry are pushing for limits on the use of flex duct. The I-codes from the International Code Council seem to get most of the attention, but there’s another set of model codes developed by the International Association of Plumbing and Mechanical Officials (IAPMO). The Uniform Mechanical Code (UMC) is their code for heating, cooling, ventilating, and refrigeration systems. As with the I-codes, it gets updated every three years.

Among the changes recommended for the 2018 UMC is this:

603.4.1 Length Limitation. Factory-made flexible air ducts and connectors shall be not more than 5 feet (1524 mm) in length and shall not be used in lieu of rigid elbows or fittings.

That’s a pretty strong limitation. If passed, adopted, and enforced, I believe it would have a positive effect on air distribution for HVAC systems. And although that proposal is labeled “Length Limitation,” it actually goes further than that by saying you have to use rigid elbows and fittings, too.

At one point, there was an exception for residential buildings to the proposal above. In one of the UMC meetings, however, that exception was struck from the proposal.

A few reasons to limit flex, including firefighter safety

Since I’ve been…

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