Duct Sealing Issues


As we hear more and more about duct sealing programs, and the promise of energy savings to our customers, we must be aware that there’s no guarantee of reducing utility costs in every case when you seal ducts. As a matter of fact, there’s a 50/50 chance power consumption may increase.

A poorly interpreted formula in ASHRAE 152 has been applied in some energy programs to promise “deemed savings” to consumers if they will simply have their duct sealed. For over a decade programs have paid companies to knock on doors of rate payers, and offer free duct sealing. In return, some fuzzy math in a back room is on file boasting energy savings as the flag of victory is waived by would-be environmentalist believing they are saving the earth. Sorry folks, the Kw savings are not panning out.

Let’s take a look at valid reasons why the legitimate practice of sealing ducts, and doing nothing else, actually increases utility bills almost as often as it may save a couple bucks a month. If you’re an HVAC contractor that simply slaps a little pookey on the duct system with a change out, you might want to stop doing that.

Duct System Capacity

The problem is most duct systems are extremely undersized and severely lack in adequate airflow. Without adequate airflow, the system does not meet manufacturer’s specifications and cannot operate as designed. That 18-SEER may be running like a 9-SEER. (This happens more often than you can imagine.) NCI studies show the average US residential HVAC system operates at a total external static pressure of .82-in. This is equal to human blood pressure of 210 over 130. (Normal is 120 over 80) Yes, we have a major problem, considering that most residential fans are still rated at only .50-in. of pressure.

Shouldn’t We Seal Ducts?

While ducts should not leak, and most do, it seems that sealing ducts should be a gimme. Just like pipes, duct should not leak. The difference is when pipes leak, the ceiling falls in or the house blows up. The results are not so disastrous when ducts leak, although the cost may be greater over time. Ducts must be sealed every time, but only after we have added more duct system capacity.

The Cure

In most cases, the best way to add duct system capacity is to add an additional large return duct and modify the filter system to reduce filter pressure drop. There is a simple quick test that can be done in just a few minutes to determine if additional capacity is needed: measure total external static pressure. Then compare that pressure to fan rated pressure. If measure pressure is more than 10% higher than rated pressure, measures must be taken to reduce system operating pressure.

It’s that simple. If only duct-sealing programs would take an hour and carefully instruct participants to add static pressure measurement to a duct-sealing job, then take appropriate measures to remedy the problem, energy savings could actually be verified. Consumers would no longer suffer from the ill effects of “wish a watt” or deemed savings, but actually enjoy verified energy savings. The tables could be turned and energy savings through duct sealing could be increased substantially.

More important, as contractors, we have a direct responsibility to those we serve to deliver what we promise. This is an individual choice. Will you accept the challenge?

Other Hazards of “Simply” Duct Sealing

When ducts are sealed, pressures in a building change.

Say a mechanical room has a visible supply duct leak near the discharge of a furnace. Of course, that supply duct leak should be sealed. That’s a good thing right? Sure it is; unless there’s a hidden return duct leak behind the furnace.

If the supply leak is sealed and the return leak remains, a negative pressure will be created in the mechanical room every time the fan operates. A negative pressure of as little as .015-in. may cause the flue to back draft. The carbon monoxide from the flue will be pulled into the HVAC system and be distributed throughout the home. On a cold night carbon monoxide levels may skyrocket, and the occupants may not wake up.

If you’ve been following several carbon monoxide court cases recently, I hope you don’t suffer the fate of several unfortunate contractors and inspectors tangled in severe legal battles with their freedom at risk.

Leave the Pack Behind

Simply sealing ducts is like wearing booties. Years ago, duct sealing differentiated you from your competitors. Today, everyone’s jumped on the band wagon.

One way to step ahead of the pack is to take the time to measure total external static pressure when replacing equipment. Involve your customers in the process and teach them about system performance to help them understand the difference between you and the pookey boys that give away duct sealing and the unfulfilled promise of energy savings.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company with technical and business level membership organizations. If you're an HVAC contractor or technician interested in a free procedure showing you how to measure total external static pressure, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.

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