Making the Case for Zero-Leak Ducts

Until this year, a fourth ring for a Duct Blaster didn’t exist. But it was recently released (the one at right is signed by Gary Nelson of the Energy Conservatory). Extreme duct tightness can be verified by an iPhone app that converts the manometer pressure into cfm (the manometer has program­ming only up to ring 3), reading “Low” to indicate total leaks of 1 cfm or less.

The Duct Blaster system consists of a calibrated "fan" that connects to the HVAC duct system, and a digital manometer that converts pressure readings into volumetric airflow in cubic feet per minute (cfm).

When measuring the tightest duct systems, the user installs "ring 3," the one with the smallest hole. This enables the manometer to record leaks as small as 10 cfm, equivalent to about 1% air leakage for a 2 1/2-ton central air conditioning system. When meticulous installers put together ducted systems that leak less than this, the display flashes the word "LO," which is a signal to install the next higher numbered ring.

Our crew knows that when metal parts show up on the jobsite, for example, they are wiped down with denatured alcohol in preparation for receiving future sealant. And when supply boots are cut in, they're caulk-sealed and foamed in place before the crew moves on to the next one.

All adjustable elbows that are installed anywhere there is a planned deviation from a straight run of flexible ductwork are taped, secured, and coated "nickel-thick" with mastic and left ready for flexible duct installation after curing.

And all plenum attachments and joints are made with lots of screws and collared takeoffs, using jugs of mastic and reinforcing mesh as needed.

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