October 2011: Duct Testing and Sealing
Duct Testing and Sealing
With the advent of furnaces in the mid-1900s, also came duct work to provide comfort in rooms and areas throughout the home. With these advances came larger houses with more interior partitions and separate zones. One can think of ducts as big, blowing/sucking forces within the home. If they are too positive, the air is pushed out of the envelope, and if too negative, unconditioned air will be drawn into the room or zone. We want most areas to be as to a close to a reading of zero as possible. If too positive or negative, plan for relief and attempt to bring the space back within a more normal range. Obviously if too negative, then exhaust pressures in the Combustion Appliance Zone (CAZ) can be detrimental for combustion appliances. This is the whole purpose we test in and do a worst case scenario, measuring air pressures/forces from room to room or zone to zone, to see how all ventilation systems (ductwork too) are interacting with regard to the CAZ.
When sealing ductwork, returns and ducts in unconditioned areas one should always be given the highest priority. Mastic is the best agent at sealing and bonding to any substrate. If the gap is greater than 1/2 of an inch than some sort of backing should be employed along with mesh and several coats over an extended drying period. It always seems that the worst leaks are often in the hardest to reach areas and one should always start at the return and work outward in a methodical order prioritizing leaks. Surfaces need to be free of dust and repaired to accept proper sealing methods. Some of the more troublesome areas are: huge panned areas, panned areas running parallel in the band joist, returns using floor and wall cavities and returns in garage ceilings.