Stopping Air Leaks in HVAC Systems

With energy loss through HVAC ducts running 30 percent or more, it is well worth the time and effort to track down and repair the leaks.

Many studies have shown that energy losses in heating/cooling ducts in most homes can run 25 percent to 30 percent — or more. The majority of these losses can be traced to air leaks in the equipment or ductwork that can easily be found and repaired. Although typical costs for remediation run $350 and up, the savings will usually be recouped within just the first year.

Finding air leaks can be time-consuming, but there are some tricks that will not only speed up the job but make the inspection much more accurate. One is the use of a blower door test: A large fan at the front door will pull air out of an otherwise closed-up house, creating drafts from leaks that are much easier to detect than the typical slow infiltration of air. (The blower door also reveals any leaks in the building envelope itself.)

Another tool called a duct leakage detector or a flow hood blows air into the duct system through a register after all other returns and registers have been sealed. With both tools, special instruments indicate any differences in pressure or where air leaks occur.

Here's a list of what you'll need to repair such leaks in your customers' HVAC systems:

Materials and Tools:

  • battery-operated drill gun, drill bits and magnetic hex-head screwdriver bits
  • ladder
  • flashlight
  • fool for installing duct zip ties tightly
  • sheet metal screws (hex head)
  • duct zip ties (for flex duct)
  • duct mastic and brush
  • tape mastic (aluminum tape with 15-mil butyl backing; can be used in areas up to 200 degrees F)

Equipment Air LeaksLeaks in HVAC air handlers can usually be seen, felt and heard. Common places to find them are at ill-fitting cover panels or those with missing screws; where air-conditioning pipes enter the housing (grommets may be missing or leaky); the entrance of the attached air-filter insertion point (base); and plenum and return-air connectors. Air leaks might also occur if whole-home air filters have been improperly installed. Use this checklist to inspect and repair these energy wasters:

1. Make sure that all panels and connections are secured with screws and that all panels are in place.

2. Straighten bent panels and repair their seals.

3. Stop all leaks with liquid or tape mastic or with permagum, which stays pliable and can be removed for access later.

4. Make sure that all interior insulation is in place and tightly secured to the metal walls and panels.

Unfortunately many air handlers and furnaces have air leaks all over them because of poor manufacturing practices. We cannot rebuild these in the field. They can be sealed with permagum, or with silicone for a permanent seal.

For repairing simple leaks in the air ducts:

1. Make sure that all metal fittings are firmly screwed together on four sides, then covered with liquid or tape mastic, and fully re-insulated.

2. The s-cleat and drive connections on ductwork are notorious for leaking. All these should be sealed with duct sealer mastic.

3. On flex duct with no metal ends, secure the inner lining to the metal boot firmly with duct zip ties, then secure the insulation and outer wrap. Zip ties are fine for the exterior insulated wrap, but duct zip ties alone on the inner liner will not secure the duct to the slick metal fitting. Use sheet metal screws with washers at the spiral wire to prevent the inner liner from slipping off. (Codes demand that insulated flex duct be secured with UL 181-B-FX tape.)

4. Cover all connections of accessories (such as humidifiers) with liquid or taped mastic. (Remember that many of these must be removed later for service.)

Leaks From Duct DamageDucts are often damaged when people come in contact with them in storage areas, such as attics. Such damage can result in major or minor leaks, depending on how bad it is, and it requires a thorough inspection to find it:

  • Metal ducts or flex ducts that aren't properly secured with screws or duct zip ties can fall apart, resulting in major air losses.
  • Fiberglass ducts can be crushed or punctured.
  • The outer covering of flex ducts can tear, resulting in energy losses and even an accumulation of moisture between the duct's inner and outer liners, which leads to worse problems.

Any major damage may require replacement of the section of ductwork, but minor damage and rips may be sealed using 15-mil butyl tape mastic with a thick adhesive on the back.

Plenum or Mixer Box Connections 1. Make sure that all seams are fixed tightly and screwed together (when necessary).

2. Make sure that all connecting boots are tight at the seams and sealed with liquid or tape mastic.

3. Make sure that all insulation is in place and well-secured.

Air Diffuser Boxes and Grilles. 1. Make sure that all seams are fixed tightly and screwed together (when necessary).

2. Make sure that all connecting boots are tight at the seams and sealed with liquid or tape mastic.

3. Make sure there are no leaks between the discharge grille facing and the space between the wall studs or ceiling joists.

4. Make sure that all non-grille surfaces are insulated.

Air ReturnsLeaks in air returns can be hard to locate because the negative pressure is difficult to both see and hear. Finding and repairing such leaks requires a close inspection. And non-ducted returns (as in spaces between wall studs or ceiling joists) are notorious for being sources of both indoor air pollutants and leaks from outside or unconditioned sources. That's why it is best to fit all air returns with a tight metal or plastic lining, or thoroughly coat them internally with mastic to cover all leak sources.

Once you've secured all connections to the ducts, as listed above, verify that all seams are tight and covered with mastic, and that there are no leaks between the return grille facing and the space between the wall studs or ceiling joists.

Filter HousingsWe often find separate filter housings (such as for electronic air filters) located in unconditioned areas. Because filters are frequently changed or cleaned, they can start to leak air. Here, the use of any sort of mastic would become a hindrance to any sort of future servicing, so we recommend repairing only any damaged seals. Permagum is good for this job. It is easy to put on and easy to remove.

An annual check of the customer's HVAC system will catch any new problems before they start running up energy bills and affecting indoor air quality. These inspections and repairs are a good investment in a comfortable and energy efficient home.

Jim Wheeler has more than 25 years of experience in installation and repairing heating and cooling systems. He is also a writer and educator on efficient and high-performance HVAC equipment.

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