Troubleshooting a Roof Jack

A key element in the operation of your gas furnace is the roof jack. Be alert to any possible problems with your roof jack, used to vent furnace flue gases from your home. Learn more below about troubleshooting a roof jack to improve the operation of your furnace and keep your family safe and healthy.

Roof Jack Assembly

The roof jack is a double walled pipe system. The inside pipe is 5 inches in diameter. The flue gases exit through this pipe. The exterior pipe, 8 inches across, provides combustion air to the furnace.

Furnace Problems due to Roof Jack

Many furnace problems can occur if the roof jack pipe assembly becomes blocked in either or both pipes. These include furnace smothering and unbalanced draft. If the roof jack seams crack or split, the air intake and flue venting will also stop working correctly.

Furnace Ignition Smothering

Blockage in the outer pipe can lead to furnace ignition smothering. The furnace will light, but then will stop burning fuel a few moments later. Make sure the roof jack outer pipe assembly is clear of debris. Debris can enter the pipe during roof construction or due to high winds blowing external materials into the pipe assembly. Slide the air inlet up to check for debris in the interior pipe as well.

Unbalanced Draft

More common in M1G furnaces than M1M furnaces, unbalanced draft can result in the air inlet pipe acting as a flue and venting out exhaust gases rather than supplying fresh air for ignition and fuel burning.

Roof Jack Damage

If the roof jack's seams become split or cracked, the balance between air intake and flue exhaust will not be maintained. The furnace can lose air for combustion or will smother due to incomplete venting of flue fumes. The roof jack can become damaged in cold areas due to build-up of ice inside the pipes. Apply an all-weather roof jack cap to prevent snow and ice build up on the roof jack. If average snowfall in your area is greater than 7 inches, add a roof jack extension to the outer end of the roof jack assembly.

Attic Leakage due to Roof Jacks

The roof jack's exterior sits in a rubber collar over a hard plastic frame to prevent leakage of rain or snow into your home's attic. Inspect the roof jack exterior assembly yearly after 5 years. You can catch deterioration of the rubber collar before leaks occur.

Replacing the Rubber Collar Gasket

Replace the rubber collar gasket with the same type as was originally installed. Get the correct size, as the roof jack's main pipe is 8 inches in diameter, rather than 3 inches or 4 in the case of other external piping. This repair should prevent further leakage, unless the plastic frame has cracked or split. In that case, replace both the plastic frame and the rubber collar gasket. It is recommended to seal the rubber collar gasket to the roof jack's exterior pipe with a water-resistant polyurethane caulking, to further reduce the chance of water penetration. Apply this caulking to the roof screws or nail heads that attach the roof jack to the shingles.