From ASHI @ Home by Carson Dunlop
The function of the venting system on a natural-draft, gas-fired, forced-air furnace is to carry the products of combustion safely and quickly out of the house (before they cool) using the natural buoyancy of the warm gases (convection). The products of combustion get heavier and wetter as they cool. We want to avoid their cooling to the point where they linger in the house or chimney, and we want to get them outside before they condense. The products of combustion temperature are around 350°F after they are diluted. We want to keep the products of combustion above the dew point (about 125°F to 130°F) until they leave the top of the chimney or vent.
The components of the venting system include –
- the exhaust flue components of the appliance, including the draft hood
- the vent connector
- the chimney or vent
The venting system of the appliance is integral and not much can be seen of it. The draft hood may be part of the appliance or may be part of the vent connector at the top of the appliance. The draft hood, incidentally, is also called the draft diverter. The vent connector joins the appliance to the chimney.
The important points include the following:
- Keep the products of combustion inside the exhaust system. Leaks are bad.
- Keep the exhaust path as short and as straight as possible (although the vertical run of a chimney can be an asset in maintaining draft).
- Slope the vent connector and chimney so the exhaust gases rise as they leave the chimney, and never have to dip.
- Keep the chimney as warm as possible.
- Where two or more appliances are manifolded together, do not obstruct the flow from one appliance with the other.
- Keep vent connectors and chimneys away from combustible materials.
- Make sure extra air (dilution air) is available to maintain the draft and keep the burner working properly.
- Make sure the vent connector and chimney sizes are neither too great nor too small for the appliance.
- When venting several appliances, unless special consideration is given, only gas appliances should share a flue. There are circumstances when other appliances (oil and even wood) may be acceptable on the same chimney flue, but these are special situations and should only be inspected and approved by a specialist.
Vent connectors are typically single-wall metal pipes on conventional gas furnaces. They are usually made of galvanized steel or aluminum. They run from the appliance to the chimney, and the shorter the vent connector, the better. Vent connectors are also called exhaust flues, C vents, vent pipes, stack pipes, flue pipes, chimney connectors, or breachings. The vent connector allows the exhaust gases to move from the furnace to the chimney.
In this discussion, we’ll go over a few common conditions found with vent connectors.
Vent Connector too long
The horizontal run of the vent connector should be as short as possible. This is an installation issue. Exhaust spillage into the house is the implication. The greater the near-horizontal length of the vent connector, the more difficult it is for exhaust products to travel from the appliance to the outdoors. Look for vent connectors to be less than 5 feet long. Where they are 5 to 10 feet long, the situation may be acceptable, but it’s marginal. Vent connectors more than 10 feet long should be described as suspect and investigation by a specialist should be recommended.
The vent connector length should be no more than 75% of the chimney height. Most vent connectors are single-wall. When a double-wall B-vent is used, it can be up to 100 percent of the chimney height. Recommend further investigation where vent connector lengths are suspect. This is a complicated issue and you should not take a firm stand you can’t defend with authority.
This vent connector for the furance is 5 to 10 feet in length, and is marginally acceptable- under 5 feet would be preferable
Vent connectors should be adequately supported with non-combustible material so they are intact and maintain their slope. Manufacturers set support requirements, often saying supports are needed every 4 feet and at every elbow. The cause of poor support is most often an installation issue. Mechanical damage or amateurish homeowner modifications are other possibilities. The implications are products of combustion finding their way into the house. Common sense is appropriate here. If the slope is appropriate, support is probably adequate.
Sections of vent connectors should be well secured. Three screws is common. Other connection mechanisms may be acceptable. Poor connections are usually an installation issue. The implication is leakage of combustion products into the home. Visually examine all connections to ensure that they are intact. Sags or changes in direction often indicate a poor connection. Watch for disconnected vent connectors. These are most common where the vent connector joins the chimney or the furnace. Check the entire circumference of each connection.
Lack of proper support at the corners have resulted in condensation and rust in this vent connector.
Inadequate clearance from combustibles
Look for a 6-inch clearance from combustibles on vent connectors unless a B vent is used, in which case a one-inch clearance is all that is required. B vents are doublewalled with an air space between the walls. This is an installation issue. If the clearance from combustibles is not adequate, there is a fire hazard. Watch for the clearance from walls and ceilings. Many authorities consider drywall combustible because of the paper surfacing.
Vent connectors should not go through walls or ceilings to connect to remote chimneys. While these arrangements may be allowed in special circumstances, they should be written up as suspect. Combustible clearances can sometimes be reduced with protective materials.
There is an inadequate combustible clearnace between the rigid foam insulation and this vent connector.
We have quickly introduced the topic of vent connectors as part of the home inspection, and have outlined a few of the conditions that are typically found. More information about vents and chimneys, and many other conditions can be found in the ASHI@HOME training program.