Older furnace equipment typically draws its combustion air from within the mechanical room and exhausts flue gases through a vent that terminates at the roof level. Replacing an older furnace with a new, high-efficiency condensing unit typically involves installing a new plastic exhaust vent and combustion air pipe that terminate on the outside of the home (Figure 3).

In practice, installing this two-pipe venting can be challenging particularly where the existing furnace appliance is located in the centre of a finished home or where the creation of new wall or roof penetrations is difficult or undesirable. As a result, "band-aid" solutions are often applied in an attempt to satisfy the venting and combustion air requirements of high-efficiency furnaces.

The following practices are not consistent with quality furnace installations and are not compliant with the FortisBC Furnace and Boiler Replacement Program requirements:

Combing Venting Materials

The new venting must comply with ULC-S636² and consist "entirely of factory-made parts, each designed to be assembled with the others..." in order to be compliant with CSA B149.1 - Natural Gas and Propane Installation Code. Combining venting materials from different manufacturers including pipes, fittings, primers, and cements is not ULC S636 compliant. Poor seals at venting joints is a safety concern and may result in flue gas leakage.

Venting Through an Existing Chimney

In many installation scenarios, routing the new plastic piping through an existing chimney is more favourable than other pathways. While this practice can be acceptable under some conditions, the venting must be adequately supported along its length (Figure 4). Per CSA B149.1 - Natural Gas and propane installation code, the new venting must be supported independently of the furnace appliance. Additionally, the entry point of the plastic venting into the chimney should be sealed such that the penetration is made airtight (Figure 5).

Combustion Air Pipe Termination

While it is well understood that flue gas venting must terminate at the exterior, the combustion air pipe is frequently terminated within the mechanical room, the attic, or within an existing combustion air duct (Figures 6, 7). These approaches are not compliant with the FortisBC Furnace and Boiler Replacement Program. The new plastic combustion air pipe is required to terminate on the outside of the building either above the roof or beyond an exterior wall.

Solutions

• In two and three-story homes, the most common venting route is horizontally through an exterior wall (Figure 8). Sometimes a combustion air opening already exists and the hole can be reused
  for the new two-pipe venting once the existing duct/pipe has been removed. In situations where a new hole through the wall is required, the combustion air pipe can be routed adjacent to the existing furnace exhaust venting and terminated with a concentric vent termination kit flush to the wall or with two pipes with elbows up and down, respectively (Figure 9).

• In jurisdictions that do not permit side wall venting where the only route is horizontal, venting may have to exit an exterior wall and travel up the outside of the building - often through a soffit, terminating above the roof. Note that in this configuration the exterior venting must be insulated along its length and a custom, longer concentric vent termination will be needed to maintain a single flashing at the roof level.
• It is often practical to install the new venting vertically by using the existing B-vent route through the floors and roof above. A concentric termination kit can then be used at/through the existing B-vent flashing complete with a new storm collar.
• Where the home has a crawlspace, the new venting can be routed through the floor and out through the above grade basement foundation. Note that routing this way will require special drain/ vent kits to deal with the condensation that will collect in this configuration (supplied by the furnace manufacturer).

Building Science Considerations

The installation of a high-efficiency furnace may result in lower energy bills and a more comfortable home. However, it can also lead to moisture and safety related issues if the unintended consequences of the upgrade are not considered. For example, the installation of new plastic venting often requires additional wall or roof penetrations that must now be waterproofed and airsealed.

The commonly observed practice of sealing the perimeter of new venting at the wall cladding (siding/stucco) is not sufficient - additional weatherization work is required. All new holes in the air/ vapour barrier (polyethene sheet) and moisture barrier (building paper/sheathing membrane) must be sealed to adequately prevent rainwater leaks and air leakage condensation. The recommended detailing for a sidewall vent penetration (exterior and interior) is shown at the end of this document (see Appendix). For further information on the various interactions that can occur between the HVAC system, building enclosure, and other building systems refer to the Best Practices Guide for Air Sealing and Insulation Retrofits³ published by BC Housing.

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² Vent systems must be composed of pipe, fittings, cements, and primers that are listed in ULC-S636 - Standard for Type BH Gas Venting Systems

³ Best Practice Guide: Air Sealing and Insulation Retrofits for Single Family Homes available at https://www.bchousing.org