Description

Single pipe exhaust systems rely on air drawn from within the home for combustion. This often means that passive vents must be installed to provide makeup air for the furnace causing unnecessary heat loss. Furthermore, it is difficult to control furnace combustion in this configuration and this may cause poor system efficiency and lower equipment service life. There is also the risk of depressurizing the home if more air is exhausted than supplied: combustion gases can spill out of the furnace into the home threatening occupant safety.

Challenges

ROUTING: Unlike a standard or mid-efficient B-vented furnace, a high-efficiency gas furnace requires PVC⁹ venting. Locating a 2-pipe route to the exterior can be challenging, particularly when the furnace is in the middle of a finished home.

INSTALLATION DETAILS: Horizontal PVC venting must be supported every 6' along the route.

PENETRATIONS: Every venting route will ultimately penetrate the exterior vapour barrier, insulation
and rain screen.

SIDE YARDS: Some Authorities Having Jurisdiction (AHJs) do not allow side yard venting in close proximity to neighbouring houses.

Solutions

• There are three commonly used vent terminations and each has a preferred function. Note that BC Housing recommends venting combustion gases and terminating new vent pipes at the roof where possible¹⁰.
  • HORIZONTAL: The most common venting route is horizontally through the exterior wall. Finding a clear joist space or dropping the vent below the finished ceiling should be considered. A common solution is to remove the redundant combustion air pipe, so that this space can be used for PVC venting. In this case, a concentric vent termination kit can utilize the existing hole through the exterior wall. If the decision is to find a new route, a two hole penetration through the exterior wall can be terminated in two ways: with a termination kit flush to the wall or with two PVC pipes with elbows up and down to restrict exhaust air back-drafting through the combustion pipe.
  • VERTICAL: Another common venting route is through the ceiling or roof. This can be more easily achieved in single level ranchers or in two-story homes by using the B-vent route through the floor above, particularly in situations where there is either no gas DHW tank or a condensing DHW is in place or being considered. When replacing the existing B-vent route, a concentric termination can be used at the existing B-vent flashing.
  • DOWN: A less common venting route is down and out - typically, this is used in a crawlspace application. Special drain / vent kits are needed to deal with condensation in this configuration, which are typically supplied by the manufacturer.

• Where the venting penetrates the exterior vapour barrier, insulation, and rainscreen, attention must be given to insulation repair, air sealing, and drip flashings to ensure that the venting does not compromise the building enclosure.
• In jurisdictions that do not permit side yard venting where the only route is horizontal, venting may have to exit the building enclosure and travel up the exterior of the building - often through a soffit, terminating above the roof. There are two considerations in this situation: 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. If options are limited, applications can be made to the AHJ for a variance and they may approve sidewall venting under certain circumstances.