High Efficiency Furnace Venting – What You Need to Know

High efficiency (condensing) furnaces are usually vented in one of two ways:

  1. Through the side of the home, known as sidewall venting.
  2. Through the roof of the home.

The type of venting setup used for a condensing furnace is dependent on many factors:

  • Existing venting systems
  • Local codes governing placement of vents (particularly for sidewall vents)
  • Location of the furnace
  • Loss of venting performance by other appliances (if performing a furnace retrofit)
  • Airtightness of home

In this article, I’ll go over all of the factors that you need to consider when venting a high efficiency furnace. I’ll also talk about some of the tradeoffs of different venting systems.

Using existing venting systems

Using existing venting systems for a condensing furnace is usually not an option. 

Older homes in the US were designed for use with non-condensing furnaces. The vents for those systems typically consisted of clay tile chimneys or double-wall metal chimneys. These types of vents cannot be used for condensing furnaces.

Why can’t clay tile or metal chimneys be used for condensing furnaces?

Clay tile and metal chimneys cannot be used as exhaust pipes for condensing furnaces due to the contents of condensing furnace’s exhaust.

Condensing furnaces contain acidic water in their exhaust. The acidic water will damage clay tile and metal chimneys.

Additionally, existing clay tile or metal chimneys were designed for use in natural draft systems. So they won’t be able to sufficiently vent the low-temperature exhaust from a condensing furnace.

The exhaust from condensing furnaces will need to be piped directly outdoors. For the exhaust pipe, PVC piping is usually used since it does not corrode.

Local codes governing the placement of vents

Many municipal and state government codes govern the placement of intake and exhaust venting. Especially for sidewall venting.

Here are a few examples of local codes that govern the placement of sidewall vents:

  • Exhaust pipe must be a minimum of 2 ft from any window, door, or deck
  • Exhaust pipe should not be installed above a walkway
  • Exhaust and intake pipes should be out of the reach of children
  • Exhaust pipes cannot be pointed in the direction of prevailing winds or neighboring homes
  • Exhaust and intake pipes must be on the same wall so they experience the same pressures

Additionally, many homeowner associations restrict the locations of vents for aesthetic purposes.

Location of the furnace

The location of the furnace will have a huge impact on how it can be vented.

For example, if the furnace is located in the basement then venting it through the roof might not be feasible. In this scenario, a better location to vent your furnace may be the sidewall of your home, since it would be much closer.

If your furnace is in your attic, then venting it through the roof would make the most sense, since the roof is nearby.

In any situation, you’ll want to find the shortest, most direct path to the outside for venting your furnace. However, local codes may not allow you to vent your furnace in the easiest way possible. So you’ll need to come to a compromise for your venting setup—one that satisfies all local codes, performance, and safety requirements.

Loss of venting performance by other appliances

Many older non-condensing furnaces share a common vent with other appliances. For instance, a non-condensing furnace may share a common vent with a gas water heater. 

If the furnace is removed, the water heater will experience a loss in venting performance.

The loss in venting performance is due to the loss in the draft from the removal of the non-condensing furnace.

In this situation, the chimney was designed for both the water heater and the non-condensing furnace. With the removal of the furnace, the water heater won’t produce enough draft on its own to remove the exhaust gases from your home. This may result in the backdraft of harmful flue gases into your home.

Here are a couple of possible solutions to mitigate the loss of venting performance after the removal of a non-condensing furnace:

  • Install a chimney liner that is properly sized for just the remaining appliances
  • Install an inducer fan to blow the exhaust fumes out of the chimney

In any situation, you’ll need an HVAC professional that’s familiar with the local codes to retrofit your home with venting equipment.

Airtightness of home

The airtightness of your home will have an impact on the venting of a condensing furnace.

Generally, if your home is airtight, then you’ll need a two-pipe setup for your furnace venting. A two-pipe setup is required because your furnace will need a direct path to the outside for intake air.

Older, less-airtight homes can sometimes use a singe-pipe setup. Single-pipe setups use indoor air from a sufficiently ventilated area such as an attic or crawl space. If the central furnace is installed in one of these areas then it must comply with local building codes regarding combustion air intakes.

Single versus two-pipe venting

High efficiency furnaces can use either single or two-pipe venting setups:

  • A single-pipe (central) furnace uses indoor air for combustion air intake.
  • A two-pipe (direct-vent) furnace has an intake pipe that draws in outside air for combustion air.

Single-pipe venting pros and cons

Single-pipe venting pros:

  • Increased fresh air ventilation. Using a single-pipe venting setup creates negative pressure inside your home. Fresh air will come into your home through the cracks around your doors, attic, windows, etc.

  • Lower cost. A single-pipe venting system requires less material and labor to install. There will also be fewer penetrations through your home to get the pipe outside.

  • More reliable. In a single pipe system, there isn’t a chance for the intake vent to be blocked by something outside. 

Single-pipe venting cons:

  • Less efficient. With a single-pipe system, your home will experience cold drafts of outside air that trickle in through the cracks around your home.

  • Home depressurization. With the intake vent inside, your home will experience a slightly negative pressure. This means that dirt and particles from the outside will infiltrate your home’s interior.

  • Less humidity control. When outside air creeps into your home, it will be harder to maintain a consistent humidity level.

Two-pipe venting pros and cons

Two-pipe venting pros:

  • More efficient. With a two-pipe system, the furnace won’t create negative pressure inside your home. This means that more warm air will stay inside your home.

  • Better humidity control. A two-pipe furnace won’t introduce any outside air drafts into your home, so it will be much easier to maintain your home’s humidity level.

  • Consistent air supply. A two-pipe furnace has a direct venting system to intake fresh air from the outside. So it will never have pollutants from inside the home in its combustion air.

Two-pipe venting cons:

  • Decreased fresh air ventilation. With a two-pipe system, your home will generally receive less fresh air than a single-pipe system. To combat this, some homeowners install a heat recovery ventilator (HRV).

  • Higher cost. A two-pipe system requires more materials and labor to install. Also, extra penetrations must be made through the wall or roof for the intake pipe. To mitigate the need for additional penetrations, some installers use concentric pipes—where the exhaust pipe is routed inside of the intake pipe.

  • Lower sound levels. With a direct-vent system, the air intake will be outside your home. This means that you’ll hear less of the air “whooshing” sounds inside your home.

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