Everything You Need to Know About Efficiency Furnace Venting

A high-efficiency furnace is an energy-efficient alternative to traditional furnaces. These furnaces are designed to reduce energy consumption and lower heating costs. However, to ensure that a high-efficiency furnace operates safely and efficiently, it requires a unique venting system. 

In this article, we will discuss everything you need to know about high-efficiency furnace venting, including types of venting systems, proper installation, and maintenance. I’ll also talk about some of the tradeoffs of different venting systems.

High Efficiency (condensing) Furnaces are Typically Vented in Two Ways:

  • Through the side of the house known as the side vent.
  • Through the roof of the house.

The Type of Vent Configuration used for a Condensing Furnace depends on many factors:

  • Existing ventilation systems
  • Local regulations for the arrangement of the ventilation slots (especially for side vents)
  • location of the furnace
  • Loss of ventilation capacity due to other devices (if you retrofit the furnace)
  • house waterproofing

Use of Existing Ventilation Systems

The use of existing exhaust air systems for a condensing furnace is usually not an option.

Older homes in the United States were designed for use with non-condensing furnaces. The vents of these systems usually consisted of clay brick chimneys or double-walled metal chimneys. These vent types cannot be used for condensing furnaces.

Why can’t Clay Bricks or Metal Chimneys be used for Condensing Furnaces?

Clay brick and metal chimneys cannot be used as flue pipes for condensing furnaces due to the contents of condensing furnace flue gases.

Condensing furnaces contain acidic water in their exhaust gases. Acidic water damages clay bricks and metal chimneys.

Additionally, existing clay brick or metal chimneys were designed for use in natural draft systems. They will therefore not be able to adequately evacuate the low-temperature exhaust gases from a condensing furnace.

Exhaust gases from condensing furnaces must be routed directly to the outside. PVC pipe is usually used for the exhaust pipe because it does not corrode.

Local Regulations for Vent Placement

Many local and state government codes regulate the placement of inlet and outlet ducts. Especially for side ventilation.

Here are some examples of local codes governing the location of side vents:

  • The exhaust pipe must be at least 60 cm away from windows, doors or terraces
  • The exhaust pipe must not be installed over a walkway
  • Exhaust and intake pipes should be kept out of the reach of children
  • Exhaust pipes must not be directed towards the prevailing wind or towards neighboring houses
  • The exhaust and intake pipes must be on the same wall so that they are subjected to the same pressures

Additionally, many homeowner associations restrict the placement of air vents for aesthetic reasons.

Location of the Furnace

The location of the furnace has a big impact on how well it can be vented.

For example, if the furnace is in the basement, venting through the roof may not be possible. In this scenario, the side wall of your home might be a better place to vent your furnace as it would be much closer.

If your furnace is in your attic, it would make more sense to vent it through the roof since the roof is nearby.

In all situations you want to find the shortest and most direct route outside to vent your furnace. However, local regulations may not allow you to vent your furnace in the simplest of ways. You must therefore find a compromise for your venting configuration that meets all local code, performance and safety requirements.

Loss of Ventilation Performance due to Other Devices

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

When the furnace is removed, the water heater loses venting capacity.

The loss of venting performance is due to draft loss from shrinkage of the kiln without condensation.

In this situation, the chimney was designed for both the water heater and the condensing furnace. With the furnace removed, the water heater will not create enough draft to remove exhaust fumes from your home. This allows harmful fumes to enter your home.

Here are some possible solutions to mitigate the loss of vent performance after removing a non-condensing furnace:

  • Install a chimney liner that is just the right size for the remaining appliances
  • Install an inducer fan to blow exhaust gases out of the chimney

In all situations, you need an HVAC professional who understands local codes to retrofit your home with air handling units.

Airtightness of Home

The airtightness of your home affects the ventilation of a condensing furnace.

Generally, if your home is airtight, you will need a two-pipe configuration for your furnace vent. A two-pipe setup is required because your furnace requires a direct route to the outside for air intake.

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

One or Two-Pipe Ventilation

  • High efficiency furnaces can use one or two tube vent configurations:
  • A one-pipe (central) boiler uses room air for combustion air intake.
  • A two-tube furnace (direct ventilation) has an intake tube that draws in outside air as combustion air.

Advantages and Disadvantages of Single-Tube Ventilation

Advantages of Single Tube Ventilation:
  • Increased fresh air supply. Using a single pipe vent configuration will create negative pressure in your home. Fresh air enters your home through the cracks around your doors, attics, windows, etc.
  • Lower costs. A single pipe ventilation system requires less material and labor to install. There will also be fewer penetrations in your home to get the pipe outside.
  • more reliable. With a single tube system, there is no chance of the inlet port being blocked by anything outside.
Disadvantages of Single Tube Ventilation:
  • Less efficient. With a single pipe system, your home will experience cold drafts from the outside seeping through the cracks around your home.
  • Pressure relief at home. With the intake port inside, your home will be subjected to a slight negative pressure. This means that dirt and particles from the outside enter your home.
  • Less humidity control. When outside air enters your home, it becomes more difficult
  • to maintain constant humidity.

Advantages and Disadvantages of Two-Pipe Ventilation

Advantages of Two-Pipe Ventilation:
  • More efficient. With a two-pipe system, the furnace does not create negative pressure in your home. This means more warm air stays in your home.
  • Better moisture control. A two-pipe furnace does not bring drafts into your home from the outside, so it is much easier to maintain humidity levels in your home.
  • Constant air supply. A two-tube furnace has a direct ventilation system to draw in fresh air from the outside. It will therefore never have pollutants from inside the house in its combustion air.
Disadvantages of Two-Pipe Ventilation:
  • Reduced fresh air supply. A two-pipe system generally gets less fresh air into your home than a one-pipe system. To counteract this, some homeowners install a heat recovery fan (HRV).
  • Higher cost. A two-pipe system requires more materials and labor for installation. In addition, additional penetrations through the wall or the roof must be made for the inlet pipe. To reduce the need for additional penetrations, some installers use concentric tubes where the exhaust tube is routed inside the intake tube.
  • Lower noise levels. With a direct ventilation system, the air intake is outside of your home. This means you will hear less breath noise in your home.

Types of Venting Systems:

There are two types of venting systems used for high-efficiency furnaces, including:

  1. Direct Venting System: A direct venting system uses a two-pipe system to draw air from outside for combustion and exhaust gases out through another pipe. This system is ideal for homes with tight building envelopes and high-efficiency furnaces with sealed combustion chambers.
  2. Power Venting System: A power venting system uses a single vent pipe to exhaust gases out of the furnace. The system uses a fan to force the exhaust gases out of the vent pipe. This system is ideal for homes without access to a chimney or homes with long vent runs.

Proper Installation:

Proper installation of a high-efficiency furnace venting system is crucial to ensure that the furnace operates safely and efficiently. Here are some essential factors to consider when installing a high-efficiency furnace venting system:

  1. Venting Material: The venting material should be rated for high-temperature applications and should be approved by the furnace manufacturer.
  2. Venting Size: The venting size should be appropriate for the furnace’s output capacity to ensure proper airflow.
  3. Slope: The venting system should slope towards the furnace to prevent condensation buildup and blockages.
  4. Clearances: The venting system should have proper clearances from combustible materials, such as wood, insulation, and drywall.
  5. Location: The venting system should be installed in a location that provides adequate clearance for service and maintenance.


Regular maintenance of the high-efficiency furnace venting system is essential to ensure proper operation and prevent potential hazards. Here are some maintenance tips to follow:

  1. Inspect the venting system regularly for signs of damage, such as cracks or corrosion.
  2. Clear any blockages, such as debris or bird nests, from the venting system.
  3. Keep the area around the furnace and venting system clear of combustible materials.
  4. Have a professional HVAC technician perform annual maintenance on the furnace and venting system.


Q: Can I use my existing chimney for a high-efficiency furnace venting system?

A: It depends on the type of high-efficiency furnace you have. Direct venting systems require a dedicated venting system, while power venting systems can use an existing chimney with proper modifications.

Q: Can I install a high-efficiency furnace venting system myself?

A: No, high-efficiency furnace venting systems require specialized knowledge and skills to install properly. It’s best to hire a licensed HVAC professional to install the venting system.

Q: How often should I have my high-efficiency furnace venting system inspected?

A: It’s recommended to have a professional HVAC technician inspect the venting system annually.


In summary, knowing what you need to know about high-efficiency furnace venting is crucial in maintaining a comfortable and safe home environment. Understanding the different types of venting systems, proper installation, and maintenance requirements will ensure that your high-efficiency furnace operates safely and efficiently. By following these guidelines and having a professional HVAC technician inspect and maintain your furnace, you can enjoy the benefits of a high-efficiency furnace for years to come.

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