Can Tankless Water Heater and Furnace Share Vent?

Proper venting is crucial for gas tankless water heaters. It ensures the combustion process is done effectively while appropriately drafting out exhaust gases and moisture. A big question for most homeowners is: Can tankless water heater and furnace share vent?

Read on to find out!

Gas furnaces and direct vent water heaters can share a vent. A consolidated venting system for your water heater and furnace optimizes space use in your home. However, it must be done properly to prevent risk for fire hazard or carbon monoxide poisoning. Ultimately, it is best to have a reputable plumber install the venting system.

Now, let’s take a closer look at this kind of venting system and everything that is involved.

Shared Water Heater and Furnace Venting Options

Whether to set up a shared venting system or not depends on the type of water heater and furnace you have. Shared venting only works for gas water heaters and standard-efficiency furnaces but not for high-efficiency furnaces and power vent water heaters.

There are two main types of shared water heater and furnace venting systems: concentric vent and standard atmospheric vent.

Concentric Vent

A concentric vent system is a “pipe in pipe” design whereby the intake combustion air and the exhaust gases pass through separate channels of the same unit. There is a separate intake pipe and exhaust pipe and a common concentric pipe that terminates to a single or on roof penetration. This installation option is aesthetic and optimizes space use. You could use a concentric termination kit to set up this venting system.

Standard Atmospheric Vent

This is a common type of venting for residential gas water heaters that can be set up to be shared with a standard gas furnace. It utilizes a vertical metal duct vent that exhausts outside your home. Like direct venting, hot air rises through the vent pipe and safely exits your home through the process of convection. It has a single vent pipe that mainly serves as a flue exhaust pipe.

Can I Use Existing Vent for Tankless Water Heater?

It depends on the size and design of the existing vent. If the vent size, design, and material are in line with the tankless water heater’s manufacturer’s recommendations, then you can use it. Most direct vent water heaters can be vented into an existing vertical vent pipe or chimney.

Most houses already have a masonry chimney that you can use to vent your tankless water heater. However, keep in mind that if not properly set up, using a masonry chimney to vent a tankless water heater runs the risk of the exhaust gases back drafting, which could lead to carbon monoxide poisoning.

If the flue is too big, the exhaust gases may condense before ascending the flue pipe. Also, masonry chimneys, especially clay-lined ones, are absorbent, which can inhibit good draw.

Also, some have holes that further interfere with prevents accession of the exhaust gases. As a result, carbon monoxide may cause a backdraft to your living spaces leading to carbon monoxide build-up and poisoning.

Below are measures to take to safely and effectively when using an existing masonry chimney to vent your tankless water heater:

Masonry chimney
  • The chimney should not be more than and 4-inch wide
  • Install a properly sized stainless-steel liner on the inner side of the chimney
  • Fit an increaser on the largest hole in the chimney and on the flue base
  • Install smoke caps on the small holes on the chimney
  • Fit a 90-degree elbow on the two increasers and ensure that they are aligned with each other

The venting requirements for a power vent water heater are quite particular. A power vent water heater is a Category IV appliance that uses PVC vent pipes and operates on positive vent pressure from a blower or fan

As such, they cannot share a venting system with gravity-centered appliances such as direct vent water heaters and standard furnaces. Therefore, it is best to install a new venting system for a power vent water heater per the manufacturer’s recommendations.

How Far Can You Vent a Tankless Water Heater?

You can either vent your tankless water heater through the roof or a side wall, depending on the type of venting system. Direct vent water heaters can only be vented vertically via the roof, while power vent water heaters can be vented vertically through a roof or horizontally to a sidewall.

What Kind of Vent Does a Tankless Water Heater Use?

There are three main vent types for tankless water heaters:

  • Power vent. The venting system is fitted with a blower or fan that serves a two-fold purpose. It sucks indoor air into the combustion chamber and propels out exhaust gases through a vent pipe. Tankless water heaters with a power venting system require a larger space with sufficient air. Therefore, power vent water heaters are best installed in the basement. Since the blower provides momentum to expel the combustion gases, power venting pipes can either be horizontal or vertical.
  • Direct vent. This venting system has two vent pipes – one for taking in air and the other for exhausting air. This venting system takes up limited space and requires minimal air from the immediate environment. Therefore, direct-vent water heaters can be installed anywhere in the house, including basements and closets.
  • Concentric vent. A concentric vent has a similar operating mechanism as a direct venting system. However, it has separate intake and outtake pipes that are housed in one assembly. Instead of having two pipes terminating into the sidewall or roof, you only have one.


Although gas water heaters and furnaces can share a venting system, most manufacturers recommend against its failure, to which your warranty may become void. It is best to have separate venting systems for the two appliances. Each product comes with specific venting system instructions provided by the manufacturer.

If you must have a shared venting system for reasons such as space limitations, consult with a professional plumber or HVAC specialist and have them install it. That way, you are sure that the shared venting system will pass inspection and, more importantly, prevent carbon poisoning or fire hazards.

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