Tankless Water Heater Venting
Installation Tips

Either you have purchased a tankless water heater or searching for relevant information about on-demand heating units, installation requirements for the venting system are something you should carefully explore. The proper selection of the heater type can help you reduce utility expenses, lower energy consumption, and save you time on installation.

Before you decide on buying an on-demand heater, consider the following guide, tips, and venting requirements.

Venting options in tankless water heaters

  • Indoor or outdoor
  • Power vent or direct vent
  • Horizontal or vertical termination
  • Two-pipe or one-pipe venting
  • Stainless steel or PVC venting

Do I need venting for my water heater?

Every gas-powered tankless water heater that is installed inside the house requires a venting system, while an electric type does not. Vents are designed to remove exhaust gases outside safely.

This includes traditional tank-type gas heaters also.

Every gas-powered unit is burning natural gas or propane to produce heat. This is happening inside the combustion chamber by utilizing the gas burner. As the unit releases energy to heat water, it also generates exhaust gases.

Released exhaust gases, such as carbon monoxide and NOx, are not safe for people; actually, they are dangerous and must be removed into the outside atmosphere where they dissipate.

Tip: Buy carbon monoxides detectors to keep your family safe. Install a detector on every level and in every bedroom.

To remove the gases safely, tankless water heaters usually utilize one of the following venting types: power or direct vent. This applies to tankless models designed for indoor installation, while the outdoor models do not require a venting system because they are already installed outside.

What do you need to know about tankless venting

Power vent vs. direct vent

Gas tankless water heaters can use indoor or outdoor air for combustion, and the exhaust gases must freely travel to the outside atmosphere, with no obstructions and leaks.

As mentioned before, there are two types of venting systems used on tankless water heaters: power-vent and direct-vent.

Power-vent water heaters use indoor air for gas combustion, while direct-vent utilize outside air.

A power-vent model comprises an exhaust fan and only one pipe, which is exhaust, while a direct-vent model has two, one for air intake and the other for exhaust.

While both tankless water heaters require sufficient air for gas combustion, power-vented models must be placed in a large room or room connected to another with louvers or have open doors, or the walls have provisions that will always bring the fresh air in.

If the water heater is installed in a confined space (if space volume is less than 50 ft. per 1000 BTU/Hr), that space must have two air openings with the specified sizes and distances from the floor and ceiling.

In a direct-vent setup where the tankless utilizes two pipes, you can either install two separate pipes (two-pipe system) or a one-pipe system with the concentric (coaxial) setup, also known as a pipe within the pipe.

The coaxial type consists of an insulated intake pipe with 5 and 4 inches in diameter and exhaust pipes with 2 and 3 inches, which enable the vent to remain cold to the touch, therefore, preventing the risk of burn injuries.

According to some experts, such systems are easier to install; they require less space and are safer to use. Even if there is a leak in an exhaust pipe, the toxic gas is contained within the concentric vent so it does not pollute the air and affect your health.

Indoor or outdoor

Tankless water heater indoorTankless-indoor

Indoor tankless water heaters must release exhaust gases to the outside atmosphere using one of the vent types mentioned above: power or direct vent. They cannot be installed outside unless they use a dedicated kit (where applicable).

Outdoor models have vent slots on the front panel and do not require additional vents.

Tankless water heater outdoorTankless-outdoor

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Typical installation and how to configure vents

Tankless-indoor concentricTankless-indoor concentric
Tankless-indoor 2 pipeTankless-indoor 2 pipe

The most typical sizes of tankless vents are 3" and 4". The vents are connected to a vent adapter that comes with the unit.

The typical vent installation of the tankless indoor water heater is horizontally through the sidewall or vertically through the roof. When you compare to the standard vent installation of the tank-type water heaters, tankless brings more flexibility and is cheaper, especially with the sidewall installation when remodeling the house.

If you decide to go with the horizontal installation of your non-condensing unit, you would have to slope the vent downward and away from the heater. The vent will exit the tankless unit vertically, then turn using a ninety-degree elbow and then straight, pointing downward (1/4" per foot) and outside. At the end of the exhaust, there will be another ninety-degree elbow. This is the simplest setup.

The vertical installation has an upward slope, and it is equipped with the condensation trap, a rain cap at the end, or an elbow.

These two methods are used to protect the water heater from damages caused by acidic condensate.

If you have a condensing tankless water heater, slope the venting ¼” per foot and toward the heater. The condensate should not drain through a heater but the trap.

Also, it is recommended to install vent screens to prevent foreign objects from entering the terminal.

If you live in a region with a warmer climate, outdoor models are a great option because no venting is required, and they free up indoor space for other use. Such types are often installed on the outside wall of the house and inside the recess box, protecting it from the cold weather and elements.

You are even safe if you live in a region with below-freezing temperatures because tankless are often equipped with a system that automatically provides heating.

Non-condensing models must use "Category III" or "Category IV" stainless steel vents (PVC is not suggested) due to the higher exhaust temperature. In contrast, condensing heaters can be vented with PVC and PP pipes, which are cheaper and easier to install (and handle).

Note: Every manufacturer specifies the maximum lengths that you can run the venting. The number of elbows is limited because they shorten the total vent distance.

If one tankless heater does not produce enough hot water for your home, you can combine more than two units into one system. As they can use a manifold to share the same vents, individual vent penetration is not needed, saving you time and money during installation. Plus, it is more practical and aesthetically nicer to have fewer wall openings.

Tankless-common ventingTankless-common venting

Useful tips

  • Avoid dips and sags in horizontal venting by installing recommended supports (pipe hangers).
  • Venting should be installed as direct as possible and with a minimum number of pipe bends.
  • Venting connections should form an airtight seal to prevent gas leaks.
  • Connecting the venting system to an existing chimney is not recommended.
  • Avoid vent termination near other vents and exhaust.
  • Follow the manufacturer's instructions regarding the distances to other objects (windows, doors, ground, walkways, and such).
  • Combustion air must be free of corrosive chemicals.
  • Check the vent system at least once annually for leaks, blockages, and damages.
  • White smoke coming out from the exhaust vent is normal.
  • Avoid using longer than recommended vents and an oversized vent pipe.
  • Vents terminating at the roof should have a rain cap or other type (90+45 bends).

Warning: Improper venting of any gas water heater can lead to carbon monoxide leaks and poisoning.

Manufacturers of vent components

  • Centrotherm
  • Metal Fab
  • Ubbink
  • Heat-Fab
  • IPEX


There are three the most used venting systems in residential water heating: atmospheric, direct-vent, and power-vent. They use flue ducts or a chimney to move the exhaust gases from the water heater to the outdoors. Depending on the venting system, flue ducts can be metal or plastic.

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