Water Heater Venting

water heater venting

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Water heaters which are fueled by liquid propane (LP) or natural gas need to have a venting system.

The gas burns through a process known as combustion, which causes heat, moisture, and exhaust gases (such as carbon monoxide which is very poisonous).

The ventilation system in the water heater ensures the removal of these dangerous byproducts from the home.

This makes it a critical safety device. The water heater typically determines the kind of ventilation system to be used.


This article is meant to be an overall guide on the principles behind water heater venting, and not instructions.

You should not fix or troubleshoot any issues with the venting system yourself, as any mistakes can create severe health problems.

If you think there’s an issue with your heater ventilation, call an expert immediately.

Water Heater Venting: Fundamentals

All venting systems on water heaters make use of a vent pipe or duct (also known as a flue or chimney) to take exhaust gases outdoors from the heater.

Depending on the vent system, this duct may be plastic or metal. They may lead outdoors directly, or connect to a bigger vent pipe that is also linked to another gas appliance. This is known as a common vent setting.

Most times, the common vent is heated by the exhaust from the bigger appliance which improves the water heater vent flow.

Though common venting systems can be very effective when properly installed, problems of back-drafting are very likely when they are not correctly installed.

Hence, some area codes no longer permit common venting systems; rather power venting or direct venting has become mandatory.

Apart from the venting system, propane and gas heaters also need a supply of air to aid combustion. This could either be from the home’s atmospheric air or from a vent pipe pulling air from outside.


To keep your venting system working properly, perform water heater maintenance at least once a year.

Water Heater Venting: Preventing Backdrafting

Backdrafting is the most popular heater venting problem, where exhaust gases emitted from the heater do not exit the home through the vent but remain in the house instead.

There can be many causes for this, but it is mostly caused by poor vent system design or installation and/or air volume imbalance in the house.

The latter is caused most times by ventilation fans, such as the kitchen or bathroom vent fans, pulling air away from the home and creating a vacuum that pulls exhaust gases from the heater vent into the home.

Some venting systems make use of direct venting technology or fan-assisted ventilation to eradicate back-drafting.

Atmospheric Venting

Atmospheric venting is mostly used by the more common water heaters. The vent is made up of an upward-sloping or vertical vent pipe which normally connects to a common vent.

Hot exhaust gases from the heater normally rise up into the outdoor air through the vent, making a draw to promote the flow of air upwards.

The hotter the vent pipe gets the more power the draw.

Atmospheric vents function well (and do not require electrical power) if designed properly and the house does not have problems of back-drafting.

Vents that are poorly designed suffer from not enough draw and/or are highly likely to experience back-drafting.

Water Heater Venting: Power Venting

Water heaters that have power venting are fitted with an electric fan blower (mostly silent) fixed at the top of the heater having either horizontal or vertical vent pipes.

Since this vent does not use natural convection, it can be run to the exterior of the home horizontally.

The air is cooled by the blower so PVC pipe can be used to run the vent (instead of metal as in atmospheric venting).

It is also quite easy to install and requires the water heater to have an electrical power outlet close by for the fan.

Water Heaters with Direct Vent

In this system, air needed for combustion is drawn from a vent duct running through the roof or an external wall.

Exhaust gases exit the home through a different vent pipe or chamber of the same duct (a double-wall vent duct is required).

Essentially, direct vent systems are not susceptible to back-drafting since they “breathe” air outdoors.

They also lower the risk associated with accidental fires from flammable vapors on the water heater.

Water Heater Venting: Mobile Homes

These are similar to standard water heaters, only that they are designed for this use. Most times, manufacturers do not warrant the installation of a heater in a mobile home unless it has been specially approved for that purpose.

Installing a standard atmospheric heater in a mobile home mostly needs an external access point. Mobile homes are very likely a closed-combustion unit that has direct venting.

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