Selecting the Gas Water Heater Venting Options

Find out how to select the best water heater venting option for the safe, reliable and economical home water heating. Explore the differences between atmospheric, power-vent and direct-vent systems.


Atmospheric vent water heaterEvery natural and propane gas water heater needs properly designed venting system so it can safely remove the flue gases (byproducts of combustion) outside, and draw the fresh air in for the proper gas combustion. 

The venting system must be correctly sized to handle the total BTU input of the water heater.

The flue gases contain mainly the water vapor and carbon dioxide, which is acidic by nature, so for the safe removal from the heater they are moved outside through the metal flue, chimney or side vent.

Depending on the location of the new gas water heater, different venting system should be considered, for the most economical, safe and reliable water heating. Venting flexibility is very important when selecting the right model, so buy a water heater that allows you a variety of installation options.

Types of the venting systems

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.

Atmospheric venting

The most common type of gas water heaters is the atmospherically vented type that uses a natural way (buoyancy) to move the flue gases from the unit out. The atmospherically vented heater uses a vertical metal pipe, sometimes connected to the chimney, where the hot flue gases rise naturally through the draft hood and flue pipe, and out in the atmosphere (natural convection). Combustion air is drawn from inside the house.

The efficiency of the atmospheric vents increases with the longer ducts and temperature of the exhaust gases.

The advantage of using this type of venting is the low cost of the unit, installation, and operation. Atmospheric-vent water heaters are also the cheapest. They can operate without electricity.

Direct venting

Direct-vent water heaters are designed with the sealed combustion chamber, and they mostly use the horizontal flue pipe directly through the external wall and out. They do require a zero tolerance for the proper and safe transfer of the flue gases. They can use a separate vent pipe or double-wall vents (coaxial) to draw the fresh air in, so they can be installed almost anywhere. 

Due to its air-tight design, they are much more flexible in venting installation than the above atmospheric type and are quieter than power-vent heaters that utilize electric fans. Sealed combustion is particularly useful if the water heater is installed near flammable and corrosive chemicals and other products. Direct vent heaters are not subject to the effects of backdrafting.

Power venting

Power-vent water heaters use a mechanical fan to move the byproducts of combustion to the outdoor atmosphere. They are expensive but bring lots of flexibility during the installation. The venting system can be installed vertically, horizontally and the unit can be far away from the point of termination. Some water heaters have long vent pipes, up to 150 feet in length. Most of the time combustion air is drawn from inside the home.

Since the byproducts of combustion are acidic and with the high temperature, both tank-type, and tankless water heaters must use metal or stainless steel venting pipes, while condensing units, due to the lower temperature of the exhaust gases can use pipes made of different and cheaper materials such as PVC and CPVC, which is also easier to install.

Water heaters with the power venting are recommended for tightly built homes, homes with no chimney, where a good natural draft is hard to establish and to increase energy efficiency.

Power vented heaters are not recommended for installation close to the living room due to the noise a fan produces. They are electrically dependent as the electric fan has to be powered, so they must be installed nearby electrical outlet. Such heaters must be placed in a room with adequate air flow. They cost more than other types.

Venting layout

The venting system of the gas water heaters might consist of one or two pipes. The two-pipe system might be further divided on the concentric (pipe inside the pipe) or two separate pipes, and the vents can run horizontally, vertically or combined.

One-pipe venting water heater, such as the above atmospheric type, utilize one pipe for transferring the flue gases out while the air for combustion is brought from the surrounding atmosphere to the combustion chamber.

There are also water heaters, such as tankless outdoor models, that do not require venting, as they are installed outside. All the tank-type heaters are designed for indoor use only.

According to the experts, when choosing between venting types, it all comes down to your home design and location of the heater.

Venting problems

One of the common issues associated with the water heater venting system is backdrafting. Backdrafting is a situation when exhaust gases, instead exiting to the outdoor atmosphere, end up inside the house. This problem is often found in atmospheric-vent models because the draft method is not reliable.

What can cause backdrafting are poor vent design and insufficient draw, obstructions in the flue pipe or outside pressure. One frequent installation mistake includes insufficient pitch upward and bends close to the draft hood, so hot gases cannot rise quickly.

The most apparent symptoms of the backdrafting are:

  • Melted plastic of the water inlet/outlet rings, located on the top of the water heater and close to the draft hood.

  • Exhaust gases can condense at the top of the heater and cause corrosion.

  • Heavy condensation can also occur on the nearby cold water pipes.

To prevent the problem, do the following to check for the proper draft at a water heater:

  • Close all doors and windows

  • Turn off the furnace if it shares the same chimney

  • Start the heater and let it run for some time before the test

  • Put your hand close to the draft hood, and if you feel warm and moist air coming out, the water heater is backdrafting. You can also use a mirror, hold it close to the draft hood, and see if it is fogging up. The cobwebs can also give you a visual indication of a problem. Alternatively, you can use a matchstick to see if the smoke is pushed away from the hood.

Installation tips

Keep in mind that water heaters must be installed according to the manufacturer's instructions and local or national codes and regulations.

Gas water heaters are usually installed in the utility or mechanical room, garage, closet, confined or unconfined space, and depending on the vent type and provisions for combustion, ventilation air must be provided in the recommended volume.

For the proper operation, venting systems, which includes the vent pipe, flue baffle, and draft hood, must be inspected and cleaned at least once a year.

Proper venting is very important for the safe operation of the gas water heater as the incorrectly designed system can lead to incomplete gas combustion, problems, and injuries, including death.

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