Tankless Water Heater Venting

Tankless Water Heater Venting

Tankless water heater venting can be a quick and affordable job if you plan it right.  When many consumers learn that a gas-powered Tankless Water Heaters must be vented, they immediately imagine costly and invasive carpentry work.  However, this doesn’t have to be the case and usually isn’t.  This Tankless water heater guide takes a closer look at tankless water heater venting.

Why Tankless Water Heaters Need to be Vented

Electric tankless water heaters do not need to be vented, but gas tankless heaters do.  The reason is because, like a standard water heater, they create dangerous combustion gases when they fire.  The gases contain carbon monoxide.  However, it doesn’t have to be an expensive hassle to vent. Before you buy your tankless heater make sure you consider which power source is best for you.

Direct Venting Tankless Water Heaters

Tankless water heaters use gas very efficiently to heat water.  And virtually all of the heat, usually over 90% of it, is directly transferred into the water.  Very little of it is lost out the vent.  Because of this, the vent gases are warm but not hot.  Therefore, they can be vented directly out the side of the house rather than having to be vented through the roof. This is just one of the many advantages they have over standard tank-style water heater.

tankless water heater venting
Typical installation of a ventilation for a tankless water heater. This was the installation of a Takagi model. I cannot do the Takagi review because I need to have the unit available and open it completely.

With conventional water heaters, the exhaust gas can be well over 100 degrees F, hot enough to burn or damage plants, siding, etc.  To vent them, a flue must be run up through the roof of the home which can be difficult and expensive.  In addition, any time you put a hole in the roof, even if the protective boot is properly installed, you run the risk of a water leak.  If the boot is damaged by a storm or falling debris, a serious leak and significant damage can be the result.

Since the exhaust of tankless water heaters is relatively cool, it can be vented through the wall instead of the roof.  This is much easier and it is more affordable since it uses far less material and takes much less time to do.  If you have a high-efficiency gas furnace, it will be vented out the side of your home too, so you can take a look at that for comparison.

Installing your Tankless Water Heater for Easier Venting

To make venting easiest, install your tankless water heater on an outside wall.  If the unit is installed on a basement wall, the vent pipe can be run up just a few feet and then vented out the side with the use of a 90-degree elbow.  If the unit is installed on the first floor or above, the vent can be run straight out for the tankless water heater.  All that is needed is a small-diameter hole, PVC vent pipe, and perhaps a housing around it.  The vent should never be covered with a screen, however.  If it were covered with a screen and the screen became clogged, it could cause carbon monoxide to back up into the house.  Cage-type fittings can be placed over the vent to prevent birds from nesting in it – similar to what you see on the roofs of houses that have traditional vents.

Installing a Tankless Water Heater Outdoors

Most manufacturers of gas tankless water heaters make units that can be installed outdoors, even in cold regions.  They use extra energy because they have their own freeze prevention system.  The advantage of outdoor installation is that no venting needs to be done at all.  The wall does need to be penetrated, however, for the cold water supply and the hot water feed into the house.  Outdoor units are often installed in a recessed wall box so that their piping is run in heated space.  Other outdoor units use an electric heating cable that self-regulates to keep water pipe from freezing. The Rheem H95 is a great outdoor unit for most areas of the country.


Using a tankless water heater offers many advantages, and the ease and affordability of venting it is just one of them. When you choose a gas tankless water heater, you’ll save on energy use over a conventional water heater and on the cost and hassle of installing it

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