Your New Tankless Water Heater: Venting Considerations

You did the homework, and decided to go with a tankless water heater for your upgrade or new build project. In either case, now that you’ve bought your new tankless water heater, venting considerations should be next on the agenda if the model you selected heats water with gas rather than electricity.

Importance of Proper Venting

If any gas-burning appliance is improperly vented, problems will occur. At a minimum, condensation will occur inside and cause premature failure. Worst-case scenario is that carbon monoxide may enter your living space and poison occupants. Even though tankless water heaters are sold to do-it-yourselfers at hardware stores and home-improvement warehouses, installation should be a job for the pros.


If you’re familiar with conventional, tank-type water heaters, you know they’re normally vented all the way to the roof. When the heater is on the first floor of a two-story home, that can involve framing, drywalling and redecorating, plus a necessary loss of space. Because your new tankless water heater can be sidewall vented through the wall it’s fixed to, the intrusion factor is negligible and venting costs should be practically nil.

It is imperative that the appropriate Category III stainless steel vent piping be used. The combustion efficiency of tankless heaters almost invariably creates a highly acidic condensate that can destroy standard vent material very quickly.


As noted, tankless units are typically vented horizontally. They can, of course, also be vented vertically to a roof space. Either way, the run should be kept as short as possible. This keeps initial installation costs down, and is also better for the equipment over the long term. If your first instinct was to site the appliance where a lengthy vent would be necessary, consider changing your plans; running water pipes to and from a better location is cheaper than running a long vent from the original site.

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