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Venting a tankless water heater

Steverino Member Posts: 140
First things that come to mind are condensation & CO. Condensation wouldn't escape the chimney and run back into the unit, conceivably destroying the heat exchanger. CO failing to escape out the chimney would enter the structure, thereby exposing its occupants to this tasteless/odorless poison which, in turn, will sicken/kill them.
Most manufacturers insist that you use their brand of venting...which you should. ALWAYS follow the manufacturers instructions when installing their equipment.


  • Matt Wynne
    Matt Wynne Member Posts: 8
    Venting a tankless water heater

    I'm planning an installation of a natural gas-fired wall-hung tankless water heater. The unit has a draft induction fan and also draws combustion air from outdoors. The manufacturer says it cannot be vented into a 1-story chimney lined with 9x9 terra-cotta flue tile unless a metal liner is installed. The chimney is currently used for an oil-fired hot water heating boiler. Aside from potentially voiding the warranty, how could venting the appliance this way adversely affect the performance/condition of the unit?
  • Bob Harper
    Bob Harper Member Posts: 1,034
    induced draft versus power vented

    If it operates under positive flue gas pressure, then it cannot vent into a masonry chimney lined with terra cotta. Even if there is a properly sized stainless flex liner, unless that liner is listed to UL 1738, you cannot use it.

    Positive flue gas pressure means you blow bad stuff into the home. Condensation will eat up masonry chimneys and many stainless steel liners.

    The codes require the chimney be inspected for suitability for that class of service. That means, temp., vent pressure, sizing, thermal inertia, flow, and resistance to condensation and corrosion.

    Most likely, this unit would require an AL29-4C stainless steel liner with a condensate trap.
  • Mad Dog_2
    Mad Dog_2 Member Posts: 6,915
    HI Matt

    Unless you are abandoning the chimney and want to use it as a chaseway (which is NEVER as easy as "they" say), you are asking for a mishap. Very simply, the "other" appliance's flue pipe could become the path-of-least-resistance, venting CO and combustion product in to the building. Add a few exhaust fans OR a whole house fan and you could really pull CO around the building. Straight out the building via the most direct route. If you have outside obstacles or restrictions, put the Tankless WHERE is can be vented easily and pipe other service over to it, i.e., gas, waters, et al. Good luck. Mad Dog (Matt From BNI)

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  • Steve Ebels_3
    Steve Ebels_3 Member Posts: 1,291
    It makes no difference what you're venting

    Always, Always, Always, follow the manufacturers venting instructions to the letter. They know how their appliance was designed, they know what conditions it operates under and what conditions it creates. The venting instructions they have recommended in their manual are the conditions required for proper function and safety. Don't try to color outside the lines on this one.

    PS: While the unit may "work" OK vented into a clay lined chimney, it will probably also destroy the chimney within a couple years. Flue temps are not high enough in that type of appliance to prevent condensation. Many new boilers, oil and gas fired, are the same way. Installing a SS liner in an existing chimney is now standard practice for us on any new boiler.
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