I have built the unfinished chimney shown in the image below to get the exhaust for my neighbor's water heaters (which I believe are sealed combustion gas appliances, such as the Remeha Avanta water heater above the required height during a remodel.

enter image description here

The chimney will be enclosed and watertight. Both ducts will penetrate the chimney roof with a flat double-walled roof vent. I was planning on not insulating the chimney.

Could condensation form on these pipes and could that be of concern in the Dutch climate (that can range from -10 degrees celcius to +35 degrees celsius)?

The water heater one of these pipes is for is shown below. The fresh air is taken off the side of the apartment, only the exhaust is vented out the roof.

enter image description here

  • 1
    Is this a gas-fired water heater, and these vents for exhausting products of combustion? Nov 1, 2020 at 23:44
  • If these are gas appliances, are they condensing or sealed combustion gas appliances? Nov 1, 2020 at 23:57
  • @JimmyFix-it yes this is a gas-fired water heater and those vents are exhausting the products of combustion. Having been working with these pipes when they were running I know that the fumes coming out of them should not be inhaled.
    – Eric
    Nov 2, 2020 at 8:02
  • @ThreePhaseEel I believe they are sealed combustion gas appliances. I added more context to my questions which will hopefully make things clearer. Thanks!
    – Eric
    Nov 2, 2020 at 8:03
  • @Eric -- the bigger question is "are they condensing"...condensing gas appliances produce acidic condensate that will eat an aluminum flue like nothing at all Nov 3, 2020 at 3:14

1 Answer 1


Combustion of hydrocarbons always produces water vapor. Air provides O2 oxygen and other stuff which combines with the CH chains to make CO2, H2O, and whatever else. When the heater operates there will always be water vapor present in the exhaust duct; whether or not it condenses to liquid depends on many factors. A significant factor is the efficiency of the heater: the more efficient the heater is at putting combustion heat into the domestic water the less heat remains in the exhaust gas. Exhaust gas heat warms the walls of the exhaust duct. If there's enough heat in the exhaust to warm the duct above the dew point of the exhaust then there will be very little condensation in the duct. But if the duct is cold, and/or the heater has high efficiency, it can get a fair trickle of condensate.

Where condensation is a concern there are two approaches. One is to run the duct horizontally with slope toward the outdoors. Any condensate in the pipe runs to the outdoors and drips out the end. The other approach is to slope the duct toward the indoors and provide a condensate drain. This is the approach used when the duct goes vertical as yours does.

I don't read a word of Dutch but I looked through the installation manual anyway for the water heater you linked. This text and illustration from page 54 mention cleaning a siphon and a condensate trap. It looks to me as if this heater is designed to receive condensate drain-back, and therefore you should assemble the exhaust duct with the expectation that it'll need to carry condensate back to the heater.

from Avanta 24c water heater installation manual

The installation section on exhaust is number 4.8. It describes arrangements using room air or outdoor air for combustion, duct sizing according to length, and I saw something in there about condensate too.

  • Thanks for the help, the water heaters and exhausts are already installed and as shown in this question I made this chimney to get the existing exhausts above the roofline because of a large renovation. So basically, this design is OK as long as we ensure the condensation drain-back on the heater functions properly?
    – Eric
    Nov 2, 2020 at 15:36
  • @Eric Yes the design looks fine. Make sure the pipe joints are crafted in a way so that the liquid remains inside the pipe without depending on tape or other sealant. In other words, each additional section of pipe should slide into the end of the one that came before it. One thing to verify is that your material is acceptable - I've not done many of these, but I'm accustomed to seeing plastic, stainless steel, or thick-wall aluminum.
    – Greg Hill
    Nov 2, 2020 at 15:45
  • That makes sense, and just to be clear, I would be doing the opposite as what is suggested in this video on installing round HVAC Duct correctly. Since I'm more concerned about water conservation not being trapped than the exhaust airflow?
    – Eric
    Nov 2, 2020 at 15:51
  • @Eric That's right, it is opposite of the usual practice for forced-air HVAC duct - the exhaust gas and the condensate acid (it is not pH neutral) are going opposite directions and you can lap the pipe joints to match the flow of only one of the two. Better to lap the pipe to ensure condensate stays in and size it large enough that turbulence in the air flow isn't a problem. Sure, joints sealed with tape might leak a little bit of exhaust gas, but if the laps go the other way then they'll leak condensate for sure.. and probably leak a little bit of exhaust gas anyway!
    – Greg Hill
    Nov 2, 2020 at 18:26
  • 1
    @Eric I'd use sheet metal screws rather than drywall screws. I can't make a recommendation on tape or other sealant - I'm only experienced with solvent-welded PVC or ABS plastic and with rubber-gasketed concentric piping. I haven't worked with thin-wall aluminum nor semi-flexible rigid duct for exhaust. Consult the water heater manufacturer or a local supplier for specifics as to materials and methods.
    – Greg Hill
    Nov 2, 2020 at 21:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.