I understand that a condensing tankless water heater will recycle that extra heat and only vent the heat that it cannot use; this is why they have a greater efficiency.

But the water that condenses out of the unit, and pumped outside, is that out of the ambient air, like a dehumidifier?

In other words, will the room around the unit become less humid? Or the unit adds this extra water in the first place?

Thank you.

  • 2
    I believe the condensate from within the unit is a result of the vapors created within as a result of the combustion process. noritz.com/demystifying-tankless-water-heater-condensation
    – Alaska Man
    Commented Jan 3, 2021 at 1:08
  • Interesting. Your source says "Water vapor is a byproduct of combustion, although, in this case, the vapor contains elements such as methane, nitrogen and oxygen.". So not the ambient air.
    – Misha AM
    Commented Jan 3, 2021 at 1:27
  • No, they call it a "condensing" boiler for a reason. By getting most of the combustion gasses to condense (go through a "state change" from steam to water), you get quite a bit of better efficiency. It's perfectly normal for a condensing boiler to expel water. Like others have said, it's nasty water, not anything you could use, but is is water. Also, modern boilers get combustion air from the outside to avoid drawing in cold, outside air into the building. So nope, it's not a dehumidifier at all. Commented Jan 3, 2021 at 13:57

2 Answers 2


The condensate from natural gas burning comes mainly from the burning. Methane is the primary component of gas, and it is chemically CH4. When burned, you have:

... a problem, because MathJax does not appear to be enabled on this exchange ...

CH4 + 2 O2 --> CO2 + 2 H2O

The relevant parts here are that 2 "oxygen" molecules from the air are consumed, and (after some chemical shenanigans) 2 water molecules are produced.

This is not simple, since natural gas consists of other things than methane, and the reaction is not a single-step reaction, so carbon monoxide and other things are also by-products. But the majority of the gas is methane, and the majority of the results are dioxide and water.

The water, naturally, is a vapor since it's created in the middle of a burning flame. But the high-efficiency gas appliances really try to capture the waste heat and feed it back into the system, so as a result the water vapor is cooled to "condensate". The condensate is not pure water (a lot of those impurities and by-products mentioned above also condense) and so you should not drink it (seriously, you could die), or even allow it to run through your sewer drain (seriously, it will eat a hole in your poo pipes, and nobody wants an that) without neutralizing it.

  • I see. Thank you. Can that water be routed straight outside, into the ground?
    – Misha AM
    Commented Jan 3, 2021 at 18:37
  • If you do so, it will affect the pH of your soil. It will also contaminate it, at least a little. If you have a grass lawn, you'll probably be okay. Beware of kids and gardens, though.
    – aghast
    Commented Jan 4, 2021 at 2:11

no. it takes air from outside, burns gas, cools the exhaust to the water temperature condensing some moisture some then expels the exhaust (at 100% humidity) and some liquid condensate.

Importantly it doesn't release dry air, and it doesn't release it indoors.

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.