Water discharging from vent

[jonesy77]

haven't found anything specific to my issue, so I figured I'd give it a shot here.

I have an outdoor, tankless water heater. It is mounted in a recessed wall on the back of our house. It has a a short, horizontal vent "mouth."

Last night when my wife was in the shower and she turned off the water, I happened to be standing by the back window and noticed a fairly decent amount of water "belch" out of the vent. I am also able to replicate the experience on a smaller scale, using the bathroom faucets; i.e. upon turning off the hot water, after about a 5 second delay, about a tablespoon of water will come out of the vent. Any ideas?

 

[Breplum]

Worth investigating further but, well may be ok. H2O is a product of combustion, but tankless water heaters like you have are so inefficient that normally the moisture vaporizes. The more effecient (94 to 96% AFUE) condense a lot of water that has to be piped/drained away.
If you haven't had the unit descaled on schedule, then have that along with a service visit from a pro.

 

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[Reach4]

I would put a garden hose thread pressure gauge onto a nearby hose spigot, and see what the pressure does when you trigger one of these events. I suspect thermal expansion is bringing the pressure up, perhaps to 150psi.

I understand this is a tankless WH, but the timing of this moisture makes me think pressure relief.

 

[Fitter30]

Think it's the relief. If it came from the burner it wouldn't fire and water would be running out the bottom.

 

[jonesy77]

I checked and cleaned the pressure relief valve and it is functioning. I haven't noticed a pressure spike, but I will check the pressure. I'm wondering if there is debris in the condensate line that is preventing it from draining.