Exhaust Vent Knocking Noise - Navien NPE-240A Tankless

dellery

Been hearing what we thought was a dripping sound in the exterior wall but source narrowed down to the the vertical exhaust pipe carrying the exhaust from a Navien NPE-240A tankless hot water heater.

The sound is triggered only AFTER there is a short burst of warm water at any faucet, and only if there has been no faucet use for 20-30min or longer. It sounds like a loud drip or knocking/tap, one tap every 3 sec. and continues for about 1 minute. Then quiet. It is loud enough to hear throughout our open plan house, and on all floors.

The unit is installed in the basement, and the exhaust leads out to the roof through an exterior wall. Three feet horizontal, two elbows, and about 40ft vertical. There is no backflow damper (should there be?). We are in Vancouver, BC Canada.

The vertical pipe has metal brackets on each of the two floors it passes through. These have been tightened and padded, but do not eliminate the root cause which is the knocking.

Help please!! What could these sounds be ?

JUGHNE

Possibly because the vertical pipe is secured at each floor. The pipe expands between the supports and tries to push thru one of them.
You might loosen the top support and see if that helps.

PVC drain pipe drilled thru wall studs will do that as it slips thru the holes as hot water is drained down from a sink etc.

dellery

Thank you. Will loosen the brackets that we have exposed (having to cut the baseboards open). I also noticed that the pipe opening through the roof is not covered nor curved so rain will go in. Wonder if that will cause a problem.

JUGHNE

If the exhaust pipe is the source of the noise because of expansion, you want a good support on the bottom and then a looser slider type support on the upper to allow movement.

I am not familiar with your unit.
Someone here knows much more about it as far as a roof cap being needed.