Why Does My Hot Water Tank Cupboard Smell?
Struggling with odors near your hot water tank and vent pipe? Learn why aerators fail, how siphon pipes affect venting, and what fixes are possible.
Moving into a new home often brings plumbing surprises. One issue many homeowners face is unpleasant smells coming from a cupboard with a hot water tank and venting system. In this case, the cupboard has a vent pipe with a membrane aerator (also called an Air Admittance Valve, or AAV) installed. These devices are designed to stop sewer gases from entering the living space.
However, there is also a siphon pipe from the bottom of the hot water tank feeding directly into the same vent pipe. This raises a key question: Does this extra connection make the aerator useless by allowing odors to escape?
Question:
If a siphon pipe from a hot water tank connects directly into a vent pipe with an aerator, does this make the aerator irrelevant and allow odors to escape?
Answer:
Yes, this setup can bypass the aerator and allow sewer odors to escape. An aerator (AAV) only works if all gases pass through it. If another pipe ties in below it, gases can escape before reaching the membrane. This can make the aerator ineffective and cause the cupboard to smell.
Air Admittance Valves are meant to be installed on fixture drains, not mixed with hot water tank relief or siphon lines. If the siphon pipe connects directly into the vent, it should be reviewed by a licensed plumber.
The fix may involve re-routing the siphon discharge to a proper drain with an air gap, while keeping the AAV serving only the fixture drain it was intended for.
For more guidance, see this guide on plumbing vent systems. You may also review the International Plumbing Code venting section.
I’m Chris Mayer, writing for Plumbing 101.
