Geyser Drip Tray Overflowing in Summer? It's Probably Your Vent Pipe

Every November to March, plumbers across Gauteng and KZN field the same panicked call: "My geyser is leaking — there's water pouring out the overflow but only when it rains." In about 80% of cases, the geyser is fine. The drip tray is collecting rainwater entering through the vent pipe.

How the vent pipe is supposed to work

SANS 10254 requires every geyser to have a vent pipe (also called the expansion pipe) that runs from the temperature-pressure safety valve to outside the building, discharging downward at least 150 mm above ground. Its job is to safely dump scalding water and steam if the geyser overheats.

The problem: that pipe terminates open to the sky, often on a north-facing wall or under an eave. During driving Highveld thunderstorms — especially with wind — rain enters the open end, runs back down the pipe, and discharges into the drip tray. From there it overflows into the ceiling.

How to tell vent rainwater from a real leak

• Only happens during or after rain → vent pipe (or roof leak, see below)
• Continuous slow drip in dry weather → faulty T&P valve, expansion vessel needed
• Hot water in the drip tray → real overheating issue, call a plumber immediately
• Tray fills overnight without rain → cold-water inlet leak or tank rupture

Stick your hand in the tray water. Cold + recently rained = vent pipe. Warm in dry weather = call someone.

The two-minute diagnosis

1. Locate where the vent pipe exits the wall (usually a 22 mm copper pipe).
2. Look at the open end during the next storm. If water visibly flows in, you've found it.
3. Some installers terminate the vent inside the roof void — a code violation. If you can't find an external outlet, that's your problem.

Permanent fixes (in order of cost)

• Bird-guard cap with downward baffle — R150 part, stops 90% of rain ingress while still allowing safe steam discharge. Must NOT seal the pipe.
• Relocate the outlet under a deeper eave or to a sheltered wall — R800–R1 500.
• Install a tundish + indirect discharge — the vent drips into a visible funnel, which then drains to outside. SANS-compliant and rainproof. R1 500–R2 500.

Don't do this

• Don't cap the vent pipe. People die from this. A blocked vent on an overheating geyser turns the cylinder into a pressure bomb — the Mythbusters episode is not exaggerated.
• Don't extend the pipe with a 90° elbow facing up. Same problem with extra steps.
• Don't ignore a wet ceiling. Even "just rain" rotting your trusses for three summers is an expensive fix.

If the drip tray itself is missing or has no outlet pipe to outside, that's a separate SANS 10254 violation worth fixing before the next storm.