A leaking or dripping overflow pipe on your hot water cylinder is one of those things you shouldn't ignore. While a small drip might seem minor, it usually points to an underlying pressure or temperature problem that — left unchecked — can cause serious damage or even a safety hazard. The good news is that understanding what's happening is straightforward, even if the actual fix often needs a licensed plumber.
What Does the Overflow Pipe Actually Do?
Your hot water cylinder has a temperature and pressure relief (TPR) valve — sometimes called a pressure relief valve (PRV) — fitted as a safety device. When water heats up, it expands. If pressure or temperature inside the cylinder gets too high, the TPR valve opens and releases water through the overflow pipe (usually a copper or CPVC pipe running to a drain or outside). This is completely normal in small amounts during heating cycles.
However, if the overflow pipe is dripping constantly, running steadily, or discharging large amounts of water, something needs attention.
Common Reasons the Overflow Pipe Leaks
- Faulty or worn TPR valve — The valve's internal seal can degrade over time, causing it to weep or drip even when pressure is normal. TPR valves typically have a service life of around 5–10 years.
- Excessive system pressure — If your mains water pressure is too high (above the valve's rated pressure, commonly 850 kPa for NZ cylinders), the valve will open to relieve it.
- Overheating thermostat — If the cylinder thermostat is faulty and the water temperature climbs too high (above roughly 99°C), the TPR valve will discharge as designed.
- Thermal expansion — In a closed system (where a check valve or backflow preventer stops water returning to the mains), expanding water has nowhere to go except out through the relief valve. An expansion vessel or pressure-limiting valve may be needed.
- No cold water expansion relief — Older or modified systems sometimes lack adequate expansion provision, causing repeated valve discharge.
What You Can Safely Check Yourself
Before calling a plumber, there are a few safe observations you can make:
- Check when it drips — Does it only drip during or just after a heating cycle? A small amount of discharge during heating can be normal. Constant dripping is not.
- Look at the overflow outlet — Is water trickling out slowly, or is it a steady stream? Note this for your plumber.
- Check the age of your cylinder and valve — If your cylinder is over 10–15 years old, components may simply be at end of life.
- Check your water pressure — You can hire or buy a simple pressure gauge from Bunnings or Mitre 10 that attaches to a tap. NZ mains pressure should ideally be between 350–500 kPa at the property boundary. Anything consistently above 500–600 kPa at the tap warrants investigation.
Do not attempt to manually operate, adjust, or replace the TPR valve yourself. This is restricted plumbing work under NZ law.
What Is Restricted Work — and Why It Matters
In New Zealand, work on hot water cylinder valves, pipework connections, and pressure systems is restricted plumbing work under the Plumbers, Gasfitters and Drainlayers Act 2006. This means it must be carried out by — or under the supervision of — a licensed plumber registered with the Plumbers, Gasfitters & Drainlayers Board (PGDB).
This isn't just red tape. TPR valves are a critical safety device. An incorrectly fitted or bypassed valve can result in catastrophic cylinder failure. Always use a licensed tradesperson for this work.
Safety note: Never cap, plug, or restrict the overflow pipe. Never remove or disable the TPR valve. These actions create a serious risk of cylinder explosion.
What a Licensed Plumber Will Do
When you call a plumber, they will typically:
- Test the TPR valve — They may manually lift the test lever briefly to check it seats correctly afterwards. If it continues to drip after testing, the valve needs replacing.
- Check system pressure — Using a calibrated gauge, they'll measure static and dynamic pressure and may install or adjust a pressure-limiting valve (PLV) if pressure is too high.
- Inspect the thermostat — They'll check the cylinder thermostat is set correctly (typically 60°C to prevent Legionella, per Tenancy Services Healthy Homes guidance and good practice) and functioning properly.
- Assess expansion provision — They'll check whether your system needs an expansion vessel or other provision for thermal expansion.
- Replace the TPR valve if needed — A straightforward valve replacement is a common job, though it does require draining part of the cylinder and correct re-commissioning.
All work should comply with NZS 3500 (the NZ plumbing and drainage standard) and the NZ Building Code, particularly clauses G12 (Water Supplies) and G13 (Foul Water).
How Much Might It Cost?
As a rough guide only, a TPR valve replacement typically costs somewhere in the range of $150–$400 including labour and parts, depending on your location and cylinder type. If additional work is needed (pressure-limiting valve, expansion vessel, thermostat replacement), costs will be higher. Always get a quote upfront.
When to Call a Professional
Call a licensed plumber promptly if:
- The overflow pipe is running continuously or discharging large volumes of water
- You notice the overflow pipe is hot to the touch (suggests the cylinder is overheating)
- There is any sign of water damage around the cylinder
- Your cylinder is more than 10 years old and has never had the TPR valve serviced
- You're unsure of the cause
You can find qualified, local plumbers in your area right here on tradedirectory.co.nz.