A dripping pressure relief valve (PRV) on your hot water cylinder is one of the more common plumbing issues NZ homeowners encounter. In many cases it signals a straightforward fault, but it can also point to a more serious pressure problem that needs professional attention. Here's how to understand what's happening and what steps to take.
What Does a Pressure Relief Valve Actually Do?
The pressure relief valve (sometimes called a temperature and pressure relief valve, or TPR valve) is a critical safety device fitted to your hot water cylinder. Its job is to release water if the pressure or temperature inside the cylinder rises to a dangerous level, preventing the tank from rupturing.
Under normal operation, the valve should be completely closed and dry. If you notice water dripping or trickling from the valve's discharge pipe (usually a copper pipe running down to a drain or outside), something is causing it to open — and that needs investigating.
Common Reasons a PRV Drips
- Thermal expansion – As water heats up it expands. If your system has a check valve or backflow preventer on the cold supply, expanded water has nowhere to go and pressure builds, causing the PRV to weep. This is very common in NZ mains-pressure systems.
- Faulty or worn valve – PRVs have a rubber seat that degrades over time. An old or gritty valve may not reseat properly after a pressure spike and will continue to drip.
- Excessive system pressure – If your mains water pressure is too high (above the cylinder's rated pressure, typically 700–850 kPa for most NZ cylinders), the PRV will open regularly to compensate.
- Thermostat set too high – If the cylinder thermostat is faulty or set above the recommended 60–65 °C, the temperature relief function may be triggering the valve.
- Failing expansion vessel – Some systems include a pressure expansion vessel to absorb thermal expansion. If its bladder has failed, pressure spikes will cause the PRV to drip.
What You Can Safely Check Yourself
Before calling a plumber, there are a few things a homeowner can observe safely:
- Check when it drips – Does it drip only when the cylinder is heating (thermal expansion), or constantly? Dripping only during heating cycles strongly suggests an expansion issue.
- Look at the age of the valve – PRVs should be tested and replaced every 5–8 years as a general guide. Check if yours has a date stamp.
- Check your mains pressure – You can hire or buy a simple pressure gauge that attaches to a tap. MBIE Building Performance guidance and NZS 4603 recommend mains pressure to a cylinder should generally not exceed the cylinder's rated working pressure.
- Check the thermostat setting – The recommended storage temperature for hot water cylinders in NZ is 60 °C to kill Legionella bacteria, as recommended by health and building performance guidance. Going significantly higher than this is unnecessary and increases pressure.
⚠️ Safety note: Do not attempt to manually lift the PRV test lever repeatedly to "clear" the drip. This can damage the valve seat further and cause it to fail to reseat at all, leading to a continuous flow. If the valve is actively spraying hot water, keep clear — the water can be scalding.
What a Licensed Plumber Will Do
Replacing a PRV, adjusting system pressure, or installing an expansion vessel is restricted plumbing work under the Plumbers, Gasfitters and Drainlayers Board (PGDB) regulations. This work must be carried out by a licensed plumber. Attempting it yourself is illegal and could void your home insurance.
A licensed plumber will typically:
- Test system pressure with a calibrated gauge
- Replace the PRV if it is faulty or past its service life
- Install or replace an expansion vessel if thermal expansion is the cause
- Fit a pressure-reducing valve (PRV) on the toby/main if mains pressure is too high
- Check the thermostat and element for correct operation
- *Issue a Certificate of Compliance (CoC) for any regulated plumbing work — always ask for this
Typical Costs (As a Guide Only)
Costs vary by region and the scope of work required. As a very rough guide:
| Job | Approximate Cost Range |
|---|---|
| PRV replacement only | $150 – $350 incl. GST |
| PRV + expansion vessel | $350 – $700 incl. GST |
| Pressure-reducing valve installation | $300 – $600 incl. GST |
These are indicative only — always get at least two quotes from licensed plumbers.
Does This Require a Building Consent?
Replacing a like-for-like PRV on an existing cylinder is generally a Schedule 1 exempt repair under the Building Act, but any significant alteration to your hot water system may require consent. Always confirm with your local council if you are unsure, and ensure your plumber provides a CoC regardless. See MBIE Building Performance for guidance on consent exemptions.
When to Call a Professional
Call a licensed plumber promptly if:
- The valve is dripping constantly or discharging a significant flow of water
- The discharge water is very hot or steaming (possible thermostat failure)
- The valve is more than 8–10 years old and has never been replaced
- You notice the cylinder making rumbling or popping noises
- You have already tried basic checks and the dripping continues
Ignoring a dripping PRV can lead to water damage, higher power bills, and in rare worst-case scenarios, a dangerous pressure build-up. Don't put it off.
You can find qualified, licensed plumbers in your area on tradedirectory.co.nz.
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