Draining your hot water cylinder before repairs is something many NZ homeowners can do themselves — but there are important safety steps to follow, and some of the repair work itself must be carried out by a licensed plumber. This guide walks you through the process clearly so you can prepare your cylinder safely and know when to call in a professional.
Understand What You Can and Cannot Do Legally
In New Zealand, draining a hot water cylinder is generally considered preparatory work and is not restricted. However, any work involving the hot water system's plumbing connections, pressure relief valves, tempering valves, or element replacement is classified as restricted plumbing work under the Plumbers, Gasfitters and Drainlayers Act 2006. This work must be carried out by a licensed tradesperson registered with the Plumbers Gasfitters & Drainlayers Board.
Always confirm the scope of your repair with a licensed plumber before starting. You can find qualified local plumbers on tradedirectory.co.nz.
Safety First — Read This Before You Start
- Hot water scalds. Water inside a cylinder can be stored at up to 60–70°C. Never open drain valves without first turning off the heating element or gas supply and allowing the water to cool.
- Pressure hazards. Hot water systems operate under mains pressure. Releasing pressure incorrectly can cause injury.
- Electrical safety. If your cylinder is electric, the element must be switched off at the switchboard before draining — running an element dry will burn it out instantly and is a serious fire risk.
- Check WorkSafe NZ guidance if you are ever unsure about working safely around hot water systems.
Tools and Materials You'll Need
- Flathead screwdriver or drain valve key
- Garden hose long enough to reach a drain or outside
- Bucket (optional, for catching residual water)
- Towels or rags
- Adjustable pliers (if needed)
Step-by-Step: How to Drain Your Hot Water Cylinder
Step 1 — Turn Off the Power or Gas
For electric cylinders, locate the dedicated circuit breaker in your switchboard labelled "hot water" and switch it off. For gas califont or gas storage systems, turn the gas supply off at the appliance or at the meter. Do not skip this step.
Step 2 — Turn Off the Cold Water Supply
Locate the cold water inlet valve on your cylinder (usually a gate valve or ball valve on the pipe feeding into the bottom or side of the cylinder). Turn it off. If you cannot find an isolating valve at the cylinder, you may need to turn off the mains water supply at the toby (street valve) — your local council can advise on its location.
Step 3 — Allow the Water to Cool
If the cylinder has been recently heated, wait at least 1–2 hours before draining. For a fully heated cylinder, waiting 3–4 hours is safer. This significantly reduces the scalding risk when water begins to flow.
Step 4 — Relieve the Pressure
Open a hot tap somewhere in the house (a laundry tub or bath tap works well). This breaks the vacuum inside the system and allows water to drain freely. Leave this tap open throughout the draining process.
Step 5 — Connect a Hose to the Drain Valve
Locate the drain valve at the base of the cylinder — it typically looks like a hose bib or a small tap. Attach a garden hose securely and run it to a suitable drain point outside, such as a garden or stormwater drain. Avoid draining onto areas where hot water could cause damage or injury.
Step 6 — Open the Drain Valve
Slowly open the drain valve using a screwdriver or by hand. Water will begin to flow. Test the temperature carefully — if it feels very hot, close the valve and wait longer before continuing.
Step 7 — Allow the Cylinder to Drain Fully
A standard NZ hot water cylinder (typically 135–300 litres) can take 20–45 minutes to drain fully. Flow will slow to a trickle when it is nearly empty. Once drained, close the drain valve and remove the hose.
After Draining — What Happens Next
Once the cylinder is drained, your licensed plumber can carry out the necessary repairs — such as replacing a sacrificial anode, swapping out a heating element, or servicing the pressure relief valve (PRV) or temperature and pressure relief (TPR) valve. These components are critical to safe cylinder operation and must comply with NZ Building Code clause G12 (water supplies).
When refilling, reverse the process: close the drain valve, turn the cold water supply back on, and wait until water flows steadily from the open hot tap before closing it. Only then should you restore power or gas.
When to Call a Professional
Call a licensed plumber if you need to:
- Replace or test the pressure relief valve or TPR valve
- Replace a heating element (also requires an electrician for wiring)
- Repair or replace pipe connections to the cylinder
- Install a tempering valve (required for rental properties — check the current requirements with Tenancy Services or a licensed plumber)
- Deal with any leaks from fittings or the cylinder body
For electrical element wiring, a registered electrician (registered with the Electrical Workers Registration Board) must carry out and certify that work.
Find trusted local plumbers and electricians at tradedirectory.co.nz.
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