How Do I Bleed Air Out of My Hot Water or Heating System?

Trapped air in a hot water cylinder or hydronic heating system is one of the most common causes of noisy pipes, uneven heat, and poor system performance in New Zealand homes. The good news is that bleeding air out is often a straightforward job a homeowner can tackle — but there are important safety steps to follow, and some situations where you'll need a licensed plumber.

Why Does Air Get Trapped in the System?

Air enters plumbing and heating systems in several ways: through fresh water supply (water naturally contains dissolved gases), after maintenance or repairs, through a leaking pump seal, or simply over time as the system ages. In hydronic (wet) heating systems — where hot water circulates through radiators or underfloor pipes — trapped air is particularly problematic because it stops water flowing freely. In hot water cylinders, air pockets can cause banging or kettling noises and reduce efficiency.

Common signs you have an air problem:

• Radiators that are warm at the bottom but cold at the top
• Gurgling, banging, or knocking sounds from pipes or the cylinder
• Reduced hot water pressure or flow
• The circulating pump sounds louder than usual

Safety First

Before you start, keep these points in mind:

• Hot water scalds quickly. Water in a heating system can be very hot — always use a cloth or gloves and open bleed valves slowly.
• Never bleed a system while the boiler or heat pump is actively firing. Turn the heat source off and allow the system to cool slightly first.
• If your system runs at mains pressure, be aware that water will escape under pressure when you open a valve. Have a container and cloth ready.
• Check WorkSafe NZ guidelines if you're ever unsure about working safely around hot water systems.

Tools You'll Need

• A radiator bleed key (available from Mitre 10 or Bunnings — inexpensive and worth keeping on hand)
• A small container or cup
• A cloth or old towel
• A flat-head screwdriver (some modern bleed valves use a slotted screw instead of a square key)

How to Bleed a Radiator (Hydronic Heating System)

This is the most common bleeding task homeowners do themselves.

1. Turn off the heating system and let it cool for 20–30 minutes so the water isn't scalding.
2. Identify the bleed valve — it's usually a small square or slotted fitting on the top corner of each radiator.
3. Place your container under the valve to catch drips.
4. Insert the bleed key and turn it slowly anti-clockwise (about a quarter turn). You'll hear a hissing sound as air escapes.
5. Wait until water starts to dribble out — this means the air has cleared. Close the valve immediately by turning it clockwise. Don't overtighten.
6. Repeat for each radiator, starting with the one closest to the boiler or heat pump and working outward.
7. Once done, check the system pressure gauge on your boiler or pump unit. If pressure has dropped below the recommended range (check your system's manual), you may need to top up via the filling loop — or call a plumber.
8. Restart the heating and check that all radiators are heating evenly.

Bleeding a Hot Water Cylinder

NZ homes commonly have a low-pressure (open-vented) cylinder or a mains-pressure cylinder. The approach differs slightly.

Low-Pressure (Open-Vented) Cylinders

These are gravity-fed from a header tank, usually in the roof space. Air is generally self-venting through the open vent pipe. If you're getting noisy pipes, the issue may be a partially blocked vent pipe or a failing element — worth having a plumber inspect.

Mains-Pressure Cylinders

These operate at full mains pressure. Some have a manual relief or purge valve that allows air to be flushed out by briefly opening a hot tap fully and letting water run until the flow is smooth and bubble-free. If you're unsure of your cylinder type or can't locate a purge point, contact a licensed plumber — working on mains-pressure systems incorrectly can cause water damage or injury.

BRANZ has useful guidance on hot water system types common in NZ homes.

Checking and Topping Up System Pressure

After bleeding, hydronic systems often need a small amount of water added to restore correct operating pressure. This is done via a filling loop — a braided hose connecting the cold mains to the heating circuit. Open the valves slowly, watch the pressure gauge rise to the level specified in your system manual, then close the valves. If you're not confident doing this, it's a quick job for a plumber.

Preventing Air Build-Up

• Bleed radiators annually, ideally at the start of winter before you rely on the heating.
• Have your system serviced every couple of years by a qualified tradesperson — they'll check the pump, expansion vessel, and pressure relief valve.
• If air keeps returning, it may indicate a leak drawing in fresh (air-containing) water, or a failing expansion vessel. These need professional diagnosis.

The MBIE Building Performance website has information on the NZ Building Code requirements (including G12 — Water Supplies) relevant to hot water systems.

When to Call a Professional

Some tasks are restricted work under NZ law and must be carried out by a licensed plumber or gasfitter registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB):

• Any work on gas-fired boilers or water heaters
• Replacing or adjusting pressure relief valves, tempering valves, or expansion vessels
• Connecting or disconnecting pipework
• Diagnosing persistent air problems that may indicate a system leak

Even for non-restricted tasks, if you're not comfortable or the problem keeps coming back, it's always worth getting a professional in. You can find qualified local plumbers on tradedirectory.co.nz.

Sources & Further Reading

• BRANZ – Hot Water Systems
• MBIE Building Performance – NZ Building Code
• Plumbers, Gasfitters and Drainlayers Board – Licensed Tradespeople
• WorkSafe NZ – Working Safely Around Hot Water
• Energywise – Hot Water Efficiency
• Mitre 10 – Plumbing Tools & Supplies