What an expansion tank does and when to replace it

An expansion tank is a small sealed vessel attached to your boiler system that absorbs extra water pressure as the water heats up and expands. Without it, pressure builds until the relief valve opens and water spills out — a sign your tank may be failing. Most expansion tanks last 5 to 10 years before the internal bladder (a rubber membrane inside) wears out and stops working.

You need a replacement when you see water pooling under the tank, when the relief valve opens repeatedly during normal heating, or when the tank feels heavy and waterlogged instead of light and hollow. A failed tank will not stop your boiler from working, but it will cause pressure swings that stress the whole system and eventually damage the pump or relief valve.

Replacing one is a job you can do yourself if you are comfortable with basic plumbing and have the right tools. The process takes 30 to 60 minutes and costs between $150 and $400 for the tank itself, depending on size and your boiler type.

Key Takeaways

  • Turn off the boiler and let it cool completely before touching any part of the system, then drain the old tank by opening the isolation valve below it.
  • The expansion tank size must match your boiler output — a 12-litre tank works for most domestic boilers, but check your boiler manual or the old tank label to be sure.
  • Disconnect the old tank by unscrewing the union nut at the top, then screw the new tank into the same fitting and reconnect the isolation valve.
  • After installation, you must pressurize the new tank with a bicycle pump or compressor to the correct air pressure — usually 1.5 bar — before refilling the boiler.
  • Bleed air from the system by opening the bleed screw on the pump or highest radiator until water flows, then check the boiler pressure gauge and top up if needed.

Gather the right tools and materials before you start

You will need an adjustable wrench or two (one to hold the fitting, one to turn the nut), a bucket to catch water, and a bicycle pump or air compressor with a pressure gauge. Some tanks come with a Schrader valve (like a car tire valve) already fitted; others need one added. Check what your new tank has before you buy it.

You also need to know the correct air pressure for your new tank. This is usually printed on the old tank or in your boiler manual — most domestic systems use 1.5 bar, but some use 1.0 or 2.0 bar. If you cannot find it, 1.5 bar is a safe starting point for a standard combi boiler. Write this number down before you start.

Have a cloth or towel ready to catch drips, and keep the boiler manual or a photo of the old tank setup nearby so you can see how the pipes connect. If your boiler has a filling loop (a small flexible hose with isolation valves), you will use that to refill the system after the swap.

Turn off the boiler and drain the old tank

Switch off the boiler at the control panel and let it sit for at least an hour so the water cools. Do not work on a hot system — you risk scalding and damaging seals. Once it is cool, locate the isolation valve directly below the expansion tank (a small tap or lever on the pipe leading down from the tank).

Turn the isolation valve clockwise to close it, stopping the flow of water into the tank. Open the drain valve at the bottom of the tank by placing a bucket underneath and turning it counterclockwise. Water will flow out; let it drain completely. If there is no drain valve on the tank itself, you may need to open the drain cock on the boiler or the lowest point of the system.

Once the tank is empty, use a wrench to loosen the union nut at the top of the tank — the large nut that connects the tank to the pipe above it. Turn it counterclockwise by hand or with the wrench until it is loose enough to unscrew by hand. Have a cloth ready to catch any remaining water inside the fitting.

Remove the old tank and fit the new one

Unscrew the union nut completely and lift the old tank away. Set it aside — do not throw it away yet, as you may need to refer to its label for pressure settings or size information. Wipe the threads on the pipe fitting with a clean cloth to remove any debris or old tape.

Take your new expansion tank and check that it has a Schrader valve (the small valve on top that looks like a bicycle tire valve). If it does not, you will need to add one before installation. Wrap the threads of the new tank with plumber's thread seal tape (also called PTFE tape or Teflon tape) — wrap it clockwise around the threads three to four times to create a watertight seal.

Screw the new tank into the fitting by hand first, then use a wrench to tighten the union nut. Tighten firmly but do not over-tighten, as this can crack the fitting. The tank should sit straight and not twist or lean. Once it is find, reconnect the isolation valve below the tank by turning it counterclockwise to open it again.

Pressurize the new tank before refilling the boiler

This step is critical and often skipped — it is why many new tanks fail quickly. The expansion tank must contain air at the correct pressure before water enters it. If you skip this, the tank will fill with water instead of air, and it will not absorb pressure when the boiler heats up.

Locate the Schrader valve on top of the new tank. Attach a bicycle pump or air compressor with a pressure gauge to the valve, just as you would inflate a tire. Pump air into the tank until the gauge reads the correct pressure — usually 1.5 bar for a domestic boiler. Check your boiler manual or the old tank label to confirm the exact figure for your system.

Once you reach the correct pressure, disconnect the pump and check that no air leaks out when you remove it. If air hisses out, the Schrader valve may be faulty — you may need to replace it or have the tank serviced before use.

Refill the boiler and bleed air from the system

Locate the filling loop on your boiler — a small flexible hose with two isolation valves, usually near the bottom of the unit. Open both valves slowly by turning them counterclockwise. You will hear water flowing into the system. Watch the pressure gauge on the boiler; stop when it reaches 1.0 to 1.5 bar (the normal operating range for most boilers).

Close both isolation valves on the filling loop by turning them clockwise. Now you need to bleed air that may be trapped in the system. Locate the bleed screw on the pump (a small screw on top of the pump unit) or on the highest radiator in your home. Place a cloth or small container under it and turn the screw counterclockwise slowly — do not remove it completely.

Air will hiss out, followed by water. Once water flows steadily without air bubbles, close the bleed screw by turning it clockwise. Check the boiler pressure gauge again; if it has dropped below 1.0 bar, open the filling loop again briefly to top it up to 1.0 to 1.5 bar.

Turn the boiler back on and check for leaks

Switch the boiler back on at the control panel. Listen for any unusual sounds — a gurgling or banging noise means air is still trapped in the system. If you hear this, repeat the bleeding process on the pump or radiators until the noise stops.

Watch the pressure gauge for the next 10 to 15 minutes as the boiler heats up. The pressure will rise slightly as the water expands — this is normal and is exactly what the expansion tank is designed to handle. The pressure should not exceed 2.5 bar; if it does, the tank may be over-pressurized or faulty.

Inspect the area around the new tank for any drips or leaks where the union nut connects to the pipe. If you see water, turn off the boiler, let it cool, and tighten the union nut a quarter turn with a wrench. Refill and test again. Small weeps often stop once the system has warmed up and the seals have settled.

Frequently Asked Questions

What size expansion tank do I need?

Most domestic boilers use a 12-litre tank, but the correct size depends on your boiler output and the total volume of water in your system. Check the label on your old tank or your boiler manual — it will state the size in litres. If you cannot find it, a 12-litre tank works for most combi and system boilers up to 30 kW output.

Can I replace the tank myself or do I need a plumber?

You can do it yourself if you are comfortable with basic plumbing and have the right tools. The most important step is pressurizing the new tank to the correct air pressure before refilling — this is where most DIY installations fail. If you are unsure about any step, a plumber can do the job in under an hour for a call-out fee plus labour.

What happens if I forget to pressurize the new tank?

The tank will fill with water instead of air and will not absorb pressure when the boiler heats up. This causes the relief valve to open repeatedly and water to leak out, or the pressure to spike dangerously high. You will need to drain the system, empty the tank, pressurize it correctly, and refill — essentially doing the job twice.

How do I know what air pressure to use?

Check the label on your old tank or your boiler manual — the correct pressure is usually printed there. Most domestic boilers use 1.5 bar, but some use 1.0 or 2.0 bar. If you cannot find the figure, start with 1.5 bar and monitor the boiler pressure gauge when you turn it back on; if the pressure spikes above 2.5 bar, the tank may be over-pressurized.

Why is my new tank leaking at the union nut?

The seal may not be tight enough, or the threads may not be wrapped with enough plumber's tape. Turn off the boiler, let it cool, and tighten the union nut a quarter turn with a wrench. If it still leaks, you may need to drain the system, unscrew the tank, add more thread seal tape, and reinstall it.