What a boiler expansion tank does and when to replace it

A boiler expansion tank is a small sealed vessel that absorbs extra water pressure when your heating system warms up. As water heats, it expands; the tank gives that expanded water somewhere to go instead of building dangerous pressure inside your pipes and boiler. Over time — usually 5 to 10 years — the internal bladder or diaphragm that separates air from water wears out, and the tank stops working. You'll notice the boiler cycling on and off more often, pressure gauges climbing higher than normal, or relief valves leaking water.

Replacing an expansion tank is a job most homeowners with basic plumbing confidence can do in an afternoon. You'll need to drain part of the system, unscrew the old tank, install a new one, and refill. The tank itself costs between $150 and $400 depending on size; labour from a plumber runs $300 to $600. If you do it yourself, you save the labour cost but need to be comfortable working with water, valves, and a wrench.

Key Takeaways

  • An expansion tank absorbs pressure from heated water and fails after 5 to 10 years, causing the boiler to cycle more often or pressure gauges to climb.
  • You will need to shut off the boiler, close isolation valves, drain the system partially, and unscrew the old tank before installing a new one of the same size.
  • Tank size matters: a 2-bedroom home typically needs an 8-litre tank, while larger homes need 12 to 20 litres; check your old tank or boiler manual to confirm.
  • After installation, you must refill the system slowly, bleed air from radiators, and check that pressure sits between 1 and 1.5 bar on the gauge.
  • If you are not comfortable draining the system or working with pressurised water, a plumber can complete the job in under two hours.

Identifying the expansion tank and checking its condition

The expansion tank is usually a small cylindrical or oval vessel mounted on or near the boiler, often in a cupboard or utility room. It may be painted to match the boiler or left bare metal. Look for a small air valve (like a bicycle pump valve) on the top or side — this is the telltale sign you've found it. Check your boiler manual or the tank itself for its size, printed on a label or stamped into the metal.

To confirm the tank has failed, turn off the boiler and let it cool completely. Unscrew the air valve cap and press the valve stem with your finger, as you would to check a car tyre. If water sprays out instead of air, the internal bladder has ruptured and the tank must be replaced. If only air comes out, the tank may still be serviceable, though low air pressure (below 0.5 bar) also signals failure. Write down the tank size and model number before you order a replacement.

Tools and materials you will need

Gather these items before you start: an adjustable wrench or two (one to hold the fitting, one to turn the nut), a bucket or large container to catch water, a cloth or old towel, a new expansion tank of the same size as the old one, and a pressure gauge if your boiler does not have one built in. You may also need a hose to connect to the drain valve if your boiler sits far from a floor drain.

If the old tank is corroded or the fittings are stuck, have penetrating oil (such as WD-40) on hand and let it soak for 15 minutes before attempting to unscrew anything. A heat gun can also help loosen stubborn fittings, though do not use it near plastic pipes or the boiler itself. Wear work gloves and safety glasses — boiler systems can hold hot water even after the boiler is switched off.

Draining the system and removing the old tank

Switch off the boiler at the control panel and let it cool for at least 30 minutes. Locate the isolation valve (or ball valve) on the pipe leading into the expansion tank — it looks like a small tap with a lever handle. Turn the lever 90 degrees to close it. If there is no isolation valve, you will need to drain the entire heating system using the drain valve at the bottom of the boiler; this is messier and takes longer, so check first.

Place your bucket under the expansion tank and unscrew the cap or blanking plug at the bottom of the tank (if it has one) to let water drain out. Once the water stops dripping, use your wrench to unscrew the main fitting that connects the tank to the pipe. Hold the fitting steady with one wrench while you turn the nut with the other, working slowly to avoid stripping the threads. Once loose, unscrew by hand and set the old tank aside.

Installing the new expansion tank

Before you screw the new tank on, check that the threads on the pipe fitting are clean and free of debris. Wrap the male threads (the part that screws into the tank) with plumber's thread tape — wrap it clockwise three times around. This creates a watertight seal and makes future removal easier. Screw the new tank on by hand first, then tighten with your wrench, holding the fitting steady as you did when removing the old one.

Do not over-tighten; stop as soon as you feel solid resistance. If you turn too hard, you risk cracking the tank or stripping the threads. Once the tank is find, open the isolation valve again by turning the lever handle back to its original position. The system is now ready to refill.

Refilling the system and checking pressure

Locate the filling loop — a flexible hose with two small valves, usually near the boiler. Open both valves slowly; water will flow into the system. Watch the pressure gauge on the boiler as you fill. Stop when the gauge reads between 1 and 1.5 bar. Close both valves on the filling loop once you reach the target pressure.

Turn the boiler back on and let it run for 10 to 15 minutes. As the water heats, the pressure will climb slightly — this is normal. Check that it does not exceed 2 bar; if it does, you overfilled the system and must drain a small amount using the drain valve. Once the boiler has cycled once and the pressure has stabilised, switch it off and let it cool. Check the pressure again when cold; it should be back to 1 to 1.5 bar.

Bleeding air from radiators and final checks

After refilling, air pockets often get trapped in radiators, making them warm unevenly or not at all. Bleed each radiator by opening the small bleed valve (usually a square socket on the top corner) with a radiator key or small wrench. Hold a cloth underneath to catch water. Open the valve a quarter turn and listen for a hiss of air; once water flows steadily, close the valve. Work through all radiators in the house, starting with those closest to the boiler.

Once all radiators are bled and warm evenly, check the boiler pressure one more time. It should sit between 1 and 1.5 bar when cold. If pressure keeps climbing above 2 bar even after bleeding, the new tank may be the wrong size or the air charge inside it may be incorrect — contact a plumber to check. If pressure drops below 1 bar within a few days, you may have a small leak at the new tank fitting; tighten it slightly or call a plumber to reseal it.

Frequently Asked Questions

What size expansion tank do I need?

Tank size depends on your boiler's output and the total volume of water in your heating system. A typical 2-bedroom home uses an 8-litre tank; larger homes or high-output boilers may need 12 to 20 litres. Check the label on your old tank or consult your boiler manual. Ordering the wrong size will cause pressure problems and may damage the boiler.

Can I replace the expansion tank myself if I have never done plumbing before?

If you are comfortable using a wrench and working with water, yes. The job involves only a few bolts and basic steps. However, if you are nervous about draining the system or working with pressurised water, a plumber can finish the job in under two hours for a reasonable cost. It is better to hire help than to damage the boiler or create a leak.

How do I know if the expansion tank is the problem and not something else?

Press the air valve on top of the tank with your finger after the boiler has cooled. If water sprays out, the bladder is definitely broken. If only air comes out but the boiler is still cycling too often or pressure is climbing, the tank may have low air charge. A plumber can test the air pressure inside with a gauge to confirm.

What happens if I do not replace a failed expansion tank?

The boiler will cycle on and off more frequently as pressure builds up, wasting fuel and wearing out the pump and controls faster. Over time, the relief valve may leak continuously to release excess pressure, wasting water and money. In worst cases, pressure can climb high enough to damage the boiler or burst a pipe.

Do I need to bleed the entire system or just the radiators?

Bleed only the radiators. The boiler and pipes usually purge air on their own as the system heats up. If a radiator stays cold at the top after bleeding, air is still trapped; bleed it again. If all radiators are warm and the boiler pressure is stable, the system is working correctly.