What an expansion tank does and when it needs replacing
An expansion tank is a small sealed vessel attached to your boiler that absorbs extra water pressure as your heating system warms up. When water heats, it expands—without somewhere for that expansion to go, pressure builds and can damage pipes, valves, and the boiler itself. The tank contains air on one side and connects to the water side of the system on the other, acting as a cushion.
Expansion tanks fail when the internal air bladder ruptures or loses pressure over time, usually after 5 to 10 years. Signs of failure include a pressure gauge that climbs too high when the system heats, water leaking from the boiler's relief valve, or a tank that feels heavy and waterlogged when you tap it. If you notice any of these, the tank needs replacing.
Replacing an expansion tank is a job you can do yourself if you are comfortable with basic plumbing and have the right tools, but it involves draining part of the system and working with pressurized water. If you are unsure at any step, or if your boiler is under warranty, call a heating engineer—a mistake here can damage your boiler or create a safety hazard.
Key Takeaways
- An expansion tank absorbs pressure from heating water and fails when its internal bladder ruptures, usually after 5 to 10 years of use.
- You will need to drain the system, disconnect the old tank, check the air pressure in the new one, and reconnect it before refilling.
- The tank size and air pressure setting depend on your boiler's capacity and system design, so check your boiler manual or ask a heating engineer before buying a replacement.
- If your boiler is under warranty or you are not confident working with pressurized systems, call a professional rather than risk damage.
Gather the right tools and a replacement tank
Before you start, you need the correct replacement tank. Expansion tanks come in different sizes—typically 8, 12, 18, or 24 litres—and the size you need depends on your boiler's water capacity and the system design. Check your boiler manual for the recommended tank size, or measure the old tank and buy the same size. If you cannot find the manual, a heating engineer can tell you the correct size in one phone call.
You will also need a few basic tools: an adjustable wrench or two, a bucket to catch water, a pressure gauge (usually included with new tanks or available cheaply), a bicycle pump or air compressor, and towels. If your boiler has isolation valves on either side of the expansion tank, the job is much simpler—you can isolate the tank without draining the whole system. If there are no isolation valves, you will need to drain the boiler down to the level of the tank connection, which takes longer.
Before you buy, check whether your boiler is still under warranty. Many manufacturers void the warranty if anyone other than a registered engineer touches the system. If that applies to you, call a professional instead.
Turn off the boiler and drain the system
Switch off the boiler at the control panel and let it cool for at least an hour. Do not work on a hot system—you risk scalding and the pressure will be higher, making the job harder and more dangerous.
If your expansion tank has isolation valves on both sides, close them now by turning the handles clockwise. If there are no isolation valves, you will need to drain the whole system down to the level of the tank connection. Locate the drain valve at the bottom of the boiler (usually a small tap), attach a hose, and run it to a bucket or drain. Open the drain valve and let water flow until the level drops below the tank connection point. This can take 20 to 30 minutes depending on system size. Once drained, close the drain valve.
If you have isolation valves, you can skip the full drain. Instead, open the small bleed screw on top of the expansion tank to release any remaining pressure, then place a bucket underneath before you disconnect the tank.
Remove the old tank and check the new one's pressure
Using your adjustable wrench, disconnect the pipe connection at the bottom of the expansion tank. Water may still drip out, so keep your bucket ready. Once the pipe is free, unbolt the tank from its bracket or mounting point and lift it away.
Before you install the new tank, you must check its air pressure. New tanks arrive pre-pressurized, but the pressure can drop during storage or transport. Use a pressure gauge and bicycle pump or compressor to check the air side of the tank. The correct pressure depends on your system—it is usually between 1 and 1.5 bar, but check your boiler manual or the label on the new tank itself. If the pressure is too low, use the pump to add air until it matches the specification. If it is too high, press the valve to release a little air.
Getting this pressure right is important. If it is too low, the tank will not cushion the pressure properly. If it is too high, the tank will not absorb enough expansion and pressure will still climb.
Install the new tank and refill the system
Position the new tank in the same location as the old one and bolt it securely to its bracket. Reconnect the pipe to the bottom of the tank using your wrench—tighten it firmly but do not over-tighten, as you can crack the fitting.
If you drained the whole system, open the drain valve again and refill slowly through the filling loop (a small lever or valve near the boiler, usually marked with a blue handle). Watch the pressure gauge on the boiler as you fill—stop when it reaches about 1 bar. If you isolated the tank with valves, straightforward open both isolation valves to reconnect it to the system.
Once the system is refilled, bleed any air from the highest radiator in your home by opening its bleed valve with a radiator key until water runs out. This removes air that entered during the drain and refill.
Test the boiler and check for leaks
Turn the boiler back on and let it heat up to normal operating temperature. Watch the pressure gauge—it should rise to between 1.5 and 2 bar as the system warms, then stabilize. If the pressure climbs much higher than that, the expansion tank may not be working correctly, and you should turn off the boiler and call a heating engineer.
Inspect the connection where the tank pipe joins the boiler and the tank itself for any drips or leaks. A small weep is normal when ready after installation, but steady dripping means the fitting is not tight enough. If you see a leak, let the system cool, drain it back down, and tighten the connection with your wrench.
Leave the system running for 24 hours and check the pressure gauge again. It should remain stable. If pressure climbs steadily or drops, there may be a leak elsewhere in the system, and you should call a heating engineer to investigate.
When to call a professional instead
Call a heating engineer if your boiler is under warranty, if you do not have isolation valves and are not comfortable draining the system, if you cannot find the correct tank size, or if the pressure gauge behaves strangely after installation. You should also call if the old tank is corroded or stuck and will not disconnect, or if you notice water pooling around the boiler after refilling.
A heating engineer can also install isolation valves if your system does not have them, which makes future tank replacements much simpler. This is a small job that usually costs less than a full replacement and is worth doing if you plan to stay in the house for several more years.
Frequently Asked Questions
How do I know if my expansion tank has failed?
A failed tank usually shows itself as a pressure gauge that climbs too high when the boiler heats up, water spraying from the relief valve, or a tank that feels full and heavy when you tap it. You can also check by pressing the valve on top of the tank—if water comes out instead of air, the bladder has ruptured and the tank needs replacing.
Can I replace the expansion tank myself if I have never done plumbing before?
If you are comfortable with basic hand tools and can follow instructions carefully, yes—the job is mostly disconnecting and reconnecting pipes. The trickiest part is draining the system if you do not have isolation valves. If you are unsure at any point, or if the tank is stuck, stop and call a heating engineer rather than risk damaging the boiler.
What happens if I install the wrong size expansion tank?
A tank that is too small will not absorb enough expansion, and pressure will climb too high. A tank that is too large will waste space and money but will not damage anything. Always check your boiler manual or ask a heating engineer for the correct size before you buy.
Do I need to bleed air from the system after replacing the tank?
Yes. Even if you only isolated the tank with valves, some air usually enters the system during disconnection. Bleed the highest radiator in your home using a radiator key until water runs out. If you drained the whole system, bleed all radiators starting from the lowest and working upward.
How often do expansion tanks need replacing?
Most expansion tanks last 5 to 10 years before the internal bladder fails. The lifespan depends on water quality, system pressure, and how often the boiler cycles. Once one fails, you will see the signs quickly—pressure climbing too high or water leaking from the relief valve.