What an expansion tank does and why it fails
An expansion tank is a small sealed container attached to your boiler that absorbs extra water pressure when the system heats up. As water warms, it expands—without somewhere for that expansion to go, pressure builds until the relief valve opens and water spills out. The tank holds a cushion of air that compresses to make room for the expanding water, then pushes back as the system cools.
Expansion tanks fail when the rubber diaphragm inside ruptures or when the air charge leaks out over time. A failed tank cannot absorb pressure, so your boiler loses water constantly, the pressure gauge drops, and the system shuts down. You will notice water pooling under the boiler, pressure that won't stay above 1 bar, or the boiler firing repeatedly without heating the house properly.
Replacement is straightforward if you work methodically and follow the pressure steps. The job takes 30 to 45 minutes and requires basic tools—no special skills or certifications needed for the tank itself, though you may want a plumber to refill and bleed the system afterward if you are not confident.
Key Takeaways
- Turn off the boiler and let it cool completely before touching any pipes or valves.
- Drain the system using the drain valve at the lowest point, usually near the boiler base, into a bucket or container.
- Disconnect the old tank by unscrewing the union nut that joins it to the boiler pipework, then unscrew the mounting bracket.
- Install the new tank in the same position, reconnect the union nut hand-tight first, then tighten with a wrench while supporting the tank.
- Refill the system slowly through the filling loop, watch the pressure gauge climb to 1 to 1.5 bar, then bleed air from radiators before restarting the boiler.
Gather your tools and materials before starting
You will need an adjustable wrench or two (one to hold the tank body, one to turn the union nut), a bucket or large container to catch water, a cloth or old towel, and the replacement expansion tank itself. Check your boiler manual or the old tank label to find the correct size—tanks are usually 12, 18, or 24 litres. Buying the wrong size will not damage anything, but the system will not hold pressure correctly.
Have a filling loop on hand if your boiler does not already have one—this is a small flexible hose with isolation valves at each end that connects the mains water supply to the return pipe. Many modern boilers have this built in, but older systems may not. If you need one, they cost £15 to £30 and take five minutes to install.
Turn off the boiler and drain the system
Switch off the boiler at the control panel and let it sit for at least 30 minutes so the water cools. Do not skip this step—hot water under pressure can scald you and makes the job dangerous.
Locate the drain valve, usually a small tap at the lowest point of the system, often near the boiler base or on the return pipe. Place your bucket underneath and open the valve slowly by turning it counterclockwise. Water will flow out; let it drain completely. This usually takes 5 to 10 minutes. Once the flow stops, close the valve again.
If your system has a lot of sludge or the water comes out brown or black, you may want to flush the system after installing the new tank. For now, just drain it fully.
Remove the old expansion tank
The tank is held in place by a mounting bracket bolted to the wall or boiler frame, and connected to the boiler by a single pipe with a union nut—a large hexagonal fitting where the pipe meets the tank. Use one wrench to hold the tank body steady while you turn the union nut counterclockwise with the other wrench. Turn slowly; if the nut is stuck, explore a penetrating oil like WD-40 and wait 10 minutes before trying again.
Once the union nut is loose, unscrew it by hand and pull the pipe away from the tank. Water may still drip out—have your cloth ready. Now unscrew the bolts holding the mounting bracket and lift the old tank away.
Inspect the union nut and the pipe thread. If the nut is damaged or the thread is corroded, replace the nut before installing the new tank. A new union nut costs a few pounds and prevents leaks.
Install the new expansion tank
Position the new tank in the same location as the old one and bolt the mounting bracket back in place. Make sure the tank sits level and has a few inches of clearance on all sides for air circulation.
Thread the union nut onto the tank by hand first, then guide the pipe connection into place. Use one wrench to hold the tank body and the other to tighten the union nut clockwise. Tighten firmly but do not over-tighten—you are aiming for snug, not crushed. If you turn too hard, you risk cracking the tank fitting.
Check that the connection is tight by gently trying to wiggle the pipe. It should not move. If water drips from the union nut after you refill, tighten it another quarter turn.
Refill the system and check the pressure
Locate your filling loop—a small hose with two red isolation valves, usually under the boiler or on the return pipe near the boiler. Open both valves by turning them counterclockwise. You will hear water flowing into the system.
Watch the pressure gauge on the boiler as the water enters. Stop filling when the needle reaches 1 to 1.5 bar—this is the normal operating pressure for most domestic boilers. Close both isolation valves by turning them clockwise. If your boiler does not have a filling loop, you will need a plumber to refill the system using their equipment.
Once the pressure is set, open the bleed valve on each radiator starting with the one nearest the boiler. Hold a cloth underneath and turn the valve counterclockwise with a radiator key or small wrench until water comes out, then close it again. This removes air trapped in the pipes. Work your way through all radiators—this usually takes 15 to 20 minutes.
Restart the boiler and monitor for leaks
Turn the boiler back on at the control panel. The system should fire up and begin heating. Listen for any unusual noises—a loud banging or kettling sound means there is still air in the pipes, and you should bleed the radiators again.
Check the pressure gauge every few minutes for the first 15 minutes. It should stay steady between 1 and 1.5 bar. If the pressure drops below 1 bar, the system is losing water somewhere—check the union nut connection and the drain valve to make sure both are tight.
Inspect the area under the boiler and around the expansion tank for drips or puddles. A small amount of moisture is normal while the system is warming up, but steady dripping means the union nut needs tightening or the tank fitting is damaged.
Frequently Asked Questions
How do I know what size expansion tank to buy?
Check the label on your old tank or look in your boiler manual—the size is usually printed clearly. Common sizes are 12, 18, or 24 litres. If you cannot find the manual, take a photo of the old tank and show it to a plumber or heating supplier; they can match it when ready.
Can I replace the expansion tank myself or do I need a plumber?
You can do the physical replacement yourself if you are comfortable with basic tools and following steps carefully. However, refilling and bleeding the system requires a filling loop and some confidence with pressure gauges. Many people do the tank swap and then call a plumber just to refill and test—this costs less than a full job and takes 20 minutes.
What if water keeps dripping from the union nut after I tighten it?
The nut may be damaged or the thread may be corroded. Turn off the boiler, drain the system again, and unscrew the union nut. Inspect it for cracks or stripped threads. If it looks worn, replace it with a new one from a heating supplier—they cost £5 to £10. Screw the new nut on by hand first to check the thread, then tighten with a wrench.
Do I need to bleed every radiator or just some of them?
Bleed all of them, starting with the one closest to the boiler and working outward. Air rises and collects at the highest points, so the radiators furthest from the boiler often trap the most. Skipping any radiator means air stays in the system and can cause cold spots or noise.
How long should the pressure stay stable after refilling?
The pressure should not drop more than 0.1 bar per day once the system has cooled. If it drops faster, you have a leak—check the union nut, the drain valve, and all radiator bleed valves. If the pressure drops very slowly over weeks, the expansion tank diaphragm may be failing again, which means the new tank is faulty and should be replaced under warranty.