What a boiler expansion tank does and when to replace it

A boiler expansion tank is a small sealed vessel that absorbs the extra pressure created when water heats up inside your boiler. As water warms, it expands — the tank gives that expansion somewhere safe to go instead of building pressure that could damage pipes or the boiler itself. When the water cools, the tank releases that pressure back into the system.

You need to replace the tank when it stops holding air pressure, which usually happens after 5 to 10 years. Signs of failure include a hissing sound when the boiler fires up, water leaking from the tank, or your boiler pressure gauge climbing too high too quickly. Once the internal bladder (the rubber membrane inside) ruptures, the tank cannot do its job and must come out.

Replacement is a job most homeowners can do with basic tools and a few hours of time, though you will need to drain part of the system and handle some plumbing connections. If you are not comfortable working with water systems or your boiler is still under warranty, a technician is the safer choice.

Key Takeaways

  • Turn off the boiler and let it cool completely before starting work, then close the isolation valve on the feed pipe to stop water from flowing into the tank.
  • Drain the system using the drain cock at the bottom of the boiler until the water level drops below the expansion tank connection point.
  • Disconnect the old tank by unscrewing the nut that holds it to the tee fitting, then unscrew the air valve on top to release any remaining pressure.
  • Install the new tank by screwing it onto the same tee fitting, checking that the air pressure inside matches the boiler's cold-water pressure before refilling.
  • Refill the system slowly, bleed air from the radiators, and check the boiler pressure gauge to make sure it reads between 1 and 1.5 bar when cold.

Tools and materials you will need

Gather these items before you start: an adjustable wrench or spanner (usually 15mm or 22mm, depending on your tank size), a bucket or tray to catch water, a pressure gauge that fits your tank's air valve, a new expansion tank of the same size as the one you are removing, and a cloth or rag. You may also need a pipe wrench if the connections are tight, and a small amount of PTFE tape (plumber's tape) to wrap around the threaded connection on the new tank.

Check your boiler manual or the old tank itself to find the correct size — expansion tanks are usually 8, 12, 18, or 24 litres. Buying the wrong size will not cause damage, but it will not work as well. If you cannot find the size, take a photo of the old tank and bring it to a plumbing supplier; they can match it for you.

Shutting down the boiler and draining the system

Turn off the boiler at the control panel and let it sit for at least 30 minutes so the water inside cools to a safe temperature. While you wait, locate the isolation valve on the feed pipe — this is usually a small lever or ball valve on the pipe that runs from the water supply into the boiler. Turn it fully closed (the lever should point across the pipe, not along it).

Find the drain cock at the bottom of the boiler or on the lowest part of the heating system. Place a bucket underneath and open it slowly. Water will flow out; let it drain until the level drops below the point where the expansion tank connects to the system. You do not need to drain the entire system, just enough to work safely on the tank.

Once the water stops flowing, close the drain cock. Leave the bucket in place — there will be more water when you disconnect the tank.

Removing the old expansion tank

The expansion tank screws onto a fitting called a tee, which is part of the pipe that feeds water into the boiler. Use your wrench to hold the tee steady with one hand while you unscrew the tank with the other. Turn it counterclockwise; it should come free after a few turns. Water may drip from the connection, so keep your bucket ready.

Once the tank is loose, unscrew the small air valve on top of it using a smaller wrench or an air-valve tool. This releases any remaining pressure inside the tank and makes it safe to handle. Set the old tank aside.

Wipe the threads on the tee fitting clean with a cloth. If there is old PTFE tape wrapped around the threads, pick it off. This gives you a clean surface for the new connection.

Installing the new expansion tank

Wrap 3 to 4 layers of PTFE tape clockwise around the threaded connection on the new tank. This tape seals the joint and stops water from leaking. Screw the new tank onto the tee by hand first, then use your wrench to tighten it firmly — do not over-tighten, as this can crack the tank.

Before you refill the system, you must check and set the air pressure inside the new tank. Use a pressure gauge on the air valve at the top. The pressure should match the cold-water pressure of your boiler system, which is usually between 1 and 1.5 bar. If the pressure is too low, use a bicycle pump or an air compressor to add air until it reaches the right level. If it is too high, press the valve to release some air.

Getting this pressure right is important — if it is wrong, the tank will not absorb expansion properly and your boiler pressure will climb too high.

Refilling and testing the system

Open the isolation valve on the feed pipe slowly. Water will flow back into the system. Watch the boiler pressure gauge as it fills; it should climb slowly and stop around 1 to 1.5 bar when the system is full. If it climbs faster than that or goes above 1.5 bar, close the isolation valve and check that the expansion tank air pressure is correct.

Once the system is full, you need to bleed air from the radiators. Starting with the radiator closest to the boiler, open the small bleed valve (usually a square nut on the top corner) using a bleed key or small wrench. Hold a cloth underneath and turn the valve counterclockwise slowly. Air will hiss out; keep the valve open until water starts to drip, then close it. Repeat this on each radiator in the house.

Turn the boiler back on and let it run for 10 to 15 minutes. Check the pressure gauge again — it should rise slightly as the water heats, but should not go above 2 bar. If it does, the expansion tank may not be working correctly and you should turn off the boiler and check the air pressure again.

Checking your work and when to call a technician

Over the next few days, check the boiler pressure gauge each time you pass it. It should stay steady between 1 and 1.5 bar when the boiler is off. If it climbs above 2 bar when the boiler is running, or if you see water leaking from the new tank connection, turn off the boiler and check that the tank is screwed on tightly and that the air pressure is correct.

If the pressure keeps climbing even after you have checked everything, or if you hear hissing or banging from the pipes, the new tank may be faulty or the air pressure may have been set wrong. At that point, it is worth calling a technician to diagnose the problem — trying to fix it yourself could damage the boiler.

If your boiler is still under warranty, check the paperwork before you start. Some warranties require that a technician do the work, and opening the system yourself could void the coverage. In that case, the cost of a technician visit is worth protecting the warranty.

Frequently Asked Questions

How do I know what size expansion tank to buy?

Check your old tank or your boiler manual — the size is usually printed on a label. Common sizes are 8, 12, 18, or 24 litres. If you cannot find it, take a photo of the old tank to a plumbing supplier and they will match it. Buying the wrong size will not cause damage, but it will not work as well as the correct one.

What if water keeps leaking from the connection after I install the new tank?

Turn off the boiler and let it cool. Use your wrench to tighten the nut where the tank screws onto the tee — sometimes a quarter turn is enough. If it still leaks, unscrew the tank, wrap more PTFE tape around the threads, and screw it back on. Do not over-tighten, as this can crack the tank.

Can I replace the expansion tank without draining the whole system?

Yes. You only need to drain the system until the water level drops below the tank connection point. This saves time and water. Close the isolation valve on the feed pipe first so water does not keep flowing in while you work.

What should the air pressure be inside the new tank?

It should match your boiler's cold-water pressure, which is usually between 1 and 1.5 bar. Check your boiler manual or look at the pressure gauge when the boiler is off and cold. Use a pressure gauge on the tank's air valve to check and adjust it before you refill the system.

How long does the whole job take?

Plan for 2 to 3 hours if this is your first time. Most of that is waiting for the boiler to cool, draining the system, and bleeding air from the radiators. The actual disconnection and installation takes about 20 minutes.