Summary. Expansion vessels, also known as expansion tanks, keep the pressure in a combi boiler stable while water heats and cools. They are typically located inside the boiler casing. The typical pressure a boiler system should be set to is 1 bar.

What happens when an expansion vessel fails?

If water comes out of the valve you have a definite expansion vessel fault. In this case the vessel has definitely failed and must be replaced. If no water comes out but no air either, you have an expansion vessel fault but the diaphragm may still be intact and it may be possible to re-pressurise.

Why do I need an external expansion vessel?

It’s a requirement in many closed-water heating systems because water expands as it heats up. If the extra volume of water didn’t have anywhere to go, the pressure in the system could rise to high levels, at which point the pressure relief valve would open to compensate.

How do you know if your expansion vessel is broken?

If the boiler system you are using is more than 20 years old,then it is likely that the leak is at the expansion vessel or the auto air vent. Water leaking from the pipe is often sign of a fault within the expansion vessel.

Should an expansion vessel be full of water?

You are correct. The expansion tank is usually about half full of water and half full of air–by volume. One tank full of air at atmospheric pressure will be about half its volume when pressurized. When in doubt, drain it.

How long do expansion vessels last?

An expansion vessel should last between 5-10 years.

Can a faulty expansion vessel cause pressure loss?

3-You have a faulty expansion vessel You have a faulty expansion vessel- This means your boiler pressure will exceed 3 bar during the heating up process. … When the system cools down you will find the water lost will now show on the pressure gauge as a significant drop.

What does an expansion tank on a boiler do?

The expansion tank is designed to handle the thermal expansion of water as it heats up in the water heater, preventing excessive water pressure. If water pressure gets to high it can damage valves in plumbing fixtures, joints in supply pipes and the water heater itself.

Do I need an expansion vessel?

As we all know, when water is heated it expands and wants to take up more volume. As the system is basically a closed loop, consisting of the boiler, pipework and radiators, it is necessary to provide some space for the water to expand into. This is why the expansion vessel is required.

What is the red tank next to my boiler?

Expansion vessel: This is a small metal tank, usually red in colour, often located either in or near the boiler or near the hot water cylinder, in which a rubber sheet, or diaphragm, separates central heating water from pressurised air and this can leak allowing water to escape into the ‘air only’ part.

Article first time published on

Does a combi boiler need an expansion tank?

On the central heating side, combi boilers pump the water round the heating system in a completely sealed system. The boiler incorporates an expansion tank internally, so there is no need for an external feed and expansion tank in the loft, which obviously saves a great deal of space.

What pressure should be in an expansion vessel?

Some important things to consider is the air pressure inside the expansion vessel. It rarely needs to be anything more than 1.5 bar in most domestic properties. Anything more and the water may not expand into the air at the required time and could risk the high pressure relief valve opening or worse.

How do I know if my expansion tank is working?

The tank should be full of air and therefore sound hollow. If it makes a dull thud rather than a hollow sound, your tank is full of water and needs repair or replacement. You can also gauge this by feeling the tank, which will be cool where it’s holding air and warm where it’s holding water.

What is the difference between a bladder tank and a diaphragm tank?

What is the difference between a bladder tank and a diaphragm tank? A bladder tank contains a vinyl tank inside it. A bladder tank can be replaced as and when required. On the other hand, a diaphragm tank contains a rubber flat diaphragm, a butyl tank or a vinyl tank that is mounted on it.

How often should you need to top up boiler pressure?

The pressure in the system will usually require topping up once or twice a year. If you are having to repressurise your heating system much more frequently, please contact your installer.

Can a dripping tap affect water pressure?

Leaking Pipes Even though a leak might not be that substantial, even the smallest of leakages may affect your water pressure. This is because a minor leak might be misdirecting the water supply and as a result, you will not be getting the full flow of water.

Should boiler pressure drop when off?

When your boiler is switched off, the pressure gauge should read around 1 Bar – in the green zone on the gauge. When it is in operation (demanding heat/hot water), its pressure will increase slightly, then it should drop back down.

How often should expansion vessels be inspected?

Your expansion vessel should be check at least once a year, We were shown into the boiler room and found two system boilers, the system was sealed and pressurized.

How do I know if my expansion tank has failed?

Check your relief valve on the water heater. It’s located on the side of the water heater with a tube that goes to the floor. If the relief valve is dripping, this could indicate your expansion tank has failed.

Is an expansion tank required for a water heater?

While expansion tanks are required by code in most closed-loop water heater systems, they have a reputation for failing. Common expansion tank issues include: Rubber Diaphragm Wear Out.

How much does it cost to install an expansion tank?

The job of the thermal expansion tank is to absorb the extra water volume so that the excess pressure does not cause undue wear and tear on the water heater and other fixtures in the home. The cost to install a new Thermal expansion tank can range from $279 to $443.

Do you bleed a radiator when it is on or off?

Turn off your heating. You can’t bleed a radiator when the heating is on, as it may be too hot to touch. You could also get hot water spraying out of the radiator. Use your radiator key to turn the valve at the top of the radiator.

Can you run out of hot water with a combi boiler?

A system boiler takes water from the mains supply, like a combi boiler, and heats it in the heat exchanger. … If your cylinder is too small, you’ll run out of hot water frequently. If your hot water tank is too big, your gas bill may be very high, as you’re heating water that isn’t being used.

Can a combi boiler fill a bath?

For example, a 24-27kW combi boiler can heat up to 10 radiators and provide hot water for one bathroom.

Can I turning off mains water with combi boiler?

As long as your boiler is in good working order, you shouldn’t even have to worry about this small risk. A combi boiler should shut down automatically if it detects a drop in mains pressure or if the temperature inside the system gets too high, as would happen without a supply of cold water.

What is the difference between an expansion vessel and a pressure vessel?

The most significant difference between expansion and pressure tanks is their functionality. An expansion tank handles water expansion and provides protection for water valves and heaters. On the other side, the pressure tank lengthens the lifespan of the pump.

Can an expansion tank explode?

As water heats up, it can expand dramatically, and if it’s hot enough to turn into steam, this can add even more pressure to the water heater. If the relief valve is malfunctioning, this can lead to an explosion.

Why is my expansion tank full of water?

Some tanks fail when a leak develops in the diaphragm. This usually causes the tank to fill with fluid and become “water logged.” You can check for this by pressing in the stem of the Schrader valve. If a stream of liquid comes out the tank is toast.

Will an expansion tank increase water pressure?

The expansion tank contains air, which is highly compressible. Expanding water from the water heater can flow into the expansion tank, where the air compresses, making room for the increased volume of water. Water pressure in the system doesn’t increase significantly.