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What Can Cause Header Tank Overflow?

Hannah Maza
By: Hannah Maza
Updated: 17th June 2022

The header tank is an essential part of an open vented heating system, found in the loft of properties with a regular boiler. There are several reasons why header tanks overflow and in this article we'll help you to find the cause and fix.

When it comes to a header tank overflowing, we highly recommend contacting a heating engineer to carry out the repair.

What is the header tank?

The header tank, otherwise known as the feed and expansion tank or cistern, is an essential part of an open vented central heating system with a regular boiler.

The role of the header tank is to supply water to the central heating system and also maintain the level of water, which expands when heated. They're located at the highest point of the heating system, usually in the loft, alongside the larger cold water storage tank.

As well as the tank that stores the water, header tanks are made up of:

  • An overflow pipe that runs outside
  • Connection to the mains water supply via a ball valve
  • Cold feed pipe
  • Open vent pipe

It's possible for the cold feed and open vent pipes to be a single pipe in some cases.

What can cause header tank overflow?

There are many reasons why the header tank is overflowing:

  • Issues with pump over run
  • Faulty coil in the hot water tank
  • Ball valve needs repairing or replacing

Pump over run

Pump over run is when the boiler continues to pump for a few minutes after the central heating or domestic hot water is turned off. Most modern boilers have this important feature which prevents heat from building on the heat exchanger.

This can mean that the pump is attempting to do this when the valves are closed and the only place for the hot water to go is up to the header tank. To avoid this, heating systems should have an Automatic Bypass Valve (ABV) which allows the hot water to circulate even when the valves are closed. If your system has an ABV yet hot water is still making its way to the header tank then it might be set too high.

Faulty coil in the hot water cylinder

Hot water cylinders store hot water for domestic use from taps and showers. Within the cylinder is a coil that hot water for the central heating system passes through. As the hot water passes through the coil at the top, its heat is transferred to the domestic hot water, warming it further, before it exits at the bottom of the coil – known as the flow and return.

Over time, this coil can become perforated, meaning there's hole in the coil which allows water for the central heating into the domestic hot water being stored in the cylinder. This rise in water levels can then lead to the feed and expansion tank overflowing.

You'll also be able to tell if the coil in the cylinder has perforated because the water from the taps will have a yellow or brown colour which is from the liquid for the central heating system.

If this is the case, your vented hot water cylinder will need to be replaced.

Ball valve needs repairing or replacing

The ball valve can be turned to allow either a full flow of water or prevent any water flow. Within the valve is a ball, hence the name, which has a hole in the middle and rotates when the valve turned – water will either be allowed to flow through the hole or not. If the valve is stuck they it might need to be repaired, although in some cases the best thing to do is have it replaced to avoid further issues.

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Hot water coming out of overflow pipe

The overflow pipe is there to prevent a water container from overflowing. The hole towards the top of sinks is there to prevent water overflowing onto the floor and header tanks have a pipe that works in the same way.

In the feed and expansion tank, there will be a float valve (another name for ball valves) which controls the flow of water into the tank. If hot water is coming out of the overflow pipe, turn the system off, wait for the water to cool right down and set the float valve to allow less water into the tank.


Being in a loft space that's often unheated can mean that the water in the header tank is susceptible to freezing during the winter. Frozen water in the tank can lead to frozen pipes which greatly increases the risk of leaks.

The solution is to fit an insulation jacket around the tank so that the water stays above freezing. Depending on the size of your header tank, an insulation jacket can cost between £8 – £20.

Build-up of slime

Brown sludge is common in open vented heating systems and if left can cause blockages which can reduce the efficiency of the central heating and potentially cause the boiler to overheat. Sludge with a brown colour can form when objects end up in the header tank – this used to be a major problem but header tanks now come with byelaw 30 kits which include a lid.

If you find slime in the header tank it's possible to remove it yourself by:

  • Turning off the mains water supply to the tank;
  • carefully scooping out the sludge with a bucket;
  • sponging down the tank to remove further sludge; and then
  • once done, refill the tank with cold water.

When removing the sludge, try not to disturb the water too much as it could end up making its way down the overflow pipe.

Your central heating system might also benefit from a powerflush to clear out any sludge, debris or rust that's blocking the water from flowing properly.

How to turn off the mains water supply

To turn the mains water supply off, you'll need to find the inside stop tap. It’s normally found under the kitchen sink, in the garage or in a downstairs toilet or bathroom. Once you've found it, turn it off and then run the cold water taps around your home to completely drain the system.

Would a sealed system be better for your home?

When it comes to replacing your existing boiler, you might want to consider switching to a sealed system and remove the risk of header tank overflow altogether.

Unlike open vented systems, sealed systems don't have a header tank in the loft, instead, they have an expansion vessel that's often fitted into the boiler itself. While an open vented heating system can only work with regular boilers, sealed systems can work with combi, system and regular boilers.

A heating engineer can let you know the most suitable heating system for your home based on the size of your property and the demand for central heating and domestic hot water. You can get free quotes from heating engineers based in your area using Boiler Guide.


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Hannah Maza

About the author

Hannah Maza

Hannah is our resident home heating expert with experience writing about energy-saving solutions. She continues to research the future of heating and renewable energy as the landscape changes.

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