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What Can Cause Central Heating Header Tank Overflowing?


Hidden away in the loft, you tend to forget the existence of the header tank until the day you find it overflowing and sending cascades of water into your garden. However, you shouldn’t panic when you see your header tank overflowing, as there are several reasons why it is happening, and we have the solutions to those.  Stick on to the end of this guide, and you will be able to diagnose why your header tank is overflowing and identify the solution to fix it.

What is a central heating header tank?

The header tank, also known as the feed and expansion tank (F&E), is a small cold water storage tank found in properties with an open vented central heating system. It is usually located at the highest point of the heating system, which is the loft area in most properties.

The header tank in your loft is an essential part of your home’s heating system as it serves the following purposes:

  1. As it is located at a higher level than the hot water tank, it feeds the hot water tank via gravity. It also provides a pressure head that pushes the hot water in the hot water tank to your home’s hot water outlets.
  2. It maintains the water level in the hot water tank by providing an escape for the excess water resulting from the expansion of water in the hot water tank as it is heated.
  3. It acts as a cold water reservoir should the mains water supply to your home be interrupted or cut off. However, since the header tank is open at the top, the water it supplies shouldn’t be used for drinking as it might be contaminated.

What are the causes of a header tank overflowing?

There are several reasons why your home’s header tank is overflowing. Some of these reasons include:

Pump overrun

Pump overrun is one of the main reasons behind header tanks overflowing. It is a feature that allows the boiler to continue pumping water even after the CH or DH has been turned off. This is usually a safety mechanism that prevents heat from building up in the heat exchanger and damaging it.

And since turning off the demand for central heating or domestic hot water closes the valve CH and DHW valves, the water in the hot water tank is pumped to the header tank, causing it to overflow. To avoid this, consider installing an Automatic Bypass Valve (ABV) in your heating system. The ABV allows hot water to circulate in your heating system for a few more minutes, even when the valves are closed.

Faulty float mechanism

The header tank has a float mechanism that monitors the level of water flowing into the tank and stops the inflow of water when the water reaches a certain level.  Header tanks usually have a ball valve consisting of a metal or plastic arm and a plastic ball that floats on water. When this stop valve isn’t working correctly, it will not rise to the correct level and shut off the water supply.  This results in too much water entering the header tank and overflowing through the outlet pipe.

Some of the common issues that may result in the float mechanism failing to control the inflow of water include:

  • The ballcock or float has developed a hole, which will fill up with water and sink instead of floating on the water. As the ballcock would be heavy with water, it won’t be able to rise and shut off the water supply, resulting in your header tank filling up way past the required level. Therefore, the excess water in the tank will be forced out through the overflow pipe.
  • Failed or disintegrated rubber washer, meaning the float mechanism won’t create a watertight seal against the inflow of water. This will result in water forcing its way into the header tank even when the ballcock has shut off the water supply.
  • Worn out brass nipple, which would be unable to create a seal, hence the stop valve won’t operate normally. A failed stop valve will result in your header tank continuing to fill even if the water has reached the recommended level.

Leaking or perforated coil in the hot water cylinder

The hot water cylinder contains a loop through which the hot water for central heating passes. The hot water passing through the coil transfers some heat to the surrounding domestic hot water.

However, the coil may start leaking with time, causing the central heating water to flow into the domestic hot water storage area. This increases the domestic water level, leading to the overflowing header tank.

The good thing is, you can quickly tell whether the coil in the hot water cylinder is perforated by observing the colour of the hot water coming out of your hot taps. If the water is yellow or brown, it means the coil is leaking and will need replacing.

Freezing in cold weather

Since the header tank is located in the loft, which is often unheated, it is exposed to cold temperatures and is susceptible to freezing in the winter. The frozen water in the header tank will most likely result in frozen pipes, which is a recipe for leaking. To mitigate the issue of frozen header tanks and pipes, consider installing insulation jackets around them as they will help keep the water above freezing point.

Build-up of grit and sludge

As the header tank is open at the top, it is prone to collecting dirt and other objects such as leaves and twigs. These objects accumulate and form brown sludge, which reduces the effective capacity of the header tank and cause blockages that reduce the efficiency of your central heating system.

Luckily, you can remove sludge from your header tank by turning off the mains water supply to the tank, draining it, and scooping out the sludge using a bucket. Alternatively, you can arrange for a heating engineer to power flush your central heating to clear any sludge and debris within it.

High water feed pressure

Your header tank might also be overflowing due to the pressure of the water entering it being too high. When the pressure of the water flowing into the tank is too high, the tank will fill up faster than the water is being drained from it, leading to an overflow. Although this issue is rare, it is a serious one and can be fixed by turning the water flow down using the isolation valve.

How to fix overflowing header tank

As earlier promised, here are some simple steps you can take to fix the issue of an overflowing header tank. However, remember these are temporary fixes, and if you want to get a permanent solution for the problem, consider contacting a certified heating engineer.

  1. Empty the excess water in the tank

Since the header tank overflow is caused by excess water in the tank unit, emptying the tank will most likely end the overflowing. To achieve this, open the hot taps in your home and keep them running for a while. And with time, the amount of water coming out of the overflow pipe will gradually decrease until it stops altogether.

The reason why emptying the tank works is simple. The header tank continually feeds the hot water cylinder with cold water; therefore, running the hot pipes will empty the hot water cylinder, and the cold water in the header tank will flow down to replace it.

  1. Drain the header tank

To drain your header tank, first, turn off the water supply to the tank by turning off the mains stopcock or rising main. Next, turn on your hot taps and keep them running until the tank is empty. When this is done, scoop out any sludge in the tank and clean it out. If your header tank requires any general maintenance, this would be the ideal opportunity to do it.

  1. Reset the ballcock level, and if it is worn out, install a replacement

Check the header tank’s ballcock to ensure it is at the right level, usually 1 inch below the overflow. Readjust it by carefully bending its arm to the right level if it isn’t. Also, check whether the plastic ball is leaking, and if it is, you should consider replacing it.

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