Header tank overflowing

Why is my header tank overflowing?

You may have it hidden away in your loft and forgotten about its existence until you discover that it is overflowing, sending water into your garden from your overflow pipe. There are many reasons your header tank is overflowing.

We have the solutions. Continue reading to learn how to identify the problem and diagnose the cause of your central heating header tank overflowing.

My header tank overflowing – what’s the problem?

Your central heating system header tank is essential. It is located in your loft. Also called a cold water storage tank, it supplies cold water to your central heating system. It also houses the expansion of your hot water tank if ever the water mains shut off.

The overflow pipe is connected to the header tank so that if water overflows, it drains outside the house and not back into your loft. If your header tank is full and the overflow pipe is leaking water outside your house, at least this is doing its job. Water is being safely diverted outside and your loft will not be flooded.

What is a central heating header tank?

A header tank is also known as the F&E (feed and expansion tank) and is used in properties that have an open-vent central heating system. It is located in the loft area of most properties, at the top point of the central heating system.

Your loft’s header tank is an important part of your heating system. It serves the following functions:

  1. It is at a higher elevation than the hot-water tank so it feeds the hot tank by gravity. It also acts as a pressure head, pushing the hot water from the hot tank to your hot water outlets.
  2. It keeps the water level in your hot water tank steady by allowing water to escape from the hot water tank’s expansion.
  3. It serves as a cold water reservoir in the event that your mains water supply is cut or interrupted. The water supply from the header tank can be opened at the top. This means that it shouldn’t be consumed as it could be contaminated.

What causes a header tank to overflow?

There are many reasons your home’s central heating header tank may be overflowing. These are some of the reasons you can see cold water coming out of your overflow pipe.

Pump overrun

A Header tank overflowing is often caused by a pump overrun. This feature allows the boiler’s to pump water even after the central heating and direct hot water has been shut off. This safety feature prevents heat buildup in the heat exchanger, which can cause damage.

The hot water tank overruns because the demand for central heating is turned off. Installing an Automatic Bypass Valve in your heating system will prevent this from happening. The ABV allows hot liquid to circulate in the heating system for several minutes more, even if the valves are closed.

Faulty float mechanism

A float valve mechanism which is like metal or plastic arm is used to monitor the water level in the cold water tank. It stops cold water from flowing into the tank when it reaches a certain level. A ball valve is a combination of a metal arm or plastic arm and an inflatable ball that floats on the water in header tanks.

If the stop valve doesn’t work properly, it won’t rise to the right level and shut off cold water feed. This causes too much water to enter the header tank, and then overflows through the outlet pipe.

The following are some of the problems that could lead to the float mechanism not controlling the inflow of water:

  • The hole in the ballcock, or float, has formed. It will fill up with water and sink rather than floating on it. Your expansion tank will fill up to the maximum level because the ballcock is too heavy with water. The overflow pipe will force out any excess water from the tank.
  • A failed or disintegrated rubber washer means that the float mechanism will not create a watertight barrier against water inflow. The cold water feed will remain open and water will continue to flow into the header tank despite the fact that the ballcock has turned off the water supply.
  • The stop valve will not operate properly if the brass nipple is worn out. Your header tank will continue to fill if your water level is above the recommended level.

Perforated or leaking coils in the hot water tank

The loop that passes hot water for central heating through the hot water cylinder is located in the hot water cylinder. The coil transfers heat from the hot water to the domestic hot water supply.

The coil can leak over time and cause the central heating water flow into the domestic hot-water storage area. This causes domestic water to rise, leading eventually to an overflowing header tank.

You can easily tell if the coil in your hot water cylinder has been perforated by looking at the colour of hot water coming from your hot taps. The coil will need to be replaced if the water turns yellow or brown.

Cold weather freezing

The header tank is found in the loft. It is usually unheated so it is vulnerable to freezing in winter. Frozen pipes can result from the frozen water in your header tank. Insulation jackets can be installed around pipes and header tanks to prevent them from freezing.

Construction of grit or sludge

The header tank’s top is exposed, so dirt and other items can collect. These items can accumulate and create brown sludge which reduces the effectiveness of the header tank. It also causes blockages that decrease the efficiency of your central heating system.

You can easily remove the sludge from the header tank by shutting off the mains water supply, draining the tank and scooping the sludge out with a bucket. You can also arrange for a heating engineer or heating technician to power flush your central heating system to remove any sludge or debris.

High water pressure

The pressure of water entering the header tank could cause it to overflow. Overflow can occur when the pressure in the tank is too high and the ball valve doesn’t work properly. This is a rare problem, but it can be serious. To fix the issue, turn the water flow down by using the isolation valve.

How do you fix a leaking cold water storage tank

Here are some steps to correct an overflowing header tank. These are temporary solutions. If you need a permanent solution, contact a Gas Safe registered heating engineer.

Drain any excess water from the tank

The overflowing of the header tank is usually caused by too much in the cold water tank unit. It is best to empty the tank of this excess water. Open the hot water taps in your house and let them run for a few minutes. The water pressure in the overflow pipe will slowly decrease, until it stops completely.

It is very simple to empty the cold water storage tank. The header tank continuously feeds the hot water tank with cold water. Therefore, running the hot pipe will empty the hot cylinder and allow the cold water in your header tank to flow down to replace it.

How to drain the expansion tank

First, shut off the water supply to your tank by turning off either the mains stopcock, rising main, or both. Next, drain the header tank. Turn on the hot taps, and continue to run them until the tank is empty. After this, remove any sludge from the tank and clean it up. This is the perfect time to perform any maintenance on your header tank.

Set the ballcock level and, if necessary, replace it

To ensure the correct level of the header tank’s water, check the ballcock. It is usually about 1 inch below the overflow. If it’s not, adjust it by gently bending the arm until it is at the correct level. You should also check if the plastic ball is leaky. If it is, then you need to replacing

Check the overflow pipe

Once you have done everything you want to ensure that your backup plan, the overflow pipe is connected properly and working in case the feed and expansion tank decide to try and fill up again.

Check the ball valve or float valve are moving and working freely so that it is shutting off the water supply as and when the level is at what it should be.

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