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Cold Water Header Tank Overflowing: How to Replace a Ballcock Washer

Eugene is a trained engineer and self-taught home improvement enthusiast with almost 40 years of professional and DIY experience.

An Overflowing Water Tank in Your Attic

You hear water running outside your home. You look up and see a pipe sticking out from the edge of the roof or soffit which you've never even noticed before. Water is trickling out from the pipe and down onto the ground.

That mysterious pipe is the overflow from the cold water header tank (also called a cistern) in your attic. If the tank overfills, excess water flows safely via the overflow to the outdoors. It's not likely to be a catastrophe, and the flow may only be a trickle, but if you have a water meter and pay for your water, obviously the cost of this wastage can add up.


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What's Covered in This Guide

This guide explains the basics of the pipes and valves in home plumbing systems and how to remedy an overflowing cold-water tank in the loft. This tank is commonly found in UK plumbing systems. You can use the same repair principles for remedying a leaking toilet cistern. There is some variation in the type of valves fitted to tanks. Newer tanks are fitted with diaphragm valves, whereas older tanks use a sliding valve system with a replaceable washer as shown in the photos below.

Note: This article covers replacing a washer in an old style ballcock valve (part 1 valve). This valve is actually against regulations in new installations. There are several different types of valves different to the one below, but all of them use some form of fiber/rubber washer/O-ring/diaphragm system to shut off flow.

What Is the Cold Water Tank in the Attic For?

The cold tank in the loft serves three purposes:

  1. It collects expanding water as it rises in the expansion tube when the water in the hot tank is heated or overheats. This is the case with a vented hot tank. The alternative system is a sealed hot tank with an expansion vessel to accommodate water as it expands when heated.
  2. The second function of the tank is to provide a pressure head and feed for the hot water tank (gravity feed). Water flows out from the bottom of the cold tank (the blue line in the diagram below) to the bottom of the hot tank. Because the cold tank is higher than the hot tank, the resulting pressure forces water through the tank and out the top. Water in the hot tank is heated by an electric (immersion) element and/or heating system boiler (furnace). Modern hot water systems may use a pump to provide the pressure.
  3. It acts as a storage reservoir and can supply water in the event of an interruption in supply from your water company. This cold water supplies WCs, showers, baths and hand basin taps (faucets). The water isn't drinkable and shouldn't supply the cold taps in the kitchen because it has been lying indefinitely in an open tank into which anything could have dropped (insects, spiders, and possibly birds, bats and their droppings!).

Older buildings only use the tank in the attic for pressurizing the hot water tank and water is fed from the rising main to all fixtures. This is called a "direct system". In some countries, the tank isn't common and pressure for the hot water supply is derived from a pump.

Indirect and Direct Plumbing Systems

  • Indirect Plumbing system. The cistern/tank in the loft and also the cold tap in the kitchen is fed by the rising main. All other cold taps and water fittings get their supply from it. The cistern also creates the pressure for the hot water tank
  • Direct plumbing system. All cold taps are fed from the rising main. This is less complicated, but the high water pressure can cause noise and water hammer in taps.

Domestic Plumbing Systems: Pipe Layout in House (Indirect Plumbing System)

The diagram below outlines the most basic plumbing arrangement in a house. This is an indirect plumbing system which is found in most homes.You may have extra plumbing fixtures, but the basic principle is the same, although the hot water tank can be located on the ground floor or on upper floors. Some older houses may not have a tank in the loft.

Inside your home, several gate or ball valves allow water to be shut off to various sections of the plumbing system. A gate valve has a wheel/knob which is turned clockwise to shut off flow. A ball or quadrant valve has a lever handle which is lined up with the pipe when the valve is on. To turn it off, the handle must be turned through 90 degrees so that it is perpendicular to the pipe. There may also be small, inline isolating valves for disconnecting water to each appliance. These are operated with a slotted head screwdriver.

Indirect Plumbing System

Schematic of basic plumbing system in the home

Schematic of basic plumbing system in the home

How Valves Are Used to Isolate Sections of the Plumbing

Water enters the building at ground level. Referring to the diagram above:

  • Valve 1 is located directly outside the premises on a wall or in a box in the ground. This normally has a square section on the end of the shaft and needs to be operated with a key. In colder climates, it may be in the basement. This enables the water supply entering the building to be cut off. Unlike a gate valve, this valve has a rubber seal which tightens against a seat so that water can be totally shut off. There may be another stopcock outside the boundary of the premises under an inspection cover but sometimes this is shared and cuts off water to several homes
  • Valve 2 located indoors is the main stop cock, and shuts off water where the service main enters the building. Drinking water must not be supplied from the tank in the loft, and comes directly from the service main.
  • Valve 3 shuts off the feed to the cold water tank in the loft to allow maintenance or drainage of the tank.
  • Valve 4 shuts off the cold feed to the bottom of the hot tank, removing the pressure, and this prevents hot water leaving the top of the tank.
  • Valve 5 shuts off feed to WCs, baths, showers, wash basins etc. There may only be one feed pipe from the cold tank to both the hot water tank and these fixtures, in which case there will only be one valve to shut off this supply, i.e one valve shuts off both cold water and hot water (because pressure is removed).
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You may have additional valves feeding each appliance. These valves are usually small isolating types which are operated by turning a screw or short lever.

The hot tank is vented to the atmosphere via an expansion pipe which runs up into the loft to the cold tank. This allows expansion to take place without restriction as water is heated. Also if the heating system malfunctions and water in the tank boils, steam can escape and water can flow safely back into the cold tank.

Three Different Types of Valves For Shutting Off Water

These are the three types of valves you're likely to find in your home for shutting off water.

Gate valve. Turn clockwise for off.

Gate valve. Turn clockwise for off.

Ball valve (quadrant valve). If the handle is attached correctly, the valve is off when the handle is at right angles to the fitting (as shown).

Ball valve (quadrant valve). If the handle is attached correctly, the valve is off when the handle is at right angles to the fitting (as shown).

An inline isolating valve, often used for disconnecting water to individual appliances. Valves are turned on/off with a screwdriver. This valve is shown in the on position. Off is when the slot is turned through 90 degrees.

An inline isolating valve, often used for disconnecting water to individual appliances. Valves are turned on/off with a screwdriver. This valve is shown in the on position. Off is when the slot is turned through 90 degrees.

What Is the Small Tank in the Loft for?

If you have an open or vented heating system, there will also be a small reservoir tank in your attic. This is a feed and expansion (F&E) tank and continuously tops up water in the heating system as well as collecting any water that expands upwards as it heats up. An expansion pipe runs from the boiler up to the tank and then bends downwards over the tank so that water can drain. If your central heating system overheats, this pipe can also vent steam. Overheating could be caused by faulty thermostats forcing an oil or gas boiler to remain on all the time, but more likely due to a circulating pump failure (or loss of power to the pump) on a solid fuel system with a back boiler and the radiators upstairs turned off.

The tank is normally fed by the same cold water pipe as the larger tank. This small tank also has a ball cock which can suffer from the same problems as the main cold water tank.

How Does a Ballcock Valve Work?

The tank in the loft is fitted with a ballcock valve (also known as a balltap or float valve) to prevent it over filling. This is the same as the system used in a toilet cistern. Water enters via the valve, and as the level in the tank rises, it pushes upwards on the underside of a plastic or brass float attached to a lever arm. The leverage of the arm creates sufficient force to close the valve once the tank has sufficiently filled.

Cold water tank in the loft

Cold water tank in the loft

Ballcock in a water tank. The red float attaches via an arm to the valve. Once the water level rises sufficiently, the arm pushes on the valve, closing it