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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

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

What Causes the Cold Tank to Overflow?

  • The float has developed a leak so that it fills with water, sinking, or providing insufficient force to close the valve
  • The rubber washer in the valve has deteriorated so that it doesn't seal properly
  • The brass nipple in the valve has worn to such an extent from decades of abrasion by particles in the water that it won't seal anymore. (a ridge can develop in the wall of the nipple's tip. In this case, the valve will have to be replaced

Any of these scenarios will cause the water level to rise in the tank, eventually causing an overflow.

What Tools are Needed to Replace a Washer in the Ballcock Valve?

Draper water pump pliers. Image courtesy Draper Tools.

Draper water pump pliers. Image courtesy Draper Tools.

Pipe wrench. Image courtesy Draper Tools.

Pipe wrench. Image courtesy Draper Tools.

Clockwise from top - Water pump pliers, standard jaw pliers, long nose (snipe) pliers, curved jaw vise grips, flat blade screwdriver

Clockwise from top - Water pump pliers, standard jaw pliers, long nose (snipe) pliers, curved jaw vise grips, flat blade screwdriver

A head torch is great for working in low light conditions and keeps your hands free

A head torch is great for working in low light conditions and keeps your hands free

Step 1: Turn Off the Valve on the Pipe Feeding the Tank

As described in the explanation of plumbing systems above, this is valve 3. This may be located downstairs, near the cold tank or near the hot tank. You will have to trace the piping and by a process of elimination, identify which valve controls what. If you push the float of the ball cock downwards and no water flows into the tank, you have found the right valve. Usually valve 3 (shown in the diagram above) feeding the tank is on a 1/2 inch / 15 mm pipe.

Step 2: Remove the End Cap From the Valve in the Tank

The valve on this tank is a slider type. Inside the body of the valve is a nylon or brass slider with a rubber washer embedded in the end. Water flows out through a nipple inside the valve. The slider is pushed against this nipple by the float arm and this cuts off flow.
Using the water pump pliers, pipe wrench or vise grips, unscrew the end cap from the valve body. Hold the body of the valve with another vice grips or Footprint pipe wrench to stop it turning, if the end cap appears to be tight. If it's obstinate and refuses to turn, try pouring some hot water from a kettle over the cap, this should expand it slightly and make it easier to turn.

Remove the end cap. Hold the body of the valve with another wrench (not shown) to prevent it turning on the tank.

Remove the end cap. Hold the body of the valve with another wrench (not shown) to prevent it turning on the tank.

Step 3: Remove the Split Pin

A split (cotter) pin holds the float arm in place. Squeeze the two ends together with the pliers. Now use the long nose pliers to grip the looped end of the pin and pull it out. If you rotate the pin backwards and forwards while pulling, it makes it easier. If it's really tight, tap it with a hammer (don't hit it hard, just use lots of light taps).

Squeeze the ends of the split pin together

Squeeze the ends of the split pin together

Pull and twist backwards and forwards

Pull and twist backwards and forwards

Step 4: Remove the Float Arm

There is a knack to doing this. Move the float arm into a horizontal position and then pull it downwards. You will likely need to wiggle it a bit to get it out.

Remove the float arm. Hold it horizontally and push down

Remove the float arm. Hold it horizontally and push down

This is what the end of the arm looks like. The "heart" shaped bit fits into the slider and pushes it backwards and forwards

This is what the end of the arm looks like. The "heart" shaped bit fits into the slider and pushes it backwards and forwards

Step 5: Check the Float

Over time, floats can become cracked or develop leaks where they screw onto the arm. Shake the float to see whether there's any water in it. You can buy replacement floats in a hardware store.

Shake the float to see if there's any water in it

Shake the float to see if there's any water in it

Step 6: Remove the Slider

Push a narrow blade screwdriver up into the slot in the underside of the valve. Push the slider so that it sticks out of the valve body. You won't get it all the way out with the screwdriver, so grip the end and pull it out with your fingers or nails.

If the washer looks ok and isn't excessively worn, the nipple in the valve might be worn so that it doesn't seal properly against the washer. If this is the case, consider replacing the complete valve. A new valve will have a push type fitting suitable for connecting to copper or plastic tubing, or alternatively a compression fitting. You can read more about these type of fittings in A Complete Guide to Using Plumbing Fittings for Joining PVC, PEX, and Copper Pipe.

Push the valve out with a screwdriver

Push the valve out with a screwdriver

Pull out the slider

Pull out the slider

The slider has a removable washer embedded in the end

The slider has a removable washer embedded in the end

Water comes out through this nipple. Eventually over a period of decades, grooves can form at the end and the rubber washer at the end of the slider won't seal properly. So the complete ballcock valve will have to be replaced

Water comes out through this nipple. Eventually over a period of decades, grooves can form at the end and the rubber washer at the end of the slider won't seal properly. So the complete ballcock valve will have to be replaced

Step 7: Remove the Washer

Sliders are of two types: Older types are solid brass with a screwed on end cap or groove that holds the washer in place or nylon like the one in the photo below with the washer just pushed into a space in the end. If your ballcock has a nylon slider, simply push the washer out with a screwdriver and clean out any remaining fragments of rubber.

Push out the washer with a screwdriver

Push out the washer with a screwdriver

This washer is well past its "sell by date"

This washer is well past its "sell by date"

Step 8: Replace the Washer

You can buy a washer in a store or make one from a suitably thick scrap of sheet rubber. If you are making one, use the perimeter of the slider as a guide and then pare to fit. Push the washer back into the slot of the slider, using the screwdriver to stop the rubber catching in the edges of the slot. Once you get it in halfway, lay the valve on its side and push it in fully with your thumbs.

Use the perimeter of the valve as a guide

Use the perimeter of the valve as a guide

Cut out the disk of rubber.

Cut out the disk of rubber.

Push on the edges of the washer while forcing it in, to stop it catching

Push on the edges of the washer while forcing it in, to stop it catching

Push the remainder of the washer in with your thumbs

Push the remainder of the washer in with your thumbs

Washer fitted into place.

Washer fitted into place.

Step 9: Replace the Slider, Float Arm and End Cap

Push the slider back into the valve body ensuring that the slot in the slider is aligned with the slot in the underside of the valve body. You can use the screwdriver to feel whether it is lined up. Replacing the float arm can be tricky if the two slots don't line up. Hold the arm horizontally and push upwards, wiggling it a bit helps to get it into place. Push the split pin back into place and splay the two ends slightly apart with a screwdriver. They don't need to be spread far apart. Replace the end cap and screw tight, hand tight is good enough. A smear of Vaseline on the threads helps to prevent seizing should it be necessary to remove the cap in the future. You can do this on threads of taps also when re-assembling.

Smear a little Vaseline on the threads

Smear a little Vaseline on the threads

Spread the ends of the pin slightly apart

Spread the ends of the pin slightly apart

Step 10: Turn the Water Back On

Turn the water back on. Hold the underside of the lever arm, and pull it upwards to validate that the valve cuts off water properly. Initially water will gush into the tank and the level will rise quickly. However as the tank fills, the valve will start to close and flow will dwindle, so it can take ages for water to stop dripping because of the slow rise of water level. Don't worry if this is the case.

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Pull upwards on the float arm to check whether the water flow shuts off ok

Pull upwards on the float arm to check whether the water flow shuts off ok

How Long Will This Repair Last?

I wrote this help guide in 2012, and the repair has lasted 10 years so far. So for the person who commented below that it was a waste of time and that the whole valve should be replaced, take note that it didn't cost me anything. Repairing is always better than replacing for the sake of the Earth's natural resources. It also saves money!

Alternative Type Ball Valves

There are various types of ball valves, but they all use some form of washer/seal arrangement which the ball arm pushes on. This video shows an alternative style.

I learned a huge amount of information from this book! The Collins Complete DIY Manual is an excellent guide, comprehensively covering plumbing, electrics, woodwork, heating, garden construction, paving, plasterwork, glass cutting, painting, tiling and all other aspects of home maintenance and improvement. This older version from Amazon UK (several available second hand, with "good" to "very good" ratings) contains a lot more info than the newer editions.
Note that electrical wiring information in this manual is geared towards UK users and may not satisfy more recent and stricter regulations.

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Adjusting Ballcock to Change Water Level

If there is no screw which facilitates making an adjustment and the arm is the older-style brass type, hold the arm securely in the middle so as not to damage the valve. With your other hand, grasp the arm just before the ball float. Bend the arm downwards so that it ends up being curved downwards. Water will now force the float to act on the arm and close the valve at a lower level. Newer style ballcock valves have a screw for adjusting level. Turn the screw clockwise to reduce the level to which the tank fills.

Turn the screw clockwise to reduce water level

Turn the screw clockwise to reduce water level

How to Stop a Cold Tank From Freezing in Winter

Modern lofts have their floor insulated so not a lot of heat permeates up into the loft space, especially if upstairs radiators are turned off. The result is that the cold water tank can start to freeze. If you're regularly running taps and using the shower, the main cold tank continually fills with new water, so this is less likely to happen, but pipes in the attic can freeze and burst if a home is vacant for a couple of days during extended snowy and cold periods. This is especially the case if plumbing is copper, and pipes can freeze in very cold weather even if they're lagged. PEX plumbing because it's flexible, is better at tolerating sub-zero temperatures because the plastic has more "give" and can stretch.

The smaller water tank used for topping up the central heating system and collecting water as it heats up and expands (or the system overheats and water boils) is much more likely to freeze because water isn't continually replaced. So in cold weather, it's a good idea to leave the trapdoor to your attic open and allow some heat up into the space to prevent this happening.

Be Prepared! Plumbing Tip

The Boy Scout motto is "Be Prepared!" so take a leaf out of their book by locating and identifying all the valves in your home! Work out what they do and write the function on the wall behind with a thick marker. Alternatively, tie a label around the stem of the valve. Gate valves can seize up after years of disuse so every so often (once a year), give them some exercise by turning on and back off again. If a pipe springs a leak in your home, you don't want to be running around figuring out how to turn off the water.

Conserving Water in a Toilet Cistern

In a toilet cistern, you can conserve water by placing a plastic bottle filled with water into the tank. This reduces the amount of water used every time you flush. Alternatively, you can use a piece of brick or other heavy object. Another option is to adjust the ballcock arm so that it shuts off the water valve when the water level is lower.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: Why is the overflow still dripping after replacing the ballcock?

Answer: Presumably this is because water is rising too high and reaching the level of the overflow outlet. If the overflow pipe still drips after replacing the ballcock, check first that the ball on the end of the lever can move unrestricted in the tank and there isn't anything obstructing it. If you didn't replace the ball float, this could be punctured, slowly letting in water and partially full. The result is that water would need to rise higher to create sufficient force to close the valve. Another possibility is that adjustment is necessary because the new ballcock has a different length arm. If it has an adjustment screw, turn this anti-clockwise or clockwise so that the valve shuts off when the water has reached a lower level.

Question: I have two header tanks up in my loft, one big tank and another small tank with a similar configuration. Why is tank number two overflowing to the roof?

Answer: The smaller tank is likely to be the expansion tank for the central heating system. Just like the main cold water tank, this will overflow if the ballcock washer is worn or if it is set up so that water level reaches the overflow before the float shuts off the valve.

Question: Why does the overflow from my header tank drip during the night but not during the day?

Answer: During the day, you're using water regularly, so the water level never reaches the overflow outlet. At night the water level may be slowly rising due to a trickle of water flowing into the tank from a faulty ballcock valve, so it has plenty of time to reach the overflow.

Question: We had this done three weeks ago (at great expense). There is now a quite loud and droning noise when we turn the shower on or run water for a while. The plumber has come around but of course it did not happen while he was here. Do you know why this might be happening? The plumber said it might happen for a day or two but three weeks seems a bit much. It is annoying but is it a signal of something more sinister?

Answer: The noise is presumably due to the tank filling when it drains when hot or cold water runs. The sound could be due to water hitting the existing water in the tank as it fills; vibration in pipes as water turns on and off (water hammer) or sound from the valve itself as water exits. Sometimes pipes can bang against each other so clipping them properly so they can't move may help (or sliding small sections of insulation onto them to separate them).

Replacing a section of copper tubing where it enters the tank with about a meter of PEX tubing may also help. PEX tends to give more of a cushioning effect than copper.

The jet in the valve or the valve itself may be a low-pressure type that's primarily used for filling tanks rapidly (e.g., agricultural water troughs). It may be possible to replace the jet in the valve (or maybe the whole valve would have to be replaced) with a high-pressure type that fills the tank more slowly.

Have a look at these links that discuss the problem in a little more detail:

https://www.irishtimes.com/life-and-style/homes-an...

https://www.boards.ie/vbulletin/showthread.php?t=2...

Question: I've replaced completely the 3/4" ballcock on my loft water storage tank serving my power shower. However the I'm having difficulty aligning the threaded female elbow connection to the existing pipework. How do I achieve a watertight seal when the elbow isn't fully tightening on the ballcock inlet? I have fully tightened the elbow is 90 degrees out of the previous position.

Answer: I'm not a plumber, so I don't know whether this is the recommended solution, but I had a similar problem attaching an elbow to the ballcock in a toilet cistern. I simply use more PTFE sealing tape so that the fitting starts to tighten up a couple of threads before the male part reaches the "end of the line" on the threads in the female elbow. If you discover that you've gone too far, and the fitting can't be tightened further around to the correctly aligned position, don't loosen it, to re-align as it may leak. Undo and re-apply tape again. It might be helpful to post on plumbing forums to get a second opinion.

Question: I have no cold water coming from the taps in my bathroom and the toilet cistern isn't filling. What's wrong?