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Cold Water Tank Overflowing - How to Replace a Ballcock Washer

Leaking Valves, Overflowing Water Tanks and Toilet Cisterns

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 or 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 mount up.
This hub explains the basics of the pipes and valves in home plumbing systems and how to remedy an overflowing cold water tank in the loft. 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.

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Basic Plumbing Layout in the Home

The diagram below outlines the most basic plumbing arrangement in a modern home. 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 the house, 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.

Water enters the building at ground level.
Valve 1 located directly outside the premises, or on the pavement in the street, (usually under an inspection cover) enables the water supply entering the building to be cut off.

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.
The cold tank in the loft serves two purposes, firstly 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 and baths, and feeds into the bottom of the hot water tank. It shouldn't supply taps (faucets) in the kitchen because the water has been lying indefinitely in an open tank into which any thing could have dropped (insects, spiders, and possibly birds, bats and their droppings!). You can make a cover from plastic or plywood to stop this happening.
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) 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).

Valve 4 shuts off the cold feed to the bottom of the tank, removing the pressure, and this prevents hot water 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).
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".


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.


If you have an open or vented heating system, there will also be a small reservoir tank in your attic. This is a top up reservoir for water in radiators and boiler, and also facilitates expansion in the pipes as water is heated in the system. An expansion vent pipe will also be fitted which allows water to trickle back into the tank if the heating system fails and water overheats.(caused by a pump not working or a boiler staying powered on). 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.

In-direct Plumbing System

Schematic of basic plumbing system in the home
Schematic of basic plumbing system in the home | Source
Gate valve
Gate valve | Source
Ball valve (quadrant valve)
Ball valve (quadrant valve) | Source

How a Ballcock Valve Works

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 | Source
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 | Source

What Causes a Tank to Overflow?

If the level of water in the tank becomes too high, excess water drains outside via an overflow pipe. An overflow in a cold water tank has three causes. One scenario is that the float has developed a leak so that it fills with water, sinking, or providing insufficient force to close the valve. Another possible cause is that the rubber washer in the valve has deteriorated so that it doesn't seal properly. A third scenario is that 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.

Tools Required

  • Water pump pliers or alternatively a pipe wrench (Stilson wrench) or vise grips
  • Flat blade screwdriver
  • Small pliers -
  • Long nose (snipe nose) pliers, not essential, but useful for pulling out the split pin
  • Torch - ideally a head torch
  • Knife


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 | Source
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 | Source

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 slide valve 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. 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
Remove the end cap | Source

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 its 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 | Source
Pull and twist backwards and forwards
Pull and twist backwards and forwards | Source

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 | Source
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 | Source

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 | Source

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 wont get it all the way out with the screwdriver, so grip the end and pull it out with your fingers or nails.

Push the valve out with a screwdriver
Push the valve out with a screwdriver | Source
Pull out the slider
Pull out the slider | Source
The slider has a removable washer embedded in the end
The slider has a removable washer embedded in the end | Source
Water comes out through this nipple. Eventually over a period of decades, it can become ridged at the end and the rubber washer at the end of the slider won't stop water coming out. So the complete ballcock valve will have to be replaced
Water comes out through this nipple. Eventually over a period of decades, it can become ridged at the end and the rubber washer at the end of the slider won't stop water coming out. So the complete ballcock valve will have to be replaced

Step 7 - Remove the Washer

Push it out with a screwdriver.

Push out the washer with a screwdriver
Push out the washer with a screwdriver | Source
This washer is well past its "sell by date"
This washer is well past its "sell by date" | Source

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 | Source
Source
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 | Source
Push the remainder of the washer in with your thumbs
Push the remainder of the washer in with your thumbs | Source
Source

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 | Source
Spread the ends of the pin slightly apart
Spread the ends of the pin slightly apart | Source

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 | Source

You might need some of these tools!

Adjusting Ballcock to Change Water Level

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. 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 | Source

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.

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9 comments

Marina7 profile image

Marina7 2 years ago from Clarksville TN

I love this hub and so much to learn here.


Maire O'Donohoe 19 months ago

Thanks for clear,detailed and realistic instruction#1


eugbug profile image

eugbug 19 months ago from Ireland Author

Your welcome Maire!


Padraig O Ceallachain 6 months ago

Wouldn't it be great if all instruction manuals were as clear as this. Well done.


eugbug profile image

eugbug 6 months ago from Ireland Author

Thanks Padraig, glad it helped!


Joanne Skelding 6 months ago

Excellent instructions, really helped. Thanks


Marni 8 weeks ago

Really excellent info/ advice. Thank you.


JP O'Leary 5 weeks ago

I agree with all the above comments, absolute Brillent clear instructions it helped me a lot.


Ron and Rachel 10 days ago

Really helped us a lot. Clear and concise. Thanks very much.

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    eugbug profile image

    Eugene Brennan (eugbug)311 Followers
    83 Articles

    Eugene is an avid self taught DIYer, and over 30 years has acquired lots of experience of power/hand tools, plumbing, electrics and woodwork



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