Loft Tank Ballcock Stuck

Stuck shut or stuck open? Read the symptoms

The ballcock, properly a float-operated valve, controls how much water enters the cold tank in your loft. It can fail in two opposite ways, and the house tells you which.

Stuck shut, the tank slowly empties as you use water and does not refill. The kitchen cold tap keeps working because it comes straight off the mains, but bathroom cold taps weaken, toilets stop refilling, and the hot water fades as the cylinder can no longer top up. People often report this as having no cold water from the bathroom taps while the kitchen is fine.

Stuck open, the tank keeps filling past its normal level and water runs from the overflow pipe outside, or you hear a constant hiss or trickle from the loft. That is covered in detail in our guide to a header tank that keeps overflowing. This page concentrates on freeing and setting the valve itself.

If the tank has run dry and you have an immersion heater, switching it off until the tank has refilled and a hot tap runs steadily is a sensible precaution. The cylinder normally stays full, but an element that heats with no water around it burns out quickly.

What to check before touching the valve

Go up with a good torch and look into the tank before you do anything else. Take the lid off carefully so dust and insects do not fall in.

• Is there any water in the tank at all? If it is empty, the problem is supply, not the tank outlets.
• Find the servicing valve on the pipe just before the float valve. Check it has not been turned off. The slot or lever should line up with the pipe.
• Push the float arm down gently. If water rushes in, the valve works and the float or arm was holding it shut. If nothing comes, the valve is jammed or the pipe feeding it is empty.
• In winter, feel the pipe running to the tank. If it is ice cold and you get nothing, it may be frozen, usually where it crosses the loft near the eaves. See frozen pipes: what to do.
• Check the kitchen cold tap has good flow. If the mains is weak or off, the tank cannot fill no matter what the valve does.

Which type of ballcock do you have?

Most float valves in UK lofts are made to one of the parts of BS 1212, and the type decides how you fix it. Look at the body where the arm meets the valve.

• Portsmouth valve (BS 1212 Part 1): brass body, a piston that slides horizontally and a rubber washer on the end of the piston. Common in older houses. The piston sticks with scale.
• Brass diaphragm valve (Part 2): a large round cap on the front, with a rubber diaphragm inside pressed against the seat by a plunger. The water level is set at the arm, by an adjuster or by the float position.
• Plastic diaphragm valve (Part 3): the same principle in plastic, with a plastic arm and an adjuster. The most common type fitted today.
• Equilibrium valves: designs that balance the pressure either side of the seal, so they close cleanly on a strong mains supply. Compact valves to BS 1212 Part 4 look similar but are made for WC cisterns rather than loft tanks.

How to free a stuck ballcock

Turn off the servicing valve first. If there is none, turn off the main stopcock, which will also stop the kitchen cold tap. Put a towel under the valve.

On a diaphragm valve, unscrew the large cap on the front by hand. Take out the plunger and the diaphragm behind it. Look for grit on the diaphragm and on the seat, which is the small nozzle it presses against. Wipe both clean, check the diaphragm for splits or hard patches, and reassemble. If you can, open the servicing valve briefly into a bucket with the diaphragm out to flush any grit through the pipe first.

On a Portsmouth valve, take out the split pin that holds the arm, then slide the piston out of the body. Scale on the piston is the usual cause of sticking. Clean it with fine wire wool and a little silicone grease, never oil. Replace the rubber washer on the end if it is grooved or hard. If the piston is corroded into the body, stop and replace the valve.

Once reassembled, open the servicing valve and let the tank fill. The valve should close firmly when the water reaches the set level, with no trickle.

Setting the water level correctly

The water should stop about 25mm (an inch) below the overflow outlet. Too high and there is no margin, so the slightest weep past the valve goes straight to the overflow. Too low and you waste storage and the hot water cylinder can run short on a busy morning.

On a plastic or brass diaphragm valve, turn the adjuster screw on the arm, or move the float along the arm, and watch the level on the next fill. On an old Portsmouth valve the traditional method is to bend the brass arm slightly downward to lower the level. Do not bend a plastic arm.

Check the float too. Unscrew it and shake it. If you hear water inside, it has a hole and sits too low. Replacement floats are cheap and screw on. A float wedged against the tank wall or catching on the lid will also stop the valve closing, which happens after a tank lid has been refitted slightly out of position.

When to replace instead of repair

Float valves are cheap parts. Typical merchant prices for a plastic Part 3 valve run from roughly £6 to £15, and equilibrium valves cost more. Replacement makes sense when the body is corroded, the piston is seized, the thread on the tank connector is damaged, or the valve sticks again within weeks of cleaning. Choose one suited to your mains pressure. A valve meant for low pressure will not shut properly on a strong London supply.

The job is straightforward in principle but awkward in practice: working in a cramped loft over a full tank and holding the tank wall while you tighten the back nut without cracking old, brittle plastic. If the tank itself is cracked, rusted steel or has no lid, it may be time for a new one with a close-fitting cover that keeps out light and insects, a screened overflow and insulation, as the Water Supply (Water Fittings) Regulations 1999 require.

If you would rather have an engineer do it, visits are the £60 call-out, and the repair price is agreed with you on site before any work starts. If the tank has emptied and the house has little water, see no water emergency, and if a toilet will not refill once the tank is full again, see toilet not filling with water.