Cold Water Header Tank Overflowing

Which tank is overflowing?

Most homes with a loft tank actually have two. The big one, often around 200 litres, is the cold water storage cistern. It feeds the hot water cylinder and in many houses the bathroom cold taps and toilets. The small one, often 20 to 50 litres and sometimes in a corner of the loft or the airing cupboard, is the feed and expansion tank. It tops up an open-vented heating system and gives the water somewhere to go as it expands when the heating warms up.

Both have an overflow pipe that runs out through the wall or the eaves. If more than one pipe is dripping outside, tell the tank overflows from the boiler and toilet pipes by their height, size and material, then trace which one is running. If you only see one, go into the loft with a torch and look at the water level in each tank. The one with water at or above the overflow outlet is the culprit.

Knowing which tank matters, because the causes are different. A big cold tank overflows because its float valve will not shut. A small heating tank can overflow for that reason too, and for three others that have nothing to do with the valve.

How to stop the overflow right now

The Water Supply (Water Fittings) Regulations 1999 require a servicing valve on the inlet pipe as close as practicable to the float valve, and most tanks fitted or replaced since then have one. It is usually a small valve with a screwdriver slot, or a lever, on the pipe just before the float valve. Turn the slot across the pipe, or the lever through a quarter turn, and the tank stops filling. The overflow will slow to a drip and stop within a few minutes.

If there is no servicing valve, or it has seized, lift the float arm gently and tie it up to a batten across the top of the tank with string or a strip of wood. That holds the valve shut. It is a stopgap only, and if the valve is badly worn it may still let some water past.

• Big cold tank: turning off its inlet does not affect the kitchen cold tap, which is on the mains. Bathroom cold and hot water will keep running until the tank is empty, so go easy on baths.
• Small heating tank: you can leave its inlet off for a few days. The heating will run normally as long as the system does not lose water.
• If you cannot get into the loft, turn the main stopcock off. That stops both tanks filling but also cuts the kitchen cold tap.

An overflow that runs into the loft rather than outside, or water coming over the rim of the tank and through the ceiling below, is an emergency. Turn the stopcock off first, then deal with the tank.

Why the big cold tank keeps filling

The float valve, often still called a ballcock, is the only thing that stops the tank filling. Our guide to a stuck loft tank ballcock covers cleaning and adjusting one step by step. As the water rises, the float lifts the arm, and the arm presses a washer or a diaphragm against a small seat in the valve body. If the valve cannot close fully, water trickles in past the overflow level and runs out of the overflow pipe. The usual reasons, in the order an engineer checks them:

• Water level set too high. The float arm or the adjuster screw lets the tank fill above the overflow. Easy to reset.
• Worn washer or split diaphragm. On an old brass Portsmouth valve (BS 1212 Part 1) the rubber washer on the piston wears. On a diaphragm valve (Part 2 or Part 3) the diaphragm splits or hardens.
• Grit on the seat. Common after the water company has worked on the main, as sediment lodges between the diaphragm and the seat.
• Wrong seat for the pressure. Older valves came with a high-pressure or low-pressure seat. A low-pressure seat on a strong mains supply will not close, and the overflow often starts at night when mains pressure rises.
• Punctured float. A float with water inside it sits low and never lifts the arm high enough. Shake it and listen.

When the small heating tank overflows

If the feed and expansion tank is overflowing, check its float valve first in the same way. But if you tie the float up, or close its servicing valve, and water still rises and pours out of the overflow, the water is not coming in through the float valve at all. It is coming from somewhere else in the system.

The most likely cause is a leaking coil inside the hot water cylinder. In an indirect cylinder, heating water runs through a copper coil to heat the water around it. If the coil develops a pinhole, water passes from the cylinder, which is fed from the big cold tank, into the heating circuit. The heating circuit then fills up and the excess comes out at the small tank. A clue is that the big cold tank level also keeps falling a little and topping up.

The second cause is pumping over. On some older systems the heating pump pushes water up and out of the open vent pipe that hangs over the small tank. You will usually see water spurting from the vent pipe when the heating is on, and the heating tank overflows or runs warm. The third is expansion: if the tank level was set too high when cold, it will overflow each time the heating warms up.

A leaking cylinder coil means a new cylinder rather than a repair. Hot water tank replacement cost covers what that involves, and our guide to a hot water tank that is dripping helps if water is also appearing around the cylinder.

What a float valve replacement involves

Changing a float valve is a common job and not complicated for an engineer, but in a loft it is awkward, and getting the nut on the tank wall tight enough without cracking an old plastic tank takes care. The engineer isolates the inlet, loosens the tank connector, fits a new valve, usually a BS 1212 Part 3 plastic diaphragm valve or a compact equilibrium valve, sets the water level below the overflow and checks it shuts off cleanly.

The valve itself is cheap. Typical merchant prices for a plastic float valve are in the region of £5 to £15, with equilibrium valves costing a little more. While up there, it is worth checking the rest of the tank against the regulations: a cover that keeps out light and insects, insulation, and an overflow pipe with an insect screen. Old tanks without a lid collect dust, insects and the odd dead bird, which is why the rules exist.

If you would rather not go in the loft yourself, an engineer’s visit is the £60 call-out, and the repair price is agreed with you on site before any work starts. A dripping overflow is not dangerous in itself, but a valve that has stuck partly open can fail fully open, and water coming through the ceiling below is a much bigger job. See emergency leak repairs if water is already coming through.