Loft cold water tanks: lid, insulation, ballvalve and overflow

Many older UK homes have a large cold water tank in the loft. Properly called a cold water storage cistern, it feeds water by gravity to the hot water cylinder and often to baths, showers, toilets and basin cold taps. The kitchen cold tap is usually fed straight from the mains.

A small second tank may sit next to it. That is usually the feed and expansion tank for an open-vented heating system. This guide is about the larger cold water storage tank. The heating tank is part of the heating system and should be left to a heating engineer.

How the system works

1. The mains supply pipe rises to the loft and enters the tank through a ballvalve (also called a float valve).
2. As the tank fills, a float rises and closes the valve at the right level.
3. Water leaves the tank through outlet pipes near the bottom. It flows down by gravity to the hot water cylinder and the cold outlets it serves.
4. An overflow (warning pipe) near the top carries water outside if the ballvalve fails to shut.

If you have a combi boiler or an unvented cylinder, you may have no storage tank at all. See hot water cylinders and immersions for how vented and unvented hot water differ.

What the regulations say the tank must have

In England and Wales, Schedule 2 of the Water Supply (Water Fittings) Regulations 1999 sets out the requirements. Paragraph 16 covers cold water services. In summary, it says:

• the pipe feeding a storage cistern must have "an effective adjustable valve" that shuts off the inflow below the overflowing level. That is the ballvalve;
• the inlet must have a servicing valve next to the cistern, and most storage cisterns must also have one on the outlet;
• every storage cistern must be fitted with an overflow pipe with "a suitable means of warning of an impending overflow, which excludes insects", "a cover positioned so as to exclude light and insects", and "thermal insulation to minimize freezing or undue warming";
• it must be installed to minimise the risk of contamination, and be sized and connected so that water circulates and does not stagnate.

Scotland and Northern Ireland have their own rules. Ask your water supplier if you are unsure. Older tanks often fall short of these points, especially on the lid and insulation.

The lid

An open tank in a loft collects dust, insects, insulation fibres and sometimes dead birds or rodents. A tight-fitting lid that keeps out light and insects is a requirement for good reason. Signs a lid needs attention:

• it is missing, cracked or just a loose board;
• there are gaps where pipes pass through it;
• there is debris or a film on the water.

Many modern lids come as kits with screened vents and sealed holes for the pipes. If you find dirt or something dead in the tank, do not use water from the taps it feeds for drinking or cooking. Ask a plumber to drain, clean and disinfect it.

Insulation

A loft tank needs insulating against freezing in winter and warming in summer. In practice that usually means:

• an insulated tank jacket or rigid insulation around the sides and on the lid;
• no insulation directly under the tank if the loft floor below is insulated. Leaving that patch clear lets a little warmth from the house reach the tank;
• lagging on the pipes in the loft, including the overflow where it runs through cold spaces. See pipe lagging.

We have not found an official minimum thickness for a domestic tank jacket, so we do not quote one. Follow the insulation maker's instructions, and make sure the jacket covers the sides and lid without gaps.

The ballvalve

The ballvalve controls the water level. Common problems:

• Water keeps running into the tank and out of the overflow. The valve is not shutting. A worn washer, a damaged float, or limescale is the usual cause. See overflow pipes.
• The tank fills slowly. The valve may be partly blocked with scale or debris, or the mains pressure may be low.
• A hammering or juddering noise when the tank fills. Ripples can make the float bounce.

The servicing valve on the inlet lets you shut off the water to the tank while the valve is repaired or replaced. See isolation valves. The general principles are the same as for a toilet's fill valve, which our toilet fill and flush valves guide covers. In a loft, though, you are working at height on a board over joists, often in poor light. If you are not comfortable up there, it is a short job for a plumber.

The overflow

The overflow is a safety valve and a warning. It should discharge somewhere you will notice, usually outside through the wall or the eaves. If you see water dripping or running from a pipe high on the outside wall, the loft tank is a likely source. Deal with it quickly: a constant overflow wastes water, and a blocked or frozen overflow can let the tank flood the loft.

Why a tank running dry causes airlocks

If the tank empties, air gets into the outlet pipes. When the water comes back, pockets of air can be trapped in the pipework, so taps splutter or a hot tap gives nothing. Common causes:

• the mains has been off, or a ballvalve has jammed shut;
• the tank has frozen;
• a big demand, such as filling a bath, draws water out faster than the ballvalve can refill it.

Our guide to airlocks explains how to clear them safely. If the hot water side has run dry, switch off the immersion heater and the boiler's hot water until the cylinder is full again.

When to call a plumber

• The tank is old galvanised steel and rusty, or any tank is cracked or bowing.
• You find dirt or animals in the water.
• The tank needs replacing, moving or raising. Its height affects pressure at the taps and showers.
• The overflow runs constantly and a ballvalve repair has not fixed it.
• There is water on the loft floor or a ceiling stain below.

Products at Wickes

We haven't tested these. They are examples, with what the retailer's page says.

We haven't tested this. Figures are as the maker states them (checked 13 November 2026).

Does my loft water tank need a lid?

Yes. In England and Wales, the water fittings regulations say every storage cistern must have a cover that excludes light and insects, as well as insulation and a screened overflow.

Why is water coming out of a pipe high on my outside wall?

It is often the overflow from the loft tank or a toilet cistern. The ballvalve feeding it is not shutting off. See our overflow pipes guide.