Cold Water Tank in the Loft: What It Does and Can It Go?

What the loft tank actually supplies

The tank, properly called a cold water storage cistern, sits high up so gravity can push water down to the outlets below. The main job in almost every house that has one is feeding the hot water cylinder. Cold water flows from the tank into the bottom of the cylinder, pushing hot water out of the top to your taps. That is what makes the system "vented", and it is why the hot water pressure in these homes depends on how high the tank is above the tap. Our guide to unvented vs vented cylinder explains the difference in full.

In many older houses the same tank also feeds the cold taps in the bathroom, the bath, the shower mixer and the toilet cisterns. In newer or refitted homes those are often on the mains, and the tank feeds only the cylinder. The quickest way to find out is to turn the stopcock off and see which cold taps keep running for a while. Anything still flowing is fed from the tank.

The small tank next to it

Many lofts have a second, much smaller tank beside the main one. That is the feed and expansion tank for an open vented heating system. It tops up the radiators and gives the heating water somewhere to expand into as it warms up. It is separate from your drinking and washing water, and the water in it is often dark or rusty, which is normal for heating water.

Homes with a combi or a system boiler and sealed heating usually do not have this small tank. If you have one, your heating is open vented, and taking it out means converting the heating to a sealed system as well.

What a healthy loft tank looks like

Most tanks fitted in the last few decades are plastic. Older houses may still have a galvanised steel tank, which rusts from the inside, and a few still have old cement based tanks. If yours is grey and heavy looking with no obvious brand, do not drill, cut or break it. Older tanks of that type can contain asbestos and should be checked before anyone disturbs them.

• A rigid, close fitting lid, so dust, insects and debris cannot get in
• Insulation on the lid and the sides, but not underneath
• An overflow pipe with a screen or filter, running to the outside
• A float valve that shuts off cleanly when the tank is full
• Isolating valves on the outlets, so the cylinder or bathroom can be turned off without draining the tank
• No rust streaks, no bulging sides, and a dry loft floor underneath

A tank with no lid, or a lid lying loose, is worth fixing straight away. Water from the loft tank ends up in your hot water and often your bathroom taps, and an open tank is how dead insects, loft dust and even birds end up in it.

Getting the loft tank ready for winter

The loft is the coldest part of most houses, and it gets colder once the loft floor is well insulated, because less heat rises from the rooms below. That makes the tank and its pipes the first things to freeze in a cold snap.

Insulate the top and sides of the tank and every pipe running through the loft. Leave the loft insulation out from directly underneath the tank, so a little warmth from the house below can reach it. In a hard frost, leaving the loft hatch open for a while lets some warmth up to the tank, and if you are going away in freezing weather, keep the heating on a low setting rather than switching everything off. Our guide on how to turn off hot water supply covers what to do if the house will be empty and unheated for a long spell.

Our guide on insulating hot water pipes covers the pipes around the cylinder and in the loft, which lose heat all year and are the ones most likely to freeze.

Common loft tank problems

Most loft tank faults are straightforward repairs. A tank that is leaking through its wall or base, or a steel tank that has rusted through, needs replacing. These are the ones we see most:

• Water dripping from an overflow pipe outside: the float valve is not shutting off, usually a worn washer or a float set too high. If it is the small heating tank that overflows, or the water in it is warm, our guide to a header tank overflowing explains what that means.
• No water from the bath or hot taps, but the kitchen tap works: the tank has run dry. The float valve may be stuck, or the stopcock may have been turned off.
• Spluttering taps after the tank has run low: air has got into the pipes. Our guide to an airlock in hot water pipes explains how to clear it.
• Rusty or discoloured water from the hot taps: often an old steel tank rusting from the inside, or sediment at the bottom of the tank being drawn into the cylinder.
• Weak hot water flow: the tank is not far enough above the outlets to give good pressure. See low hot water pressure.

Can you remove the cold water tank from the loft?

Yes, in most houses, but the tank never comes out on its own. Everything it feeds has to be moved onto the mains first, and the hot water has to come from somewhere that does not need a tank.

There are two usual routes. The first is replacing the vented cylinder with an unvented one, which is fed directly from the mains and gives hot water at close to mains pressure at every tap. The second is replacing the boiler and cylinder with a combi boiler, which heats water from the mains as it passes through. Both free up the loft and the airing cupboard. Our guide on converting to a combi boiler covers that route, and unvented cylinder installation covers the other.

The deciding factor is your incoming mains. Both options rely on the mains supply having enough pressure and, just as importantly, enough flow to run a shower and a tap at the same time. Many older London houses still have a narrow lead or old iron supply pipe that looks fine at a single tap and falls away when two outlets run together. Before anyone recommends taking the tank out, the mains should be measured at the property.

In flats with no loft at all, the cold tank is often built onto the cylinder itself as a combination cylinder, which has its own repair and upgrade choices.

An unvented cylinder must be installed by an engineer holding the G3 qualification, and the installation has to be notified under Building Regulations. If someone offers to take your loft tank out and fit a pressurised cylinder, ask to see their unvented card first.

Should you replace an old tank rather than remove it?

If you are happy with a vented system, or the mains flow will not support an unvented cylinder or a combi, replacing an old steel or damaged tank with a modern plastic one, with a proper lid, screened overflow and insulation, is a sensible and less disruptive job. Vented systems are simple, quiet and long lasting, and there is nothing wrong with keeping one.

If you are unsure which way to go, call us on 020 4652 5111 or send an enquiry with photos of the tank, the cylinder and the pipework around it. We have 20 engineers across London, including G3 qualified engineers for unvented cylinders, and we can measure your mains and tell you honestly whether the tank can go. Our rate is on the pricing page and agreed before we attend.