You still need the cold water tank in the loft if a vented hot water cylinder or a cold tap is fed from it, because that supply comes from the tank and not from the mains. If your home has a combi boiler, or a cylinder that runs at mains pressure, the loft tank is normally doing nothing. The tricky part is telling the two apart, because disused pipework often stays in place for years. This page explains what the tank feeds, which clues point to it still being in use, and what has to happen in England before it can be taken out.
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Do you still need the cold water tank in the loft?
You still need the cold water tank in the loft if a vented hot water cylinder or a cold tap is fed from it. Vented means the cylinder is open to the air through a vent pipe. That type of cylinder takes its water from a tank in the loft and gets its pressure from gravity, not from the mains.
A combi boiler needs neither a hot water cylinder nor a cold water tank, because it heats mains water on demand. A system boiler takes its water from the mains, so it needs no loft tank of its own. It can still be paired with a vented cylinder, though. The boiler type alone therefore does not settle the question.
Old pipework left in place is the usual reason a loft tank looks redundant when it is not. The tank may still fill a bathroom cold tap, a toilet cistern or the cylinder. Tracing where those pipes go belongs in a survey, not in a guess from the loft hatch.
What does the cold water tank in the loft actually feed?
The loft tank usually feeds the upstairs bathroom, and in some older houses it feeds every cold tap in the home. The Drinking Water Inspectorate advises using a tap fed directly from the mains for drinking, cooking and brushing teeth. In most homes that is the kitchen cold tap.
A vented hot water cylinder is fed from its own storage tank in the loft. As the stored water heats and expands, the cold feed pipe takes up the extra volume. A vent pipe runs from the top of the cylinder to a point above the tank and acts as a safety device. Approved Document G sets this out for England, and it explains why water sometimes drops into the tank. Water coming out of the vent pipe into a loft tank has its own causes.
Two tanks in the loft: cold water storage cistern or feed and expansion cistern?
Many lofts hold two tanks: a large cold water storage cistern for taps and the cylinder, and a small feed and expansion cistern that tops up the heating circuit. They are not interchangeable, and removing one has nothing to do with removing the other.
| Point of difference | Cold water storage cistern | Feed and expansion cistern |
|---|---|---|
| What it feeds | Cold taps upstairs, often a toilet cistern, and a vented hot water cylinder | The central heating circuit and the radiators, nothing else |
| Size and position | The larger of the two tanks in the loft | The small tank, usually set higher than the large one |
| Pipes you can see | A large cold feed running down to the cylinder, plus an overflow to outside | A small feed pipe and a vent pipe that curls over the top of the tank |
| Boiler pressure gauge | Not related to this tank at all | An open vented heating system has no filling pressure at the boiler; the head of water does that job |
| What removal involves | Moving the cold taps and hot water onto mains pressure | Converting the heating to a sealed system with an expansion vessel |
A house can lose one tank and keep the other. Converting the heating to a sealed system removes the feed and expansion cistern, while the cold water storage cistern stays because the cylinder still needs it. The reverse also happens.
How do you tell whether the loft tank is still in use?
Whether the cold water tank in the loft is still in use depends on what it is piped to, and that is confirmed at a survey rather than by looking at the tank. An engineer checks whether there is a hot water cylinder, whether that cylinder is vented or unvented, and where the cold feeds run. Vented or unvented hot water cylinder explains how the two differ.
Two clues are safe for you to note yourself. The first is the pressure gauge on the boiler: a sealed heating system is filled to a pressure you can read, while an open vented system has no filling pressure at the boiler. The second is which tap your drinking water comes from, since the Drinking Water Inspectorate recommends a mains-fed tap for drinking and cooking.
Capping, cutting or draining pipework in the loft is not householder work. Anything that touches the gas supply, the flue or the inside of the boiler casing must be done by a Gas Safe registered engineer under the Gas Safety (Installation and Use) Regulations 1998 as amended, as HSE states. What you may safely do yourself stays with boiler pressure, thermostat and programmer settings, the reset button and bleeding radiators.
What has to happen if the cold water tank in the loft is removed?
Removing the loft tank is the last step of a conversion, not a job on its own. Something has to replace what the tank fed. In most homes that is an unvented cylinder working at mains pressure, or a combi boiler that heats water on demand. Converting a vented system to an unvented cylinder sets out what changes in the house.
An unvented cylinder is a sealed cylinder fed straight from the mains, and in England it falls under Requirement G3 of the Building Regulations 2010. It may only be installed by a person competent for that work. It needs at least two independent safety devices, and water discharged from them must be visible but safely taken away. The work is notifiable to building control; where the installer is registered with a competent person scheme, you receive a building regulations certificate of compliance instead.
Regulation 5 of the Water Supply (Water Fittings) Regulations 1999 applies in England and Wales. For the operations listed there, notice must go to the water supplier before work begins, and consent must not be unreasonably withheld. Work carried out by a contractor approved under those regulations is exempt from the notice. Redundant pipework left behind is worth removing as well, which HSE names as a control measure against dead legs.
The cold water tank in the loft is only redundant once whatever it fed has been replaced, and that replacement is regulated work in England. Taking the tank out first leaves taps, a cylinder or the heating with no supply.
Your mains supply has to carry the house before the loft tank can go
A house without a loft tank runs on whatever the mains delivers, so the incoming supply has to be measured first. With gravity from a loft tank, pressure comes from the height of the water above the outlet. On mains pressure, the supply pipe and the incoming flow decide what the shower does.
Heatrae Sadia gives 0.15 MPa (1.5 bar) flowing pressure and 50 litres per minute as the minimum for a Megaflo unvented cylinder, with a 28 mm supply pipe preferred. High mains pressure on its own does not mean high flow. Worcester Bosch notes that the flow at each outlet drops as more outlets are opened at the same time. Minimum flow rate for an unvented cylinder explains what the figures mean for your home.
If the cold water tank stays, what keeps the stored water fit to use?
A loft tank that stays needs a close-fitting, light-proof and secure lid, so birds, insects and dust cannot get in. The Drinking Water Inspectorate also asks for insect screens on the overflow and vent openings. The tank should be insulated, out of direct sunlight and away from heat sources. Where a tank is properly sized, correctly installed and well maintained, the Inspectorate says the water is safe to use.
Oversizing causes its own problem. The Inspectorate notes that 250 litres is enough for most households, so the stored water is turned over often. Quality suffers where there is no lid, where the water warms up, or where the tank material is unsuitable.
HSE rates the legionella risk from hot and cold water systems in most homes as low, because the water is used and turned over regularly. Temperature is the main control: keep the hot water hot, the cold water cold, and keep it moving. For landlords, HSE says sampling is not usually needed in domestic systems, and health and safety law does not require a legionella test certificate. Where a survey shows the system should change, a free home survey and a written fixed price for a new boiler installation can be arranged.
Sources
- Drinking Water Inspectorate — Water storage tanks and cisterns
- Approved Document G: Sanitation, hot water safety and water efficiency (2024 amendments)
- The Water Supply (Water Fittings) Regulations 1999
- HSE — Legionella and landlords' responsibilities
- HSE — Domestic gas: frequently asked questions
- Vaillant — What is a hot water cylinder
- Glow-worm — What is a regular (heat only) boiler
- Heatrae Sadia — Megaflo Eco installation manual