What does the cold water tank in my loft do?
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Part of our tap water guide
There is a lidded tank in the loft with pipes running out of it. What is it for, and does your house need one?
Short answer: It stores cold water at atmospheric pressure and feeds the pipes connected to it. The Drinking Water Inspectorate (DWI) says that is usually the upstairs bathrooms, sometimes every cold tap, while newer homes with unvented heating likely have all taps on mains pressure. For a drinking water tank, it calls 250 litres quite sufficient for most homes.
By Andrew Tomlinson, WaterKit · Last reviewed 25 October 2026
What does the cold water tank in the loft do?
It stores cold water and supplies the pipes connected to it. The Water Supply (Water Fittings) Regulations 1999, which apply in England and Wales, define a cistern as a fixed container holding water at atmospheric pressure. Water Regs UK guidance gives four reasons for fitting one.
- Backflow protection.
- A reserve if the incoming supply is disrupted.
- Lower peak demand on the cold supply.
- Lower supply pressure.
Which taps and hot water does the loft tank feed?
It depends on how your home is plumbed.
| Set-up | What the source says |
|---|---|
| Older home with a loft tank | The DWI says the tank usually supplies cold water to upstairs bathrooms, and in some homes all the cold taps |
| Newer home with unvented heating | The DWI says all hot and cold taps are likely to be supplied directly under mains pressure |
| Combi boiler | The Energy Saving Trust says it uses no hot water cylinder |
| Heat-only (regular) boiler | The Energy Saving Trust says it has two extra header tanks for cold water, usually in the loft if there is one |
The DWI's chlorine page adds that hot water and bathroom tap water usually come from a storage tank in the loft. If in doubt, the DWI says, ask a qualified plumber to check the plumbing. Its thumb check and advice on drinking from a tank-fed tap are in our bathroom tap water guide.
Do I need a cold water tank in the loft?
Not in every home, because it depends on the system above. Water Regs UK says a cistern may provide backflow protection for installations downstream, so it should not be removed without consulting the local water undertaker. The DWI says a replacement tank must be an approved type fitted by an approved plumber, and some changes need prior notice to your water company.
What rules does the tank have to meet?
In England and Wales, Schedule 2 of the 1999 Regulations says a storage cistern needs an overflow pipe, a cover that excludes light and insects, and insulation against freezing and undue warming. In Scotland, Water Regs UK says, byelaws apply instead.
How do I keep loft tank water fresh, and who should clean it?
Keep it covered, cool and turning over, and follow the DWI's cleaning advice below. The DWI says tank water will deteriorate if there is no lid, if it warms, if too much is stored and turnover is low, if the material is unsuitable, or if an unqualified person works on it. If a tank-fed tap runs cloudy or tastes odd, see our hot water cloudy guide and tap water taste guide.
The DWI recommends an annual check of the tank and the water if it stores drinking water. The DWI says any cleaning must not affect water quality, and to follow the instructions supplied with the tank or ask your water company or a WaterSafe Register plumber.
How can I check a tank-fed tap?
Compare it with the kitchen cold tap. The WaterKit Water Test Kit has free chlorine and total chlorine pads on its 20-in-1 strip, whose lowest steps are 0, 0.5, 1 and 3 mg/L. Test each tap with a fresh strip and read it after 30 to 60 seconds. Our understanding, not the DWI's, is that chlorine fades as water stands, so a lower reading at a tank-fed tap would fit that.
- The separate rapid bacteria test is a presence screen for coliforms, not a count, read after 24 to 48 hours.
- We do not claim the test shows a tank's condition. A strip result is not a legal sample.
Best pack: 50 strips, enough to test the kitchen tap and a tank-fed tap side by side and again after any tank work, £8.99 (or £13.99 with a bacteria test), with free 2nd Class delivery on every order. See the kit
Your loft tank questions
Do you need a cold water tank with a combi boiler?
Not always. The Energy Saving Trust says a combi boiler uses no hot water cylinder and mentions header tanks only for heat-only boilers.
What does the cold water tank in the loft feed?
Usually the upstairs bathroom cold taps, and in some homes every cold tap, the DWI says.
What is a cold water storage tank used for?
Storing water for later use, plus the four reasons in the Water Regs UK list above.
How to keep water fresh in a storage tank?
Follow the DWI's list above.
How to clean cold water tank in loft? How to clean a cold water storage tank?
We do not give a method. Ask your water company or a WaterSafe Register plumber.
How to chlorinate a water storage tank?
The DWI tanks page gives no chlorination method, and neither do we. For cleaning, it points to your water company or a WaterSafe Register plumber.
Sources
- Drinking Water Inspectorate, Water storage tanks and cisterns (tank and which taps it feeds, unvented homes, the 250 litre figure, deterioration list, replacement, annual check, cleaning advice; page metadata last modified 9 June 2026; read 25 October 2026).
- Drinking Water Inspectorate, Chlorine (hot water and bathroom tap water, loft storage tank; page metadata last modified 13 August 2024; read 25 October 2026) and Advice for finding a plumber (WaterSafe recommendation; page metadata last modified 4 November 2024; read 25 October 2026).
- The Water Supply (Water Fittings) Regulations 1999, Schedule 2 (definitions of cistern and storage cistern; paragraph 16, cisterns; marked as applying in England and Wales; read 25 October 2026).
- Water Regs UK, Cold Water Storage (reasons for a cistern, removal, and byelaws in Scotland; undated, non-statutory guidance; read 25 October 2026).
- Energy Saving Trust, Boiler types explained (combi and heat-only boilers; last updated 20 May 2026; read 25 October 2026; not linked, because its website terms allow a link to the home page only and ask for notice first).
- WaterSafe Register, watersafe.org.uk (national register of approved plumbers, supported by the drinking water regulators; read 25 October 2026).
- WaterKit, Water Test Kit (20-in-1 strip, 30 to 60 second read, rapid coliform test, from £3.99, free 2nd Class delivery on every order, 30-day money-back guarantee) and results portal (chlorine pad steps, bacteria read time). The reading of the chlorine chart steps and the comments on expected readings are WaterKit's own.
Contains public sector information licensed under the Open Government Licence v3.0.