If your hot water runs out too quickly, takes ages to heat or the tank in the airing cupboard is leaking, it may be time for a hot water cylinder replacement. In the UK, a new vented cylinder usually costs £700 to £1,500 fitted, while an unvented cylinder – which delivers mains-pressure hot water – typically costs £1,200 to £2,500 including installation.
Choosing between the two depends on your water pressure, how many bathrooms you have and whether you are planning a heat pump in future. Below we compare the best cylinder options, sizes, prices and the rules installers must follow.
Vented or unvented?
- Fed by a cold water tank in the loft
- Gravity pressure – may need a pump for showers
- Simple, cheaper and no annual safety inspection
- Good for homes with low mains pressure
- Fed directly from the mains
- Strong showers on several floors at once
- Frees up the loft – no cold tank needed
- Needs good mains pressure and annual servicing
Check your mains pressure first
An unvented cylinder is only as good as the water supply feeding it. Before you commit, ask the installer to measure both pressure (in bar) and flow rate (in litres per minute) at the kitchen tap. Many manufacturers recommend at least 1.5 bar and around 20 litres per minute for good performance. If your supply is weak, an unvented system may disappoint, and a vented cylinder with a shower pump, or an upgraded supply pipe, could be the better option.
What size do you need?
| Household | Typical cylinder size |
|---|---|
| 1–2 people, one bathroom | 120–150 litres |
| 3–4 people, one or two bathrooms | 150–210 litres |
| 5+ people or several bathrooms | 210–300 litres |
A heat pump heats water more slowly and at lower temperatures, so heat pump cylinders are usually larger with a bigger internal coil. If a heat pump is on your list, choose a compatible cylinder now to avoid replacing it twice.
Direct, indirect and dual-coil cylinders
Cylinders are also described by how they are heated. An indirect cylinder has a coil connected to the boiler or heat pump, with an electric immersion heater as backup. A direct cylinder is heated only by electric immersion elements, which suits homes without a boiler, often on an off-peak tariff. A dual-coil cylinder has two coils so it can take heat from two sources, for example a boiler and solar thermal panels. Some newer cylinders also include connections for solar PV diverters.
Signs your cylinder needs replacing
- Water leaking from the base or seams, or rust-coloured water.
- Hot water running out much faster than it used to.
- Rumbling or banging noises as limescale builds up.
- The cylinder is 15–20 years old or has thin factory insulation.
- A failed immersion heater that keeps recurring.
Repair options before replacement
Not every problem means a new cylinder. A faulty immersion heater element or thermostat can usually be replaced for £100 to £250. A sticking motorised valve, which controls hot water from the boiler, is another common and affordable repair. On unvented systems, a dripping discharge pipe often points to a failed pressure relief valve or an expansion vessel that needs re-pressurising, both of which a qualified engineer can fix. If the cylinder itself is corroded or leaking from the body, though, replacement is the only safe option.
Rules for unvented cylinders
Because they store hot water under pressure, unvented cylinders must be installed by someone holding a recognised unvented hot water qualification (often called G3). The installation is notifiable under building regulations, and the safety valves should be checked by a qualified engineer every year to keep the warranty valid.
Stainless steel or copper?
Most modern unvented cylinders are made from duplex stainless steel, which resists corrosion and usually comes with long warranties, sometimes 25 years on the inner vessel. Traditional vented cylinders are commonly copper, which is cheaper and easy to work with but more vulnerable to corrosion in hard water areas. In very hard water regions, ask about a scale reducer to protect the cylinder and immersion heater.
What a good quote includes
- Cylinder make, capacity and warranty length.
- Removal of the old cylinder and, for unvented, the loft tanks.
- Expansion vessel, valves, discharge pipe (tundish) and controls.
- Any pipework upgrades and a mains pressure and flow test.
- Building regulations notification and commissioning certificate.
How long does installation take?
A straight swap of a vented cylinder usually takes one day. Converting to an unvented system, including removing loft tanks and fitting a discharge pipe to the outside, can take one to two days. Hot water will be off during this time, so plan showers accordingly.
Prices reflect typical UK installed costs for standard domestic cylinders.
Independent guide – not linked to any manufacturer or installer.