Hot water cylinders store and supply domestic hot water in system and heat-only boiler setups, and in some property types with electric-only heating. When a cylinder component fails, you lose hot water — and in some cases, the boiler fault code will reflect a zone valve or control issue rather than the cylinder itself.
This page covers the main components of both vented (traditional) and unvented (pressurised) hot water cylinders, what’s legal to replace yourself, and what requires a qualified engineer.
Vented vs Unvented Cylinders
Vented cylinders are the traditional design — an open-vented system with a separate cold water feed tank (usually in the loft). Water pressure from the tap is gravity-fed, not mains pressure. These systems are common in pre-1990 properties.
Unvented cylinders connect directly to the mains water supply and deliver mains-pressure hot water. Working on unvented cylinders requires G3 unvented qualification, not just Gas Safe registration.
Always confirm whether your cylinder is vented or unvented before ordering pressure-related components.
Immersion Heaters
An immersion heater is a direct electric heating element submerged in the cylinder, used to heat water independently of the boiler.
Failure Symptoms
- No hot water despite immersion being switched on
- Slow or incomplete heating
- Circuit breaker trips when immersion is switched on
Types
Long immersion heaters: Heat the full cylinder volume. Positioned vertically. Short (top) immersion heaters: Heat only the upper portion of the cylinder — quicker to heat a small volume. Dual immersion systems: Both a long and a short element. Economy 7 setups typically heat the full tank overnight on cheap-rate electricity.
Sizes
Standard UK immersion heaters are 11", 18", 24", 27", or 36" in the immersion pocket. Most cylinders use a 11/4" BSP fitting.
DIY note: Replacing an immersion heater is legal for competent homeowners on a vented cylinder. On unvented cylinders, element replacement should be carried out by a G3-qualified engineer.
Cylinder Thermostats
The cylinder thermostat clips onto the outside of the cylinder and controls the heat source. Standard setting: 60°C — hot enough to prevent Legionella bacteria growth while remaining safe for distribution.
Failure Symptoms
- Cylinder always cold (stat stuck open, not calling for heat)
- Cylinder excessively hot, expansion vessel discharging (stat stuck closed)
- Inconsistent hot water temperature
DIY note: Cylinder thermostats are clip-on and operate at low voltage. Replacement is DIY-legal.
Expansion Vessels
Expansion vessels absorb the pressure increase that occurs when water heats up. Water expands by about 4% when heated from cold to 60°C.
Failure Symptoms
- Pressure relief valve discharging regularly
- System pressure increasing significantly when hot, then dropping when cold
- Boiler needing to be re-pressurised frequently
If F22 or E119 recurs after topping up: Inspect the expansion vessel — charge with a tyre pump. If water comes out, the bladder has failed.
DIY note: On a sealed central heating system, expansion vessel replacement is often a competent homeowner job with proper isolation. On unvented hot water cylinders, replacement should be carried out by a G3-qualified engineer.
Pressure Relief Valves (PRVs)
Pressure relief valves are safety devices — they open and discharge water to a drain if pressure exceeds a set threshold. A correctly functioning system should never discharge through its PRV in normal operation.
If your PRV is discharging regularly, the root cause is almost always a failed expansion vessel. However, PRVs do fail. A stuck-closed PRV is a serious safety risk — replace immediately if suspected.
Zone Valves (Hot Water Circuit)
For system boilers with a hot water cylinder, a motorised zone valve directs hot water flow to the cylinder when the cylinder thermostat calls for heat. Zone valve failure presents as either constant heating of the cylinder (valve stuck open) or no hot water with the boiler otherwise working normally (valve stuck closed).
Zone valve heads (actuator motors) can often be replaced without draining the system.
Thermostatic Blending Valves (TMVs)
TMVs blend hot cylinder water with cold water before the hot water outlets to deliver water at a safe temperature. Required by Building Regulations on new or significantly modified systems.
A failed TMV can deliver scalding water (failed open) or cold water only (failed closed). Both warrant immediate replacement.