CylinderAccessories

Expansion vessel size depends on the volume of your cylinder and the pressure of your incoming mains supply, not just tank capacity alone. As a rule of thumb, an 8–12 litre vessel suits most unvented cylinders up to 300 litres, but always check the cylinder manufacturer's specification first. Fitting an undersized vessel is one of the most common causes of a tundish dripping or a pressure relief valve lifting prematurely.

On an unvented hot water cylinder, any work on the expansion vessel, relief valves or associated pipework falls under Building Regulations G3 and must be carried out by a competent person registered under a scheme such as WIAPS or Part L. This isn't just red tape — unvented systems are pressurised and can be dangerous if worked on incorrectly. A registered installer will also need to notify the work to building control.

A cylinder thermostat monitors stored water temperature and switches the immersion heater or heat source on and off to maintain your set point, typically around 60°C to guard against legionella while limiting scale build-up. Look for a thermostat accurate to within 2–3°C, as excessive drift wastes energy and can cause nuisance cycling. Strap-on and pocket-mounted versions are both available depending on your cylinder type.

A dripping relief valve is usually a sign of excess pressure in the system, often caused by a failed or waterlogged expansion vessel rather than a fault with the valve itself. Mains pressure that's too high, or a valve past its service life, can also be to blame. It's worth checking the expansion vessel's pre-charge pressure before assuming the valve needs replacing, as valves are often condemned unnecessarily.

A solar pump station circulates heat-transfer fluid between your solar thermal collectors and a twin-coil cylinder, allowing you to preheat hot water using free solar energy before your boiler or immersion tops up the temperature. Most stations include the circulation pump, flow meter, isolation valves and safety gauges in one compact unit, simplifying installation and future servicing. This can meaningfully cut water heating costs over the summer months, though output will vary with UK weather and collector size.

Yes — lids and seals perspire wear over time from repeated heating and cooling cycles, and a perished seal can allow leaks or air ingress that affects system pressure. Check periodically for cracking, hardening or visible gaps around the seal, particularly if you notice damp around the cylinder base. Always match the replacement lid and seal to your cylinder's exact make and diameter, as fitments are not universal across manufacturers.