ExpansionTanks

Sizing depends on the volume of water in the system and whether it's a sealed heating system or a potable hot water cylinder setup — as a rough guide, most domestic sealed heating systems use a 12-18 litre vessel, while unvented hot water cylinders typically need a larger expansion vessel sized against the cylinder capacity. Undersizing causes the pressure relief valve to weep or discharge repeatedly, so always check manufacturer sizing charts or ask us if you're unsure.

If the expansion vessel is fitted on the potable side of an unvented hot water system, it must be WRAS approved, as it's in contact with drinking water. Expansion vessels on sealed central heating circuits don't carry drinking water so WRAS approval isn't a requirement there, though building regs still apply to the overall unvented installation. Always check the product spec matches the application before fitting.

Fitting on a sealed heating circuit is straightforward for a competent heating engineer or confident DIYer, provided pre-charge pressure is checked and set correctly before commissioning. However, expansion vessels forming part of an unvented hot water system must be installed and commissioned by a competent person registered under Part G of the Building Regulations. Get this wrong and you risk invalidating warranties or breaching regs.

A heating expansion vessel handles the expansion of water within a sealed central heating circuit and typically has a red casing. A potable water expansion vessel, usually blue or grey, is fitted to unvented hot water cylinders and must be suitable for contact with wholesome water. They're not interchangeable, so always check the intended use before ordering.

Most expansion tanks have a Schrader valve (like a car tyre valve) where you can check the air charge with a pressure gauge — this should be set slightly below the system's minimum operating pressure, typically around 1.5 bar for heating systems, before the vessel is connected and water pressure is applied. If pressure has dropped, top it up using a hand pump or foot pump; if it drops repeatedly, the internal diaphragm may have failed and the vessel needs replacing.

Smaller expansion vessels are generally stocked items and can be despatched quickly, often for next-day delivery. Larger capacity vessels or less common sizes may need to come direct from the manufacturer, so lead times can be longer — check the product page or ask us to confirm stock before you commit to a fitting date.