HotWater Tanks

As a rough guide, allow 35-45 litres per person for a standard household, so a family of four typically needs a cylinder in the 150-210 litre range. Properties with power showers, multiple bathrooms, or combi boiler back-up will need to size up accordingly. If you're unsure, it's worth checking demand against your boiler or immersion heater output before choosing a tank.

A vented tank is fed from a cold water storage tank in the loft and operates at low pressure, making it simpler and cheaper to install. An unvented tank connects directly to the mains, giving strong, consistent pressure at every outlet without needing a header tank. Unvented systems must be fitted by a competent, qualified installer due to the pressures involved and building regulations (Part G) requirements.

Yes, if you're running an unvented or sealed system, an expansion tank (or expansion vessel) is essential. As water heats up it expands, and without somewhere for that extra volume to go, pressure will build up and can damage the cylinder or trigger the safety valve unnecessarily. Sizing the expansion vessel correctly to match your cylinder capacity extends the life of the whole system.

Most domestic cylinders are stainless steel or copper, both of which resist corrosion well and are widely used for potable hot water storage. GRP (glass-reinforced plastic) tanks are more common for larger commercial or industrial hot water storage where bespoke sizes and shapes are needed. Stainless steel is generally the preferred choice for unvented cylinders due to its strength and durability under pressure.

Look for tanks with factory-fitted insulation, typically CFC-free foam, that meets current Building Regulations standards for heat loss. Better-insulated cylinders lose less heat overnight, meaning your boiler or immersion heater doesn't have to work as hard to reheat the water. This directly reduces running costs and is worth prioritising over a slightly cheaper, less insulated model.

Vented cylinders need loft or roof space above for the cold water feed, whereas unvented cylinders can be sited in an airing cupboard, utility room or loft since they don't need a header tank. All unvented installations must comply with Building Regulations Part G3 and include the correct safety devices, such as a tundish and discharge pipe. It's always worth checking access, floor loading and pipework runs before finalising the location, particularly for larger commercial units.