IndirectHot Water Cylinders

An indirect cylinder heats stored water via a coil or heat exchanger connected to your boiler or heating system, rather than using its own immersion element as the primary heat source. This makes it more efficient where you already have a boiler running for central heating, as it shares that heat source rather than generating its own. Most indirect cylinders still include an immersion heater as a backup, so hot water is maintained even if the boiler is down.

Sizing depends on peak demand rather than just occupancy — a hotel or office block with simultaneous showers or taps needs considerably more stored capacity than a domestic home. As a rough guide, domestic properties typically use 150-250 litres, while commercial or multi-occupancy buildings often need 300 litres upwards, sometimes with multiple cylinders linked together. It's worth checking boiler output too, as an oversized cylinder paired with an undersized boiler will struggle to reheat quickly enough between draw-offs.

Both options are available. Vented cylinders rely on a cold water storage tank in the loft and are simpler to install, while unvented cylinders connect directly to the mains and deliver higher pressure hot water without needing a header tank. Unvented systems must be fitted by a competent, registered installer and comply with Building Regulations G3, as they include safety devices such as pressure and temperature relief valves.

Most indirect cylinders are manufactured from grade 316 or 444 stainless steel, or copper, both of which resist corrosion well and are suited to potable hot water storage. Stainless steel tends to be lighter and cost-effective for larger commercial capacities, while copper has a long track record in domestic installations. Whichever material you choose, check the cylinder carries appropriate certification for use with wholesome water if it's feeding taps or showers.

Pre-plumbed cylinders arrive with key fittings — valves, immersion heaters, and sometimes pumps or expansion vessels — already installed on the cylinder itself, rather than needing to be sourced and fitted separately on site. This significantly cuts installation time and reduces the chance of on-site fitting errors, particularly useful on commercial projects with tight programmes. You'll still need a qualified installer to connect it into the heating and water system, but the cylinder itself turns up largely install-ready.

Yes — indirect cylinders are generally the better choice for commercial settings such as hotels, gyms or care homes, precisely because they draw on the boiler's heating capacity rather than relying solely on an internal element. This allows faster recovery times after heavy draw-off, which matters where multiple showers or taps run simultaneously. For particularly high demand, look at cylinders with a larger coil surface area or consider twin-coil models that can also integrate a secondary heat source such as solar thermal.