IndirectSlimline Hot Water Cylinders

An indirect cylinder has an internal heat exchanger coil through which hot water from your gas or oil boiler passes, transferring heat to the stored water without mixing with it. A direct cylinder instead heats water using its own immersion elements, drawing electricity rather than boiler-heated water. If you already have a working boiler and central heating system, indirect is almost always the more efficient and cost-effective choice.

Slimline cylinders have a reduced diameter compared to standard indirect cylinders, typically bringing the width down to around 400-450mm, while keeping height and capacity similar. This makes them ideal for narrow airing cupboards, utility cupboards or tight loft spaces where a standard-diameter cylinder simply wouldn't go in. Always check both the diameter and height against your available space before ordering, as insulation jackets add a little extra to the stated cylinder dimensions.

Indirect slimline cylinders are available in both vented and unvented versions. Vented cylinders need a cold water feed and expansion tank in the loft and operate at mains-fed low pressure, whereas unvented cylinders connect directly to mains pressure and don't need a header tank, giving better flow rates. Unvented models must be installed by a competent, G3-qualified engineer and include the required safety valves and expansion arrangements under Building Regulations Part G3.

No — combi boilers heat water on demand and aren't designed to work with a stored hot water cylinder. Indirect cylinders need a conventional (heat-only) or system boiler that can circulate heated water through the cylinder's coil. If your property has a combi boiler and you want to add a cylinder, you'd need to change the boiler type as part of the installation.

As a rough guide, a 1-2 bedroom property with one bathroom often suits a 120-150 litre cylinder, while a 3-4 bedroom house with an en-suite typically needs 180-250 litres. Households with power showers, multiple bathrooms or high simultaneous demand should size up, or consider an unvented model for better recovery and flow rate. It's worth checking boiler output too, as recovery time depends on how quickly your boiler can reheat the stored water.

Most indirect slimline cylinders are manufactured from grade 316 or 444 stainless steel, or copper, both of which resist corrosion well and carry long manufacturer guarantees. They're wrapped in factory-fitted high-density foam insulation, which significantly reduces standing heat loss compared to older jacket-and-tank designs. This means water stays hotter for longer between boiler firing cycles, cutting down on unnecessary reheating and running costs.