Slimline Indirect Unvented.

A reduced-diameter cylinder fed at mains pressure and heated by a coil from a boiler or heat pump, with an immersion element as backup. It delivers mains-pressure hot water from a narrower footprint than a standard cylinder.

Capacity is preserved by increasing height, so the constraint shifts from floor area to ceiling clearance.

Apartments, converted properties and homes where the airing cupboard is narrow but a boiler is present. Anywhere the standard 545mm to 600mm diameter will not pass through a door or fit an alcove.

They are also common in loft conversions where a taller, narrower vessel fits between joists and roof structure more easily.

It can. A slimmer vessel means a smaller coil diameter, so coil surface area is the specification to check rather than the cylinder volume alone.

This matters most with heat pumps, which need substantially more coil area than boilers because they run at lower flow temperatures. Confirm the cylinder is rated for heat pump duty if that is the heat source.

Usually, provided ceiling height allows and the pipework can be adapted. The bigger practical question is access — a tall cylinder needs a clear route with enough headroom to tilt it upright at the end.

Measure the stairwell, doorways and any turns before ordering, not just the cupboard.

The same as any unvented cylinder: pressure reducing valve, expansion provision, temperature and pressure relief valve, and a correctly sized discharge pipe to a safe, visible termination.

Installation and servicing must be carried out by a G3 qualified person under Building Regulations Part G.

Generally yes, modestly, because the manufacturing run is smaller and the taller vessel uses more material for the same capacity.

Against that, a slimline cylinder often avoids building work to enlarge a cupboard, which usually more than covers the difference.