Indirect Unvented.

A cylinder fed at mains pressure that is heated by a coil connected to a boiler or heat pump, usually with an immersion element as backup. It delivers mains-pressure hot water without a cold tank in the loft.

This is the most common configuration in modern UK homes with a system or regular boiler.

Only if the cylinder is specified for heat pump use. Heat pumps run at lower flow temperatures than boilers, so they need a much larger coil surface area to transfer heat effectively. A standard boiler cylinder paired with a heat pump will underperform badly.

Look for cylinders explicitly rated for heat pump duty, which state coil area and reheat times at low flow temperatures.

Usually, provided the incoming mains can deliver adequate flow and pressure — typically at least 1.5 bar and around 20 litres per minute. The loft tank and its pipework come out, and a discharge route for the relief valves has to be found.

The discharge pipe routing is generally the deciding practical constraint in a retrofit, particularly in flats and mid-terrace properties.

A twin coil has two separate heating coils, allowing two heat sources — most commonly a boiler on one and solar thermal or a heat pump on the other. The lower coil takes the renewable source so it heats the coldest water at the bottom of the cylinder.

If solar thermal or a second heat source is a possibility later, specifying twin coil now is far cheaper than replacing the cylinder afterwards.

Usually scale build-up on the coil in hard water areas, a failed thermostat, or thermal stratification breaking down through a faulty valve letting cold water in.

In unvented systems, a waterlogged expansion vessel can also reduce usable capacity. If output has fallen gradually over years, scale is the most likely cause; if suddenly, suspect a control or valve.

Yes — the same as any unvented system. Installation and servicing are notifiable work under Building Regulations Part G and require G3 certification.

Annual servicing is also generally a warranty condition, covering the expansion vessel, relief valves and anode.