Direct Unvented Cylinders.

Unvented means the cylinder is fed directly from the mains and delivers hot water at mains pressure, with no cold water tank in the loft. Direct means the water is heated by immersion elements inside the cylinder rather than by a boiler coil.

So a direct unvented cylinder suits properties with no boiler, or where the heat source is electric — including homes on Economy 7 or with solar PV diverting to the immersion.

Yes. Unvented hot water systems are notifiable work under Building Regulations Part G, and installation must be carried out by someone holding a G3 qualification. Installing one without is both unlawful and a genuine safety risk.

The system stores water above 60°C under pressure; the safety devices exist to prevent a stored-energy failure and must be commissioned correctly.

A pressure reducing valve, an expansion vessel or internal air gap, a temperature and pressure relief valve, and a discharge pipe routed safely to a visible, frost-free termination.

The discharge arrangement is the part most often got wrong on retrofit installations — it must be sized correctly and terminate where a discharge would be noticed but cause no scald risk.

Unvented gives mains-pressure hot water at every outlet with no loft tank, which usually means better showers and simpler pipework. Vented systems are cheaper, simpler and can be worked on without a G3 qualification, but flow depends on the height of the tank above the outlet.

Unvented needs adequate mains flow and pressure — typically at least 1.5 bar and 20 litres per minute — so check incoming supply before specifying.

Size against peak simultaneous demand and reheat time rather than household size alone. As a rough guide, 150 litres suits a one-bathroom home, 210 to 250 litres a two-bathroom home, and 300 litres and above where there are multiple bathrooms or a high-flow shower.

Direct cylinders reheat more slowly than indirect ones, so err upward if the household showers at the same time each morning.

Annually. Servicing checks the expansion vessel charge, the relief valves, the immersion elements and the sacrificial anode where fitted. Manufacturers generally require documented annual servicing to maintain the warranty.

A failed expansion vessel is the most common fault and shows itself as the relief valve discharging intermittently.