Vented vs unvented hot water cylinder: which is right for me?

What is the difference between a vented and an unvented cylinder?

A vented cylinder is open to the atmosphere through a vent pipe and is filled by gravity from a cold water storage tank in the loft. An unvented cylinder is a sealed pressure vessel connected directly to the cold mains, so the hot water arrives at whatever pressure the water company delivers.

Everything else follows from that one difference. The vented tank works at a pressure set by the height of the loft tank above the tap; the unvented tank works at mains pressure and therefore has to carry a full safety group — a pressure-reducing valve, an expansion vessel, an expansion relief valve and a temperature-and-pressure relief valve — to deal with the fact that water expands by about 2% when it is heated from 10 °C to 60 °C and has nowhere to go.

How much pressure and flow does each actually deliver?

A metre of water height is worth 0.098 bar. A loft tank whose base sits 3 metres above a first-floor shower therefore supplies about 0.29 bar there, and perhaps 0.55 bar at a ground-floor tap. Mains-fed unvented systems typically run at 2–3 bar after the pressure-reducing valve.

EN 12897:2016+A1:2020 is the standard behind every indirectly heated unvented cylinder sold in Europe. It covers vessels up to 2,000 litres on supplies between 0.05 and 1.0 MPa (0.5–10 bar) and requires the controls to prevent the stored water ever exceeding 95 °C.

What does an unvented cylinder need that a vented one does not?

Three things: mains capacity, safety hardware and a discharge route. The mains supply has to deliver both pressure and volume — 1.5 bar static and 20 L/min is the practical floor, because a sealed cylinder cannot boost a weak supply the way a loft tank plus a pump can.

The safety group is not optional. The pressure-reducing valve is normally set to 3.0–3.5 bar, the expansion relief valve to around 6 bar, and the temperature-and-pressure relief valve to roughly 7 bar and 90–95 °C. The expansion vessel is pre-charged to match the PRV setting within ±0.2 bar; when it loses that charge, the surplus water goes out through the tundish, which is the single most common unvented cylinder fault.

In the UK, any unvented system storing more than 15 litres falls under Building Regulations Approved Document G3 and must be installed by a registered competent person. Most European countries apply an equivalent pressure-equipment and installer regime.

When is a vented cylinder still the better choice?

Four situations genuinely favour vented storage:

• Weak or intermittent mains. Below about 15 L/min an unvented cylinder will disappoint at every outlet, while a loft tank plus a shower pump will not.
• Very hard water. A vented cylinder can be drained, opened and descaled far more easily, and the loft tank buffers the system against mains interruptions.
• Low budget retrofits. The tank costs less, no safety group is needed, and no scheme-registered installer is required.
• Existing gravity systems where the loft tank, pipework and airing cupboard are already in place and working.

The efficiency argument does not favour either type. Standing loss is governed by insulation, not by pressure, and both are capped by the same Ecodesign formula of 16.66 + 8.33 × V^0.4 watts.

Which suits a heat pump or solar thermal system?

Unvented, in almost every case. A heat pump or solar loop charges the tank slowly through a coil, so the tank has to be correctly stratified and generously insulated; an unvented cylinder gives balanced pressure at the mixer, which matters when stored water sits at 50–55 °C rather than 65 °C and every degree of blending counts.

The one exception is a property where the mains cannot support an unvented system. There, a vented cylinder with a pumped shower circuit, or a thermal store, is the honest answer. What does not work is keeping an old boiler cylinder: its coil is far too small, as set out in what coil surface area a heat pump cylinder needs.