How open-vented and sealed unvented cylinders handle pressure and expansion, the safety controls each relies on, and what common faults point to.
Domestic stored hot water comes in two broad families: vented (open) and unvented (sealed). Knowing how each handles pressure and expansion is the key to understanding the safety controls. In the diagram, the open-vented cylinder is on the left and the unvented cylinder on the right.
The open-vented cylinder
In a conventional vented arrangement, a cold-water storage cistern feeds the cylinder and a separate open vent rises above the cistern. Normal expansion is accommodated through the cold-feed pipe; the vent is the safety path to atmosphere. Gravity pressure at an outlet depends on its vertical distance below the cistern water level, not on the whole system being at atmospheric pressure. The arrangement is simple and works at low, gravity-driven pressure.
Why the open vent must never be valved or dip
The open vent has to stay a permanently open path, rising continuously from the cylinder to its outlet over the cistern with no dips or valves anywhere along it. A dip can interfere with free venting, while a closed valve can block the route. Neither is acceptable in this safety connection. That matters because the vent's job goes beyond ordinary expansion: if a control ever fails and the stored water starts to boil, the vent is what gives the steam and surge a safe route to the cistern instead of letting dangerous pressure build in the cylinder.
The unvented cylinder
The common domestic unvented arrangement is supplied from the cold main through an inlet control group, with no loft feed cistern: typically a pressure-reducing valve and check valve, with the specified expansion capacity and relief protection. Some cylinders use a separate expansion vessel; others use an internal air volume. Hot water leaves the cylinder at the pressure those inlet controls regulate rather than at a loft cistern's gravity head, but flow at the outlets still depends on the incoming supply and the pipework, so it must be assessed, and balanced hot and cold supplies require the appropriate connections. Unvented means no open vent; a dedicated cistern supply is possible in another suitable design.
The trade-off is that a sealed cylinder cannot let expanding water escape on its own, and overheated water under pressure is dangerous, so an unvented cylinder relies on layered safety devices working together. Typical functions include expansion capacity, a control thermostat, an independent energy cut-out and suitable relief devices. The thermostat and cut-out control heat input; relief devices discharge water. The temperature and pressure relief valve discharges through a tundish, which keeps any discharge visible, and on to a safe termination.
Keep both forms of protection in mind: the vented arrangement has its open safety path and still needs the specified overheat protection. An unvented arrangement has no open vent and relies on its designed expansion capacity, heat controls and relief devices.
Competence and notification
Unvented hot water storage is covered by requirement G3 of the Building Regulations. The installer must be competent for the system and follow the applicable requirements. In England, storage over 15 litres normally needs building-control notification or competent-person self-certification; the small-storage exemption also depends on any associated electrical work being non-notifiable. It does not remove the need for safety controls.
Interpreting discharge during heat-up
First identify the valve and the cylinder’s expansion arrangement. With an external diaphragm vessel, its gas cushion accepts the increase in water volume. Insufficient charge, a failed vessel or an obstructed connection can leave inadequate expansion capacity. Other cylinders use an internal air volume instead.
Intermittent expansion-relief discharge during otherwise normal heat-up points first to those expansion checks. Continuous discharge also calls for checks of the inlet pressure controls and valve seating. The pattern guides the investigation; it does not prove one failed part. Very hot discharge from temperature protection needs urgent action under the cylinder instructions. Keep every discharge route unobstructed and have a competent person investigate.
Scald safety applies to both
Whichever system stores the water, the temperature delivered at the outlet still matters. For a fixed bath in a new dwelling in England, the hot-water supply is limited to a maximum of 48 °C. Vulnerable users may need a lower limit; where they are at risk during whole-body immersion, water must not exceed 44 °C.
Reading common vented-system complaints
A few symptom patterns come up often enough to be worth recognising on sight.
- One outlet has poor flow while the others work normally: start with that outlet and its supply branch, including local valves, strainers and restrictions. This pattern helps locate checks but does not prove that every shared component is sound.
- The usual draw-off used to last, but now turns cool early: check the cylinder control and how much of the store is actually heated. If only an upper boost heater works, a small hot layer can hide a full-store heating fault. If the system has always run short, or demand has grown, compare the draw-off with cylinder capacity and recovery instead.
- A pumped hot outlet looks milky: water clearing from the bottom of a glass upwards indicates air bubbles. Check the available head, feed capacity and the pump take-off against the manufacturer’s layout. A suitable anti-aeration connection can limit air entering the pump, but fitting one does not correct every supply fault. Appearance alone does not identify the entry route or rule out a separate water-quality problem.
- Brown water at several hot taps while the incoming cold tap is clear now: inspect stored water and hot pipework for rust or sediment, and ask about recent mains work. Clear cold makes an active whole-supply fault less likely, but earlier main deposits can remain in the hot store after the cold supply clears.