Vented versus unvented hot water systems

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.

A vented copper cylinder must also suit the head of water above it. Copper Industries makes its cylinders in the three grades described in BS 1566, set by the thickness of the copper: Grade 3 for a maximum working head of 10 m, Grade 2 for 15 m, and Grade 1 for up to 25 m. It recommends Grade 2 for most vented installations. Not every maker grades cylinders the same way, so check the cylinder's own data.

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. The guidance to the Water Fittings Regulations asks for a secondary (hot water) vent pipe of at least 19 mm internal diameter, insulated against freezing, rising high enough above the cistern's top water level that it does not discharge in normal use. A heating (primary) vent must not terminate over a cistern storing wholesome water for the taps or the hot water, and a hot water vent must not terminate over the heating's feed and expansion cistern. Water in a house's heating circuit is a fluid category 3 risk, so a heating vent discharging over the storage cistern could contaminate wholesome water. 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 set, not at a loft cistern's gravity head. Flow at the outlets still depends on the incoming supply and the pipework, so assess them, and use the right connections to balance the hot and cold supplies. "Unvented" means no open vent; some designs are fed from a dedicated cistern instead.

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. The guidance to the Water Fittings Regulations puts the tundish in the same room or space as the valve, vertically as close to it as possible and never more than 500 mm from the valve's outlet; below the tundish the pipe drops vertically for 300 mm before it falls away, and it is made of a non-ferrous metal such as copper. Schedule 2 of the Regulations sets the limit all of these devices work to: the water in a secondary hot water system must never go above 100 °C, because water held under pressure above that can flash into steam when the pressure is released. The expansion valve sits on the cold supply close to the cylinder, with no valve between them. Under the Water Fittings Regulations it may open only at a pressure at least 0.5 bar above the cylinder's normal working pressure, so it stays shut in normal use and lets water go only when the expansion vessel or the system has failed; the temperature and pressure relief valve is set higher again.

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. Smaller storage is exempt only if any associated electrical work is not notifiable either, and the exemption never removes the need for safety controls.

Interpreting discharge during heat-up

First identify which valve is discharging and how the cylinder takes up expansion. With an external diaphragm vessel, its gas cushion takes the extra water volume; a low charge, a failed vessel or a blocked connection leaves it unable to. Other cylinders use an internal air volume instead.

If the expansion relief valve discharges now and then during an otherwise normal heat-up, check the expansion provision first. If it runs continuously, check the inlet pressure controls and the valve seat as well. The pattern tells you where to look; it does not point to one failed part. Very hot discharge from the temperature protection needs urgent action, following the cylinder instructions. Keep every discharge route clear 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 shows you where to start; the shared parts could still have a fault too.
• Hot water that used to last now runs cool early: check the cylinder control and how much of the store is actually being heated. If only an upper boost heater works, a small hot layer at the top can hide a fault in heating the whole store. If it has always run short, or the household now uses more, compare the draw-off with the cylinder's capacity and recovery instead.
• A pumped hot outlet looks milky: water that clears from the bottom of the glass upwards has air bubbles in it. Check the available head, the feed capacity and the pump take-off against the manufacturer’s layout. A suitable anti-aeration connection can stop air reaching the pump, but it will not fix every supply fault. How the water looks does not tell you where the air gets in, or rule out a separate water-quality problem.
• Brown water at several hot taps while the cold is now clear: inspect the stored water and hot pipework for rust or sediment, and ask about recent work on the main. Clear cold water makes a fault in the whole supply less likely, but sediment from the main can stay in the hot store after the cold has cleared.