Why an unvented cylinder needs independent layers of overheat protection, how each layer behaves, and how outlet temperatures are kept safe.
The hazard
A typical domestic unvented cylinder takes its cold supply from the mains and has no open vent. The wider definition also permits a dedicated storage-cistern supply; the safety arrangement must suit the actual design. Water stored under pressure can be heated above 100 °C without boiling. If the controls failed and the pressure were suddenly lost, it would flash to steam with explosive force. The protection is built in independent layers so that one failure never leads straight to that outcome.
Layer 1: control thermostat
The control (operating) thermostat manages the water temperature in normal use, switching the heat source off at the set temperature. It is the everyday regulator. A timer or programmer only decides when heating runs, not how hot the water gets.
Layer 2: non-self-resetting cut-out
If the thermostat fails and the water keeps heating, a separate high-limit thermostat, the energy cut-out, shuts off the heat source. It latches off: cooling alone does not restart heating, and someone has to reset it deliberately. That stops the system quietly cycling on its safety limit and makes the overheat obvious. Find and correct the cause before the system is used again; pressing reset does not prove that has happened.
The cut-out only protects if it stops heat being added. On a boiler coil it closes the motorised valve through its wiring; on an immersion heater it switches the element off. Relying on the relief valve instead would let the water keep heating. Every heat source that can overheat the water needs its own thermostat and its own cut-out, so an immersion heater incorporates a control thermostat plus a separate non-self-resetting thermal cut-out.
Layer 3: temperature relief valve
If both thermostats fail, the temperature relief valve, fitted on the cylinder as a temperature and pressure (T&P) relief valve, is the final safeguard. Typical temperature-relief operation is in the 90–95 °C band. Check the actual valve’s specified setting and tolerance; it releases hot water that the cold supply replenishes. A combined T&P valve also opens on excess pressure, which need not involve a failure of both thermostats. Normal expansion still belongs in the expansion provision, not routine relief discharge.
Why independence matters
If the safety devices depended on the control thermostat, one fault could remove control and protection together. Each layer is a different, independent device, so a single failure does not remove all protection. Duplicating the thermostat, or fitting a self-resetting cut-out, does not give that independence.
Reading the diagram
The figure puts the cylinder at the centre with four numbered labels. Three are temperature layers and one is not. The diagram identifies functions; it is not a complete installation or wiring plan.
- 1, control thermostat: holds the stored water at its normal temperature in everyday use.
- 2, high-limit cut-out: cuts the energy supply if the thermostat fails and the water keeps heating, and stays off until a deliberate reset.
- 3, expansion provision: the expansion vessel at the bottom left, teed off the cold feed, or an internal air gap in some cylinders, takes up the extra volume in normal use. It is not a temperature layer, and discharge each time the cylinder reheats means it needs checking.
- 4, T&P relief valve: fitted on the cylinder itself, the final layer. Its discharge runs across to the tundish, an open funnel that gives a visible air break, then down to a safe, visible point.
The protection order is 1, 2, 4: thermostat, cut-out, relief valve. The relief valve backs up the cut-out; it never replaces it. The figure shows a low-level end to the discharge; other permitted arrangements have their own conditions. Whatever the arrangement, a discharge must be noticed and must not scald anyone. Unexpected water at the tundish is a warning to investigate, while a competent installer deliberately passes water during the prescribed relief-valve service test.
Protection at the taps
The layers above stop the stored water overheating. Scalding at the outlets is controlled separately: set a safe storage temperature on the control thermostat, hot enough to control Legionella, and fit thermostatic mixing valves where vulnerable users are at risk. Where stored water can exceed 80 °C, as with some heat stores and solid-fuel systems, an in-line tempering valve at the vessel outlet keeps the supply to the distribution pipework at no more than 60 °C. In new dwellings, bath outlets are limited to 48 °C by a blending valve.