Unvented cylinder safety devices: relief valve, tundish and discharge

Note 12 of 16 · free to read

The independent safety layers on an unvented cylinder, how their discharge is routed and terminated, and what a drip from it usually means.

Key points

An unvented cylinder relies on independent, layered safeguards: the control thermostat normally, a non-self-resetting cut-out if that fails, then the temperature and pressure relief valve as the last resort.
The relief valve discharges via a tundish, which gives a visible air break, then a discharge pipe that must fall continuously and stay correctly sized so it can never be restricted.
The discharge must terminate somewhere safe and visible, such as a trapped gully, a guarded low-level outlet or a suitable high-level arrangement, and must never be capped.
Discharge appearing only on heat-up usually means the expansion vessel has lost its air charge; a continual drip points to the pressure-reducing valve or a damaged valve seat.
A tripped immersion thermal cut-out means isolate, diagnose and fix the cause, then reset by hand – not simply reset and hope.

Layered, independent safeguards

Water is virtually incompressible, so as it is heated in a sealed cylinder it expands, and that extra volume has to go somewhere or the pressure rises. An unvented cylinder controls this with independent layers acting in sequence rather than one control doing everything. In normal use, the control thermostat sets and holds the temperature. If it fails, a separate, non-self-resetting cut-out disconnects the heat source once the water overheats, and has to be reset by hand once the fault is found and fixed. Only if both of those fail does the temperature and pressure relief valve open and discharge water, as the last resort. Because each layer is independent, one failure cannot defeat the others, and a relief valve that discharges routinely, rather than only during a genuine fault, is telling you something further up the chain has already failed.

Getting the discharge away safely

The relief valve's discharge runs first to a tundish, which gives a visible, physical air break so anyone can see when a valve has operated, then on via a discharge pipe that must fall continuously all the way to where it terminates, never rise or narrow. Running it uphill or undersized can restrict that path so the valve cannot discharge freely exactly when it needs to. The discharge must end somewhere safe for anyone nearby and still visible, such as a trapped gully, a guarded low-level outlet, or a suitable high-level arrangement; it must never be hidden away or capped.

Reading a discharge

Water dripping from that pipe is a fault signal, not something to ignore or cap. Discharge that appears only when the cylinder is heating usually means the expansion vessel has lost its air charge and can no longer absorb the water's expansion; a valve that drips continually, even when the cylinder is not heating, points instead to the inlet pressure-reducing valve letting by or a damaged valve seat. Either way, get it investigated rather than resetting or capping anything. The same principle applies to an immersion heater's own thermal cut-out: if its overheat button has tripped, isolate the circuit electrically, find and fix what caused the overheat (commonly a failed control thermostat), and only then reset it; resetting it repeatedly without diagnosis leaves a real fault in place.