Unvented hot water: layered safety and discharge

Note 6 of 10 · free to read

Walk an unvented cylinder from the cold inlet to the discharge: what each device does, and what water at the tundish tells you.

Schematic of an unvented hot-water cylinder with its pipework and eight parts marked by letter. A cold pipe, headed cold inlet safety group, runs through four brass fittings: A, with an angled branch and cap below it; B, with a grey domed top; C, with an arrow on its body; and E, with a small red cap on top. A red round vessel, D, hangs below the pipe between C and E. The pipe then drops and enters the bottom of the cylinder. On top of the cylinder, F is a brass valve with a red lever; its pipe runs across and down to G, an open funnel, and continues as H down to ground level, marked visible discharge to safe point. — Unvented cylinder: safety and functional layout

The illustrated domestic cylinder is unvented and fed from the mains. Its safety relies on several functions: control the incoming pressure, take up expansion, stop overheating, and make any relief discharge visible and safe. Read the figure along the water's path, from the cold inlet to the discharge. This is a component-identification diagram: the temperature-control wiring and the expansion relief valve’s discharge connection are not shown. Those connections must still follow the cylinder manufacturer’s complete arrangement.

1. The cold inlet group

The drawing shows a typical inlet-control arrangement, from the main: line strainer, pressure-reducing valve, check valve, then the expansion vessel and the expansion relief valve.

Line strainer: catches grit so the controls downstream are not damaged or held open.
Pressure-reducing valve: limits the cold inlet to the manufacturer’s specified operating pressure, which is distinct from the cylinder’s maximum pressure rating.
Check valve: holds heated, expanded water on the cylinder side, away from the cold main.
Expansion relief valve: opens if the pressure rises too far, for example when the vessel has lost its charge.

2. The expansion vessel

Water is virtually incompressible, so as it is heated in a sealed cylinder the extra volume has to go somewhere or the pressure rises. Because the check valve blocks the way back to the main, the vessel, or an internal air gap in some cylinders, takes up the extra volume as the water heats. It is the normal home for expansion; the relief valves are safety back-ups. Check the gas pre-charge with the vessel’s water side isolated and fully depressurised, following the service procedure. A reading affected by water pressure cannot replace that unloaded check.

3. The valve on the cylinder

The temperature and pressure relief valve is fitted on the cylinder itself. Its temperature function is the last safeguard against overheating; its pressure function responds separately if pressure reaches the valve’s opening setting.

The operating thermostat controls day-to-day temperature. If normal control fails, the independent cut-out must stop the heat input and remain off until deliberately reset after the cause has been corrected. Temperature relief provides a further safeguard if overheating continues. The pressure side of a combined T&P valve can operate without both temperature controls failing. A valve can also pass because its seat is damaged or dirty, so discharge alone does not identify an upstream failure.

4. Tundish and discharge pipe

D1 carries a discharge from the valve to the tundish, the open funnel that gives a visible air break. D2 carries it down by gravity to a safe termination. The tundish makes any discharge visible, including where a compliant final connection is concealed. It can carry very hot water and steam, so it is never an overflow or waste pipe.

D2 needs a continuous fall and a diameter selected for the valve outlet, the full route and its bends. An uphill or undersized run can make discharge back up. Suitable external arrangements include a trapped gully, a guarded low-level outlet or a specified high-level route. A compliant soil-stack connection can have a concealed final end; the tundish still provides the visible warning. Keep every discharge route safe and unobstructed, never capped.

What a discharge tells you

Water passing through the tundish shows discharge from the relief route. Outside an intentional test, stop heating and arrange competent investigation; never cap, block or hide it. The symptom alone does not identify the failed component.

Drips each time the cylinder reheats: check whether the expansion provision has enough usable capacity, including its charge or air gap. Also check regulated inlet pressure and valve operation before deciding what needs repair.
Weeping even when cold: compare the regulated pressure with the correct relief setting and check the valve seat. A reducer letting by, an incorrect valve or seat damage can produce this pattern.
The T&P valve passing outside a service test: stop heating and arrange a competent investigation. The flow alone does not identify the failed part.
A tripped cut-out the customer keeps resetting: another reset can restore heat while the cause is unresolved. On an immersion heater, isolate the circuit electrically, find and correct what caused the overheat, commonly a failed control thermostat, and only then reset it by hand.

Common exam traps

Calling the tundish an overflow instead of a visible safety-discharge point.
Treating the relief valve as normal expansion control.
Ignoring a dripping tundish because the cylinder still produces hot water.
Confusing the check valve, pressure-reducing valve and expansion relief valve.
Applying a vented-cylinder model to a sealed unvented system.

Go further

For the full article version, read the unvented cylinder diagram guide. For sizing practice, use the expansion vessel calculator.