Periodic checks that keep each protection layer working, and how to respond to a discharge, a tripped cut-out or overheated water.
Water stored under pressure can be heated above 100 °C without boiling, and if the pressure is suddenly lost it flashes to steam with explosive force. The service checks that every protection layer still works, because relief valves, thermostats and the expansion provision can seize or deteriorate with no sign in normal use.
The service
- Ask about symptoms: water at the tundish or the outside discharge pipe, cut-out trips, or water hotter than usual.
- Inspect for leaks, corrosion, signs of past discharge and missing labels. Check the tundish is visible and unobstructed.
- Follow the maker’s isolation and safe service conditions. For an external vessel’s pre-charge check, isolate and fully depressurise its water side, then compare the gas pressure with the specified setting. Water pressure changes the gas-side reading, so never check the charge with the water side under pressure. An internal air gap has its own recharge procedure.
- Before you heat the cylinder or do functional tests, restore the supply, refill and vent it as the manufacturer’s procedure says. Check the operating thermostat controls the stored temperature.
- Check the non-self-resetting cut-out acts on the heat source. After a trip it stays off until someone deliberately resets it; cooling alone does not restart heating.
- Carry out the set relief-valve checks: the valve opens, discharges freely and reseats, while you control the hot-water hazard. Lifting it by hand does not test the pressure or temperature it is set to open at.
- Check D1, the visible tundish, D2 and the safe termination against the specified arrangement. Confirm the required sizes and fall and that no shut-off obstructs relief. Where a soil-stack route is used, check its specified mechanical odour seal and branch as well.
- Replace a safety device only with one that matches the cylinder's approval data. A valve can thread in and still have the wrong set point.
- Record the service. Tell the user not to adjust safety controls and to report any discharge.
If you find a fault
- Water in the tundish outside a test: stop heating and investigate. Lost expansion, rising pressure, a temperature-control fault or a damaged valve seat can all cause it.
- Water above its normal range, no discharge: stop heating and investigate. It can scald before the relief valve reaches its opening setting, and the lack of discharge does not tell you which layer failed.
- A cut-out the customer keeps resetting: repeated resets restart an unresolved fault. Find and correct the cause first.
- Any discharge: never cap, plug or valve off the route. It may be relieving danger.