Closed-system heatingJuly 1, 2014
A mains pressure system does not have an open vent. It is a closed system. Naturally, when we heat water in a closed system, the water expands and exerts more pressure on the system. If this process were allowed to continue uncontrolled, the system would yield under pressure, so a pressure relief valve is fitted to the cylinder to restrict maximum
The cylinder is designed to hold a known volume of water at a known mains pressure. The rise in temperature caused by heating the water is also known and, from that, we know the increase in pressure due to thermal expansion. This is the pressure at which the relief valve opens. Remember, the cylinder is tested at 2.5 times this pressure, so the safety margin is very large.
While water is being heated in the cylinder, it is expanding and increasing pressure inside the cylinder. The relief valve, therefore, opens during heating and allows the expansion water to escape through a relief valve drain pipe, which discharges through an open end over a gully trap. Once the water is heated and the burner shuts off, expansion stops and the relief valve closes.
The relief valve also contains a thermal element that expands when heated above normal water temperature and opens the relief valve, regardless of pressure. Hence the name temperature and pressure relief valve (TPRV). The thermal element may open the valve anywhere from 93°C but never at more than 99°C.
The TPRV also contains a vacuum break device that relieves partial vacuum in the relief valve drain pipe by allowing air to be sucked in. This device will collapse and blow out if back pressure occurs due to a restriction in the relief pipe. Once the vacuum break is blown, a pin protrudes from the TPRV and cannot be reset. You must clear the relief pipe and replace the TPRV plumbing by a plumber.
A manually operated easing gear lever allows you to open the relief valve to flush out any grit or sediment that may settle on the seating and prevent the valve from working.