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With the valve correctly installed into the plumbing system (400kPa), no terminal fittings are opened and the system is filled with water and the pressure control valve is in the closed position. When a terminal fitting is opened on the outlet side, the downstream of the pressure control valve can fill a bath or a washing machine.

The water pressure drops on the outlet side (A2) causing a pressure drop under the diaphragm. The regulating spring defeats the lower pressure and forces the valve to open, allowing water to flow through the regulating seat, regulating the income mains from 700kPa to 300kPa while the water flows through the valve.

When the terminal fitting is closed the water pressure under the diaphragm builds up forcing the regulating spring back and the regulating seat shuts off.

A pressure control valve basically uses two components to operate, they are:

The big, regulating spring to open the valve and to regulate the pressure through the valve. The diaphragm to close the valve.

Pressure relief valve
What is the purpose?

To protect the hot water cylinder from over pressurising when water is heated. SABS specification 198-1992 operating pressures between 380 to 400kPa (400kPa system).

What is their application to hot water cylinders?
When cold water is heated it expands. When water expands in a concealed container, it creates pressure. When this pressure rises above the working rate of the hot water cylinder = 400kPa the pressure relief valve will open to discharge the excessive pressure. When the pressure inside the cylinder drops to below 400kPa the valve will close automatically.

This whole process is controlled by the thermostat that regulates the electric power to the element by switching on at low temperature and off when the water reaches the set temperature.

How does it work?
The incoming water mains are regulated by the pressure control valve in 400kPa system at 300kPa. This is the set pressure of the pressure control valve for a 400kPa hot water cylinder system. The pressure relief valve is set at 400kPa allowing a pressure gap of 100kPa between the set pressure of the pressure control valve (300kPa) and set pressure of the relief valve.

When the pressure inside the hot water cylinder rises, due to the heating process, the relief valve will open up when the pressure reaches 400kPa.

The expanded water is allowed to discharge to atmosphere preventing the water pressure from exceeding the system pressure.

A pressure relief valve must always be installed between the pressure control valve and the hot water cylinder. No non-return or stop valve must be installed between the hot water cylinder and pressure relief valve.

What does a pressure relief valve consist of?
The pressure relief valve consists of a forged DZR brass body with a stainless steel seat and a regulating spring with a seat washer combined into a cartridge. In the case of the pressure relief valve, the seat washer acts as the diaphragm that closes the valve. The Kwikhot pressure relief valve also incorporates a flushing mechanism for easy maintenance and a stainless steel seat for long life.

Vacuum relief valve (vacuum breakers)

What is the purpose?

To access air into the plumbing system when the water pressure drops below atmospheric temperature. SABS specification 198-1992 flowrate of 100 standard litres a minute. The vacuum does not exceed 9.0kPa negative.

What is their application to hot water cylinder installations?

Cold side
A vacuum relief valve must always be fitted on a proper anti-siphon loop between the pressure control valve and the hot water cylinder. It will prevent back-flow (siphonage) from the hot water cylinder, when the inlet pressure drops below hot water cylinder pressure. By access air into the system, the siphon could break, which:
  • Could drop the water level partly in the cylinder to the level of hot water cylinder inlet exposing the element and thermostat. This will cause the element to burn out, because the element and thermostat are not immersed in the water.
  • Could drop the water level partly in the cylinder leaving the T&P valve’s (safety) probe not immersed in the water, which could cause a potential dangerous situation as the T&P valve will lag to open under extreme temperatures if the thermostat goes faulty. this could cause a flash steam explosion.
  • Cause hot water to backflow and come out of the cold water fittings plumbed upstream of the hot water cylinder. This could result in scalding water users at cold taps.
  • It sill prevent siphonage from hand showers lying in baths and washing machines, which could contaminate drinking water

Hot Side
  • It could prevent cylinder implosion when the pressure drops below atmospheric, eg, the draw off of water exceeds the supply and by draining a cylinder.
  • Prevents inter floor siphonage, where the geyser is serving more than one floor

Pressure control valve
Pressure reducing valves are of the pressure balanced type, which means that the set pressure (outlet) is unaffected by changes in the supply pressure (inlet). The set pressure to the hot water cylinder will remain constant.

Pressure control valve to be plumbed with the arrow on the body in the direction of water flow

Pressure control valve must be of the same pressure rating as the hot water cylinder.

100kPa hot water cylinder = 100kPa pressure control valve (label colour – blue)
200kPa hot water cylinder = 200kPa pressure control valve (label colour – black)
400kPa hot water cylinder = 400kPa pressure control valve (label colour – red)
600kPa hot water cylinder = 600kPa pressure control valve (label colour – green)

Distance between the pressure control valve and hot water cylinder pipe length:

100kPa system = 1m
200kPa system = 3m
400kPa system = 15m
600kPa system = 30m

Expansion relief valve
Expansion relief valve drain to be plumbed to a suitable visible place of discharge, not to be a potential nuisance and not into drip tray
Not to be interconnected with the safety valve, drain pipe
Must be piped at a decline
Must be protected against frost
Normal expansion is + 1,5% of volume of water heated

Safety valve
Safety valve drain pipe to be plumbed to a suitable place of discharge
Safety valve drain pipe never to be interconnected with expansion relief valve drainpipe
Safety valve drain pipe never to be plumbed into pvc sever or waste pipes
Safety valve drain pipe always to be metallic and
a minimum diameter of 20mm. Never use polypropylene or any other plastic pipe.

Vacuum breaker
To prevent hot water cylinders from collapsing and syphonage, vacuum breakers must be fitted on inlet and outlet of a hot water cylinder. At a minimum of 300mm higher than the top of the hot water cylinder.

The connection for cold water supply to the geyser also allows for the hot water cylinder to be drained.

Drip Tray
SABS specification 151 and SABS 0254, code of conduct call for the use of a drip tray. Kwikot highly recommend the installation thereof. All valves to be installed above drip tray.

General Requirements
all pipes must be flushed before commissioning of the system
all valves and terminal fittings to be removed prior to flushing
an inline strainer is to be fitted, where water quality cannot be guaranteed. The strainer cartridge should be removed before the pipes are flushed
do not install a stopcock or non-return valve between the hot water cylinder and the pressure-reducing valve.
Do not install a stopcock or non-return valve between the hot water cylinder and the vacuum breaker on the hot or cold side.
All valves must be plumbed in a serviceable position. Allow a minimum clearance of 100mm for removal of parts

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