Mains Pressure Hot Water Service

The thermostat acts as a thermometer and controls the water temperature inside the water storage tank. Power or gas  is only used when heating is required, meaning the thermostat is fully automatic.

In electric hot water cylinders one, sometimes two, immersion elements are located within the storage tank, these create the heat required to heat the water. In gas systems, the burner is located at the bottom of the cylinder with a flue running through the centre of the tank to exhaust toxic emissions.

Some tanks have two elements controlled by separate thermostats, only heating the water when the water temperature goes below a predetermined temperature. The temperature-pressure relief valve exists to protect the hot water cylinder from excess pressures and temperatures, avoiding major damage to the cylinder or your home. The anode rod protects the cylinder from corrosion. The anode rod is suspended in the tank to reduce corrosion over time and prolong the lifespan of the cylinder. The drain valve is used to drain water out if the cylinder should repairs be required. 

Cold water will enter the cylinder storage tank when hot water exits, this way the water temperature in the bottom of the cylinder storage tank is colder than the water at the top.

This triggers the thermostat, allows the element to activate and heat the temperature to the set temperature again. This process repeats 24/7 to ensure hot water is always available. Gas hot water cylinder work in a very similar way, the only difference being they burn gas to heat water at the bottom of the tank and do not have an element in the water. 

Mains pressure, open vented, storage water heater supply systems supply mains pressure hot water to all outlets; hot and cold water is delivered to outlets at the same pressure. These systems have an internal or external expansion vessel that allows the heated water to expand, the system must also include a pressure relief valve in the event the expansion vessel fails.

All mains pressure hot water cylinders (>120 kPa) require this pressure relief valve to be both temperature and pressure operated. If a mains pressure unvented system is used, valves must be specified for the particular system to achieve the required pressure rating. The problem with only having a single valve for relieving pressure is that hot water is wasted as the hot water expands during the frequent heat-up cycles.

To minimise wasting hot water, an expansion control valve, set at slightly less pressure, is installed on the inlet side of the cylinder and will relieve cold water during the heat up-cycle. The expansion control valve can normally drip up to 5 litres of water per day. Both valves must have the appropriately-sizes copper drain line that discharges to a safe and visible location. 

The components of a mains pressure hot water cylinder are the storage tank, thermostat, element (electric system) or the burner (in the case of gas systems), temperature relief valve, anode rod and drain valve.

The storage tank will usually have an inner steel cylinder which is lined with glass or vitreous enamel, insulation and a steel outer lining. Vitreous enamel is a thin layer of glass fused to the surface of the metal at a very high temperature.

This high pressure hot water cylinder is becoming much more popular in New Zealand because of its seamless ability to provide hot water to a large household, without any disruptions.

Older electric hot water cylinders are often poorly insulated, which will lead to heat loss. It is worth wrapping the cylinder with insulation if the electric hot water system is less than “A” grade.

Gas hot water storage systems have a faster heat recovery time than a comparable electric hot water cylinder. Gas cylinders need to be installed in a well-ventilated area and flued to remove exhaust gases. This leads to long pipe runs and means that gas hot cylinder have higher heat losses than electric hot water cylinders. It is also not safe to wrap a gas hot water cylinder. 

For an in depth analysis of which hot water cylinder is better, please read our article here.