Class is in – hot water 101
Since the mid-1940s Australia has utilised mains pressure electric storage water heaters for use in domestic and commercial applications, Jon Palfrey explains the best practices of installation and commissioning.
This article explores the electric water heater – its operation, function, installation, and commissioning, to a system that has been an industry mainstay for many years particularly in non-reticulated natural gas areas.
Electric water heaters are designed to operate at mains pressure by connecting directly to the mains water supply. If the mains supply pressure in the area exceeds the unit’s cold water supply parameters, a pressure limiting valve must be ﬁtted. The supply pressure should be greater than 350 kPa for true mains pressure performance to be achieved.
Each electric storage water heater is ﬁtted with at least one tubular single phase immersion element located near the bottom of the tank, controlled by a contact thermostat and over-temperature energy cut out mechanism.
The elements are attached to the cylinder by a mounting plate and sealed with a rubber gasket and four screws.
The tube of the element sheath is made from copper. A resistor is connected between the sheath and the mounting plate to produce a resistance earthed element, reducing unwanted dissipation of the protective anode.
Single Element Models
This type of water heater has one element with its own thermostat, located at the base of the water heater. It can be connected to an Oﬀ -Peak (overnight) or time controlled supply, Extended Oﬀ -Peak (overnight and day) or Extended Time controlled supply, or a Continuous/Domestic electricity supply.
Twin Element Models
These have two separate elements, each with its own thermostat and over-temperature energy cut out mechanism. One is located near the base of the cylinder and designed to heat the whole cylinder.
The other element is located part way up the tank to serve as a “booster” to heat only the upper portion of the contents of the cylinder in conjunction with the main element circuit.
During normal operation only the bottom element supplies all the hot water. During periods of high demand the top (booster) element operates, to provide an additional supply of heated water.
For many electric hot water heaters, the temperature can only be adjusted by a licensed tradesperson via an adjustable thermostat.
The thermostat (bottom thermostat on twin element models) has a maximum temperature setting of 75°C (80°C for 50 litre stainless steel models) and a minimum temperature setting of 60°C. The top thermostat on a twin element model has a ﬁxed setting of 60°C and cannot be adjusted. Only adjust the bottom thermostat setting when the isolating switch is switched oﬀ at the switchboard.
WATER HEATER LOCATION
Electric mains pressure storage water heaters are suitable for either indoor or outdoor installation (power cord and plug models are suitable for indoor installation only). Key considerations for choosing the location for the heater are:
- Install as close as possible to the most frequently used outlet (with safety and service in mind)
- The water heater is accessible without the use of a ladder or scaﬀold.
- The temperature pressure relief valve lever is accessible and the front cover, thermostat and element can be removed for service.
- You must be able to read the rating plate.
- Remember you may have to take the entire water heater out later for servicing.
- The water heater must stand vertically upright on a stable base
- The base is made of corrosion resistant steel, so can be placed on the surface. It is not necessary to allow for free air circulation under the base of the water heater.
- For internal installations, the unit must be placed on a safe tray and drainage available.
HOT WATER DELIVERY
The water heater can deliver water at temperatures which can cause scalding.
It may be required by regulations (and where recommended) that a temperature limiting device be ﬁtted before the hot water outlets in any ablution area such as a bathroom or ensuite to reduce the risk of scalding.
Where a temperature limiting device is installed adjacent to the water heater, the cold water line to the temperature limiting device can be branched oﬀ the cold water line either before or after the isolation valve, pressure limiting valve and non-return valve to the water heater. If an expansion control valve is required, it must always be installed after the non-return valve and be the last valve prior to the water heater.
If a pressure limiting valve is installed on the cold water line to the water heater and the cold water line to a temperature limiting device branches oﬀ before this valve or from another cold water line in the premises, then a pressure limiting valve of an equal pressure setting may be required prior to the temperature limiting device.
If the hot water outlet is located on top of the water heater an external heat trap may need to be installed to ensure against convection and heat losses. It must be within 1m of the hot water outlet, before the ﬁrst hot water branch and have a vertical drop of 250 mm from the outlet.
Therefore if the outlet connection is located at the top of the unit and the direction of supply to ﬁxtures is vertical, a heat trap extending 250mm must be plumbed.
To achieve true mains pressure operation, the cold water line to the water heater should be the same size or bigger than the hot water line from the water heater.
Pipe sizing should be carried out by persons competent to do so, with reference to the technical speciﬁcations of the water heater and local regulatory authority requirements.
When hot water is released from a water heater, cold water comes in to replace it. The amount of cold water that can be brought up to temperature per hour is the heater’s recovery rating. The recovery rating is calculated based on the known properties of water and assumes no heat loss during recovery – that it is 100% eﬃcient.
Once installed, the power supply must not be switched on until the water heat is ﬁlled with water and a satisfactory megger reading is available.
To do that,
- Ensure all the hot water taps in the house are open;
- Open the cold water isolation valve fully on the cold water line to the water heater. Air will be forced out of the taps.
- Close each tap as water ﬂows freely from it
- Check the pipe work for leaks
- Switch on the electrical supply at the isolating switch to the water heater.
Explain to the householder the functions and operation of the water heater. Upon completion of the installation and commissioning, leave the installation instructions/owners guide with the householder.
MAJOR SERVICE EVERY FIVE YEARS
It is recommended a major service be conducted on an electric water heater every ﬁve years. Only genuine replacement parts should be used on the water heater.
The major service includes the following actions:
- Replace the temperature pressure relief valve.
- Inspect and ﬂush the expansion control valve (if ﬁtted). If required, replace the valve.
- Check the element for excessive calcium build up or corrosion and replace if necessary.
- Visually check the unit for any potential problems.
- Inspect all connections.
- Check the drain line from the safe tray (if one is installed) is not blocked.