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Both instant and storage water heaters generate hot waters, but their attributes and features differ.

There are 3 types of instant electric water heaters: mechanical, electronic and digital. The mechanical kind is the least expensive and features step-less control. Electronic type lets you control the power setting while digital type comes with a temperature display panel besides having power setting control. The instant water heater operates with a 3-3.3kw element and generally works for individual supply points, therefore if you have 2 bathrooms, you will need 2 instant water heaters. Unlike storage water heater, instant water heater does not require pre-heating before using as it generates hot water instantly. The water temperature can vary according to the water pressure. For example, if water pressure is low, the water temperature will go up and vice versa. The average life span for an instant water heater is around 3 to 5 years.

A storage water heater has an inner tank for storage of water. The capacities range from 15 litres to 100 litres. The inner tank can be made with stainless steel or copper. It operates with a 1.5kw to 3kw element and can supply hot water to more than one point (multi-point system), which means that it can supply hot water to more than 1 bathroom as well as kitchen simultaneously. A mixer is used to regulate and provide constant water temperature so it is not affected by the change in water pressure. However, the water stored need to be pre-heated before used. To save energy and prevent loss of heat, insulation is placed between the storage tank and outer casing of the heater. A storage water heater also comes with an automatic thermostat to cut off the power supply when it reaches the preset temperature. Therefore the additional electricity consumption for switching the heater on the whole day is negligible. The average life span for a storage water heater is around 5 to 7 years.

Formula for calculation of electricity consumption:

Electricity cost = Power (1.5kw or 2.5kw) x Usage Time (hr) x PUB Electricity Rate

The following is a simple illustration of the consumption of 2 types of heaters on the basis of 2 users.

User 1 & 2 - uses 5 minutes of hot water each
User 3, 4 & 5 - uses 3 minutes of hot water each
Total: 19 minutes hot water required
Electricity rate - S$0.1656

Electricity Consumption:
Instant water heater : 3kw x 19/60 x S$0.1656 = S$0.16
Storage water heater : 1.5kw x 50/60 x S$0.1656 = S$0.21

The above calculation does not take into consideration of other factors such as capacity of water heater, frequency of bath per user and original temperature of water in the tank.

For instant water heater, the recovery of hot water is instant.

The recovery rate for storage water heater varies according to the power of the heater.

Initial recovery rate:
a) Inlet temperature = 25 deg C or 77 deg F
b) Bathing temperature = 42 deg C or 108 deg F

Recovery rate gallon/hour (Small capacity - 1.5kw )

= Andris/Ti Shape (1500) / 2.42x (108 - 77)
= 20 gallon / hour
= 1.3 litres / min

Recovery rate gallon/hour (Bigger capacity - 2.5kw)

= Inox/PRO R/PRO R Slim (2500) / 2.42 x (108 - 77)
= 33 gallon / hour
= 2.1 litres / min

The following is a simple illustration on how to size and select a proper water heater.


  1. Normal bathing temperature = 40 deg C.
  2. Cold water inlet temperature = 25 deg C.
  3. TUV/SUD/PSB maximum allowed temperature = 77 deg C.
  4. Ariston maximum temperature setting = 70 deg C.
  5. US heater maximum temperature setting = 60 deg C.

Calculation of % of hot water required when mix with cold water:

[(mixed water) - (cold water)] / [(hot water) - (cold water)]
= [40 - 25] / [70 - 25]
= 15 / 45
= 33% of mixture is hot water (for Ariston heater)

Amount of mixed water produced at temperature 40ºC:

Ariston ( 30 litres ):

33/30 = 100/x
x = 91 litres

Ariston ( 50 litres ):

33/50 = 100/x
x = 152 litres

From the sizing program, we understand that the amount of the warm water (40 deg C) produced for bathing is not only depended on the capacity of the tank but also the pre-set temperature.

The below table may be used as a reference guide in determining a proper storage water heater for your home:



Inox AA-25, Ti-Shape 15, Andris R15

Shower & basin

PRO R 50H, PRO R 50H (L), Ti-Shape 30, Andris R 30, PRO R S 30, PRO R S 40, Inox AA-40, Inox AA-50

Bathtub & basin

PRO R 50H, PRO R 50H (L), PRO R 80H, PRO R 80H (L), Inox AA-50, Inox AA-65, Inox AA-80, PRO R S 56, PRO R S 80

Corner bathtub & basin

Inox AA-80, PRO R 80H, PRO R 80H (L), PRO R S 80, PRO R S 100

Our steel tank is tested to 13 bars and the pressure relief valve is set at 6.5 bars according to Code of Practice SS 146. This is a main pressure heater; therefore, there is no specific requirement on the minimum working pressure as long as the inlet pressure is able to meet the end requirement.

The minimum water pressure for an instant electric water heater is 0.25 bars and the maximum is 6 bars.

There are three safety features:

  1. Thermostat

    First level safety feature. It controls the temperature of the water inside the tank in normal operation (maximum at 60 Deg C) - It can be manually adjusted by user.

  2. Thermal cut-out

    Second level safety features. If the thermostat fails to work properly, the temperature of the water can rise and generate steam. In order to avoid this, thermal cut-out serves as an independent safety device to cut off the power. When a fault or failure increases the temperature of the tank excessively (over 85'C), the thermal cut-out interrupts the current to the heating element. Therefore, it is also known as over temperature cut off. However, manual intervention is required for resetting the power. It is however advisable to identify the cause of the problem before resetting the power of the water heater.

  3. Pressure relief valve

    The pressure relief valve used on our Ariston water heater is rated 65±10N/cm2. In other words, when the pressure in the tank is more than 6.5 bars (max 7.5 bars), the stainless steel spring in the valve directing the cold water into the water tank will be depressed in the opposite direction as the water supply. This forces the water in the tank to be released through a small diameter 4.5mm outlet found on one side of the same valve, thereby reducing the water pressure within the tank to 6.5 bars or below. The valve will remain open allowing cold water flowing into the tank to be discharged through the valve until the water pressure returns to normal.

Routine preventive maintenance of water heater can provide you many years of trouble-free heater. There are several points which you should take note:

  1. Keep free from gasoline or other flammable or combustible materials around the vicinity area of the water heater.

  2. It is not uncommon for the water heaters to generate hissing noise when in operation, even for the new heater. However, you should contact your professional plumber or our service center should the hissing or the noise level increases substantially as this noise may be caused by the heating element which required cleaning.

  3. The purpose of magnesium anode is to neutralize stray galvanic currents, thus preventing corrosion. Therefore, it is advisable to replace the magnesium anode in the Andris series, Ti-Shape series, PRO R Slim series, PRO R SIN series and Inox AA series regularly, e.g. bi-yearly. Please call our service center for providing this service with charge.

Refer to troubleshooting table under the customer service page.

  • To reduce risk of electric shock, do not attempt to repair your water heater appliance by yourself.
  • No user-serviceable parts are included with the product.
  • Keep out of reach of children.

Refer servicing to a qualified service center.