Choosing Your Water Heater Size in Singapore

Method 1: Estimate Your Usage

Determine if your family is high or low demand. If your family is ‘high demand’, consider moving up to the next level on the quick sizing chart.

Low Demand High Demand
  • No back to back showers
  • Low flow rate showers
    (< 10 lit/min)
  • No bathtubs
  • Back to back, concurrent showers
  • High flow rate showers  (> 10 lit/min)
  • Bathtubs
  • Teenagers use more hot water
Quick sizing chart
Family Size Tank Size
Kitchen only

1 person

2 people

3 people

4+ people

15 lit

15 – 30 lit

30 – 40 lit

40 – 50 lit

80+ lit

Method 2: Estimate by Duration of Usage

Use table below to calculate number of minutes of running 38℃ water you will get with different variables.

Heater Capacity vs. shower flow rate
Regular 100mm 200mm 300mm Waterfall
Heater Capacity Regular
7 lit/min
100mm
10 lit/min
200mm
12 lit/min
300mm
18 lit/min
Waterfall
24 lit/min
30 lit heater 8 6 5 3 2
40 lit heater 11 8 6 4 3
50 lit heater 14 10 8 5 4
80 lit heater 22 16 13 9 7
100 lit heater 28 20 16 11 8

Method 3: Calculate your peak demand

To size a storage water heater for your home more accurately, match the water heater’s First Hour Rating with your Peak Hour Demand.

Step 1: Estimate your Peak Hour Demand

  • Determine what time of day (morning, noon, evening) you use the most hot water in your home. Keep in mind the number of people living in your home.
  • Use the equation below to estimate your maximum usage of hot water during this hour of the day – this is your Peak Hour Demand.

 

Peak Hour Demand
= (Showerhead flow rate, lit/min) x (avg. time of running water/shower, min) x (no. of showers within one hour)

Example:

Linda’s family of 4 uses the most hot water in the morning, just before everyone sets off to school or work. They use a small 100mm rainshower of flow rate 12 lit/min. The water is running for about 4 min each shower.

Linda’s family’s Peak hour demand
= (Showerhead flow rate, lit/min) x (avg. time of running water/shower, min) x (no. of showers within one hour)
= (7 lit/min) x (4 min) x (4)
= 112 lit

Step 2: Estimate your First Hour Rating (FHR)

The First Hour Rating is the number of litres of hot water the heater can supply per hour (starting with a tank full of hot water).

Electric/heat pump   = (litre capacity) x 2
Digital with MAX x 4 = (litre capacity) x 4
Digital with MAX x 6 = (litre capacity) x 6

Example:

30 lit horizontal electric water heater FHR
= 30 x 2
= 60

40 lit horizontal digital (MAX4) water heater FHR
= 40 x 4
= 160

40 lit horizontal digital (MAX4) water heater FHR
= 40 x 4
= 160

Step 3: Find the right water heater for your family

Your Peak Hour Demand value should be less than your water heater’s First Hour Rating.

Example:

Sam’s peak hour demand is around 150 litres. He is considering the following electric heaters.

Option A

40 lit horizontal electric water heater FHR
= 40 x 2
= 80

Option B

80 lit horizontal electric (MAX4) water heater FHR
= 80 x 2
= 160

Option C

40 lit horizontal digital (MAX4) water heater FHR
= 40 x 4
= 160

Answer: Sam’s peak hour demand is 150 litres.
Option A is too small (80)
Option B (160) and Option C (160)  are greater than 150 liters and are suitable for his family.

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