Spa & Pool Heater Sizing - Temperature Rise and Minimum Requirements

There are 2 very important factors when sizing heaters for pools and spas.
Heating a spa or pool within a 'convenient' timeframe is key.

1. How quickly the body of water will heat, and
2. The minimum sized heater required to achieve the desired temperature.


How quickly does a spa or swimming pool heat?

The temperature rise created by a heater is equivalent to 1 degree celcius per kilowatt for every 853 litres of water.

The calculation is:

Temperature rise in degrees C per hour = (kW x 3412) divided by (Volume in litres x 4) 

Examples:

853 litres - 

1.5kw heater - 1.5 degrees C per hour
2kw heater - 2 degrees C per hour
6kw heater - 6 degrees C per hour
24kw heater - 24 degrees C per hour
54kw heater - 54 degrees C per hour 

For 853 litres you'll get 1 degree C temp rise per kilowatt per hour.

1000 litres - 

1.5kw heater - 1.3 degrees C per hour
2kw heater - 1.7 degrees C per hour
6kw heater - 5.1 degrees C per hour
24kw heater - 20.5 degrees C per hour
54kw heater - 46 degrees C per hour 

For every 1000 litres you'll get 0.85 degree C temp rise per kilowatt per hour. 
With a desired temperature rise of 15 degrees C from 22C to 37C, it would take a 2kw heater 8.8 hours, and a 24kw heater 44 minutes

1200 litre spa - 

1.5kw heater - 1 degrees C per hour
2kw heater - 1.4 degrees C per hour
6kw heater - 4.3 degrees C per hour
24kw heater - 17 degrees C per hour
54kw heater - 38.4 degrees C per hour 

1500 litre spa - 

1.5kw heater - 0.85 degrees C per hour
2kw heater - 1.14 degrees C per hour
6kw heater - 3.4 degrees C per hour
24kw heater - 13.65 degrees C per hour
54kw heater - 30.7 degrees C per hour

With a desired temperature rise of 15 degrees C from 22C to 37C, it would take a 2kw heater 13.2 hours, and a 24kw heater 1.1 hours

6000 litre swimspa - 

4kw heater - 0.57 degrees C per hour
6kw heater - 0.85 degrees C per hour
24kw heater - 3.4 degrees C per hour
54kw heater - 7.7 degrees C per hour 

With a desired temperature rise of 10 degrees C from 16C to 26C, it would take a 6kw heater 12 hours, and a 24kw heater 3 hours

40,000 litre swimming pool - 

24kw heater - 0.5 degrees C per hour
36kw heater - 0.77 degrees C per hour
54kw heater - 1.15 degrees C per hour 

With a desired temperature rise of 10 degrees C from 16C to 26C, it would take a 24 heater 20 hours, and a 54kw heater 8.7 hours

60,000 litre swimming pool - 

36kw heater - 0.5 degrees C per hour 
54kw heater - 0.77 degrees C per hour

With a desired temperature rise of 10 degrees C from 16C to 26C, it would take a 36kw heater 20 hours, and a 54kw heater 13 hours 

 

To determine what size heater is desired to achieve a given temperature rise in a given timeframe the following calculation is used

Kilowatts Required = (Temperature rise in degrees C x Volume in litres x 4), divided by (hours x 3412)

Examples -  
Heater size required to heat a 1500 litre spa 15 degrees in 30 minutes is 52kw 

Heater size required to heat a 1200 litre spa 20 degrees in 4 hours is 7kw 

Note that these calculations do not take into account the inevitable losses that occur during the heating timeframe. 
Factors affecting efficiency such as heat loss to atmosphere and through pipes, through spa shell to ground, etc. Wind, ambient air temperature, ground temperature affect actual results. 
A covered, insulated spa or pool will achieve far greater efficiency than one without.

Most smaller spas are 1000-1300 litres. Most larger spas are 1300-1700 litres. Often portable spas this size are fitted with 1.5 - 4kw heaters
Most standard swimspas are 5000-7000 litres. Often portable swimspas this size are fitted with 3 - 6kw heaters. Note that these spas are generally insulated and designed to run at set temp, rather than heat from cold.

In-ground / larger spas, pools and swimspas should be fitted with larger heaters.
For rapid heating of standard sized spas, to maximise convenience heater size can matter. Bigger is better if you need your spa hot fast!

 

Size your heater and maintain a lower temperature that you can comfortably heat your spa from, to desired temperature in a convenient timeframe.
You may want to maintain a 10 degree lower than desired temp in order to use a smaller heater to heat the water over a given timeframe, rather than a 20 degree differential.
Examples - 
A 1500 litre spa with a water temp of 15 degrees, a desired temp of 36 degrees (21 degree rise) and a 2kw heater will take 18.5 hours to heat. 
The same spa maintained with a water temp of 25 degrees (11 degree rise) and a 4kw heater will only take 5 hours to achieve the same temperature. 
The same spa maintained at 30 degrees (6 degree rise) and a 6kw heater will only take 1.75 hours to achieve the same temperature. 
Again the same spa maintained at 30 degrees (6 degree rise) and a 24kw heater will only take 26 minutes to achieve the same temperature.




The minimum size of heater required to achieve the desired temperature
 

A heater needs to be suitable sized to not only heat, but maintain the desired temperature in the coldest part of the year
Kw rating required = 0.06 x surface area (m2) x temp differential (between desired water temp and ambient air temp C) x wind factor
wind factor = 1 for 5.6kmh, 1.25 for 8kmh, and 2 for 16kmh
Example - A 2.4m spa has an approx internal dimension of 1.9 x 1.9 and surface area approx. 3.6m2
desired water temp 37c
with 15c air a min heater of 4.75kw is required
with 24c air a min heater size of 2.8kw is required