How much hot water is required from your Hot Water System at any one time?

Hot water systems are given a number rating which refers to the amount of litres of water that they produce at maximum capacity in the first hour or at maximum capacity plus the quantity that can be generated in the second hour.  Other hotwater systems that do not store any hot water refer to the amount of litres that it passes and heats through the system per minute.

Example 1:  An electric hot water system with a 250 litre tank size that only heats up once or twice a day only has a max of 250 litres of stored hot water.  Once this is depleted, you have to wait until the electric turns back on to heat up the water again.  This is referred to as an Off Peak Electrical Hot Water Tariff.

Example 2: You have a 130 litre hot water system or tank size.  This system has a gas energy source.  This system will turn on again on its own as soon as the temperature drops 10 degrees below your set temperature.  These systems often have names based on their first hour ability to produce hot water.  The Rheem Stellar 330 hot water heater system is a good example as it only has a 130 litre holding tank, yet it will deliver 330 litres of hot water in the first hour and 180 litre of hot water each hour there after.  This is a small sized tank but with a large output.  The tank is not restricted to pre-governed heating times like off peak electrical hot water systems.

Example 3:  Systems called Continuous Flow hot water systems will deliver hot water all day if you keep the hot water tap open.  They are restricted not in the ability to produce hot water but in the litres per minute they can produce.  They have names like Rinnai Inifinity 26 or Rheem 27 or Bosch 17e.  The numbers refer to the amount of litres the Continuous Flow Gas Hot Water Heater can produce per minute at a temperature 25 degrees above the cold water at the time.  Therefore, the theory is that if the cold water at the tap is 18 degrees and the litres per hour is 26 at a temperature increase of 25 degrees then the system will produce 26 litres a minute at 44 degrees.  In actuality, it works a little differently.  What actually happens is the system will produce water at a set temperature of say 50 degrees.  It now needs to decrease the water flow rate down a little to allow it more time for the gas to heat the water.  Instead of 26 litres per minute, in this scenario you are only getting around 23 litres per minute.

This 3rd example is important when working out the first question in choosing a hotwater system.  When calculating how much water is required at any one time remember:

Showers use around 75% hot water to cold water mix.  If two showers are running at the same time with a 9 litre/minute shower rose, 14 litres a minute are required (each shower uses 7 litre/minute flow rate x2). Downstairs usage always gets priority so if someone turns the kitchen hot water tap on (7 litres/minute) and there is zero reserve left in the system….  Shower downstairs you will always come up trumps!

Article written by and Intellectual Property of Shannon Cooper