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April 2015

Choosing the Correct Gas Water Heater

These units are activated by the water flowing through them. The water flows over a venture valve that causes low pressure on one side of a diaphragm and with water pressure on the other it pushes open the micro switch. This fires the igniters and opens the gas solenoid valve.

  • The water is now heated as it flows through the radiator above the gas burners.
  • The settings on the right-hand control knob are flow rate settings.
  • The slower the water flows through the radiator the hotter it gets.
  • The rated flow of a unit is on the cool setting i.e. a 16lt unit will flow at 16lt/min on cool. At the hot setting the flow rate is half that of the cool so the same 16lt unit will flow at 8lt/min.

Now let us take the situation where someone wants a 20lt unit that they have enough hot water

A 20lt unit will deliver 20lt/min on cool and 10lt/min on hot. If you run a basin tap at 10lt a min it is flat out and this will be effectively be the same as running it on hot. Most showers are 10lt/min and this again would be the same as running the unit on full hot. If your unit is set on cool which is 20lt/min and you are only taking off 10lt/min then it may not even fire up.

Also with the way these units work if the water is too hot and you mix in cold you are reducing the flow of the hot water making it hotter and you are reducing the effect of the venture valve that operates the unit. Too much cold water will overcome the hot water flow and the unit will switch off. So set it at the temperature you will use it at.

What about a 20lt unit running three showers of 10lt/min? Well if the three are running at the same time then approximately 7lt/min is feeding each shower and the water temperature is on say 40degrees great. Now one person gets out and instead of 7lt/min going through three showers you have 10lt/min going through two, still O.K. If now the second person gets out you will have 20lt/min trying to feed a 10lt/min shower. This is the same as setting the unit on hot. The person in the shower now adds cold further restricting the flow and the unit switches off.

So how do we choose which unit to fit?

Look at what is the maximum and minimum flow rate of water you need and pick a unit that matches it. See below;

  • 6lt Dewhot Flows at 6-3lt/min. Suitable for a basin or camping.
  • 8lt Dewhot  Flows at 8-4lt/min. Suitable for a kitchen, basin, small shower.
  • 10lt Dewhot Flows at 10-5lt/min. Suitable for shower and basin.
  • 12lt Dewhot  Flows at 12-6lt/min. Suitable for a shower, basin, occasional bath.
  • 16lt Dewhot  Flows at 16-8lt/min. Suitable for a bath, basin, larger shower.
  • 20lt Dewhot  Flows at 20-10lt/min. Suitable for a Jacuzzi and commercial use.

A unit will run more than one bathroom providing two people do not use them at the same time.

What happens if you need a larger flow rate than 20lt/min. You can cascade the system by putting a number of units in parallel.

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The logic tree is a tool that guides you through a series of questions to help find the gas water heater solution the would be most suitable for your individual home set up.

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