Let’s be honest, gas geysers can be a bit of a confusing topic.

We understand though, that the name itself can be confusing. Gas geysers are also referred to as gas water heaters, as they heat water on demand. This means that they are only heating water when your hot water tap is turned on.

This is unlike the traditional geyser, which acts as a reservoir for your water, heating it over time – gas geysers do not store large amounts of water internally.

Gas geysers are activated by cold water running through the unit i.e. as soon as you open a hot tap the geyser will switch on. It will automatically switch off again when you close the hot tap.



But how do they actually work?

As mentioned above, the name gas geyser can cause some confusion. The other name that some people call it (gas water heaters) can seem like a more fitting name since that’s exactly what gas geysers do – they heat water using a gas flame. And because they only turn on when you open the hot tap, the geyser only uses gas when you are actually using hot water.

At the heart of most modern tankless water heaters is an Electronic Control Unit (ECU). The ECU controls all of the functions of the heater including temperature control, ignition, gas flow, and modulation. It is also responsible for the safety aspects, with a flame sensor and automatic gas cut-off, in the event of a problem.



Let’s break that down a little more:

  • When you turn on a hot water tap on anywhere in your home, a flow sensor, connected to the ECU, detects the flow and the heating process begins.
    Don’t worry, a slow leaking tap is not enough flow to start the unit.
  • At the same time, the ECU starts the flow of gas and triggers the igniter. The igniter sparks until the flame sensor detects a flame. In the event that the unit fails to light within a few seconds, the ECU shuts down the gas flow to prevent any unsafe accumulation of gas.
    Some models also have a combustion fan that draws in air for the combustion process.
  • The gas burner applies heat to the heat exchanger, through which the water passes. The course of the water is typically a serpentine pattern, allowing the water to absorb as much heat as possible from the walls of the heat exchanger.
  • Some models have a secondary heat exchanger to pre-heat the water, using the heat from the burner exhaust gases. The advantage is that they provide even greater energy efficiency, however, the purchase price is a bit more than for the standard models.
  • Most units come with their maximum temperature pre-set in the factory. This is a safety precaution to help prevent possible scalding. 50°C is typical for residential applications.
  • As the flow rate and temperature of the incoming water can vary, the ECU modulates the amount of gas that goes to the burner, turning it up or down, as required. This modulation of the gas and burner intensity is what keeps the water at a set temperature.
    This functionality is based on input from a water temperature sensor on the exit side of the heat exchanger.
  • Some continuous flow systems have temperature controllers that allow the user to pick a temperature lower than the factory pre-set. These controllers can be located on the unit itself, wired to a remote location — like a kitchen or bathroom — or even wireless. The ECU takes the input from the controller to further modulate the gas flow and burner intensity, keeping the water at the user’s desired temperature.


To find out if a gas geyser is a good option for your household, contact DMS Plumbing for a professional technician to consult and advise you.

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