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Instantaneous gas hot water heaters or continuous flow gas hot water systems work by heating the incoming cold water using a large gas flame. This simple process allows them to provide instant hot water without the need for a hot water storage tank.

The basic configuration of a continuous flow hot water heater is a copper pipe coil wrapped around a large gas flame. This is known as a heat exchanger. The cold water comes in through the inlet pipe, travels around the heat exchanger gaining heat as its passing around the coil, then exits on the hot outlet side. This style of heating provides endless hot water for the heaters specified flow rate (i.e. 16 litres per minute).

process image of how a tankless gas hot water heater works

How does a tankless gas hot water heater work?

As there is no storage tank, the hot water in a continuous hot water heater should never run out. The temperature can be governed by PCB (Printed Circuit Boards) in modern hot water heaters through to old style bellow thermostats. Modern types of continuous flow hot water heaters also allow temperatures to be set specifically through control pads, where exact hot water temperature settings can be achieved, in some cases even eliminating the necessity for adding cold water to mix.

There are many brands of continuous flow hot water heaters including, Rinnai, Rheem, Dux, Bosch as well as various sizes to suit any application.

Continuous flow hot water heaters can be manifolded and even coupled with a storage tank, where the water is heated by the continuous flow hot water heater, then stored in the storage tank to allow for large volumes of hot water ready for use. This application is most used on hotels or large commercial sites requiring large volumes of hot water while still remaining cost effective. Modern continuous flow hot water heaters do not require pilot lights, therefore saving the average household approximately $70 a year (average cost to run a pilot light per annum).

Whilst they consume large amounts of gas initially to get the water up to temperature, the gas flames are designed to modulate, therefore as the heat exchanger heats up and begins heating the incoming water, less energy is required to maintain the heat, therefore becoming very cost effective to run. Continuous flow hot water heaters can be installed inside or outside. Inside heaters require adequate and proper fluing to ensure customer safety and to meet Australian Standards. They also require proper planning when installing to ensure adequate gas supply to the heater for correct and safe operation.

An undersized / oversized gas supply to a hot water heater can work inefficiently and be very dangerous. Speak to us for professional advice about sizing a continuous flow hot water heater adequately to meet your immediate and ongoing needs. Continuous flow hot water heaters are generally sized based on the number of hot outlets its needs to serve. For a two bathroom household you typically need a 20-24lt per minute flow rate (size) hot water heater, however every application is different and can sometimes depend on available gas pipe size and cold water inlets. Always ensure that all hot water heater installations are carried out by a licensed plumber and installed in accordance with AS3500 Australian Standards.

gas hot water storage available brands include Rheem, Rinnai, Dux, Everhot, Thermann


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