When gas burns in a conventional non condensing water heater, the heat generated is available in two forms:
- Around 89% of it is ‘sensible’ heat, i.e. it can be transferred through the water heater heat exchanger to the system water. Typically conventional water heaters are limited to 80%-85% efficiency to ensure that condensate does not occur and corrode the inside components.
- The balance of 11% is ‘latent’ heat, i.e. Water vapour discharged through the flue Condensing water heaters take advantage of additional efficiency from their ability to pass most of the latent heat into the heating system. A non-condensing water heater would have an efficiency of around 81% to 84%, A condensing water heater will give between 103% to 107% efficiency as the latent heat is captured and flue losses are smaller.
Water heaters can only condense when the flue gases within the appliance are at their ‘dew point’ so to ensure continuous condensing operation the heat exchange surfaces and therefore the return water must be at low temperature.
Continuous flow condensing water heaters are one of the only truly condensing products. As the appliances almost always operate in condensing mode as the incoming water temperature is always low.