How efficient is shower heat recovery?
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The performance of a shower heat exchanger is normally expressed as a heat recovery efficiency.
The Hamwells Blue 21XE, for example, has a stated heat recovery efficiency of 70.6% at 8 litres per minute, under test conditions of:
- 10°C incoming cold water
- 38°C wastewater entering the heat exchanger
- 8 L/min flow
So what does 70.6% actually mean?
The temperature difference between the wastewater and incoming cold water is:
38°C − 10°C = 28°C
If 70.6% of that available temperature difference is recovered:
28°C × 70.6% = 19.8°C
Under these test conditions, this corresponds to a theoretical temperature increase of approximately 19.8°C on the incoming cold water.
So water entering the heat exchanger at: 10°C
could theoretically leave at approximately: 29.8°C
That is a very significant amount of preheating — before the water reaches the system that provides the remaining heat.
> 10°C cold water → approximately 29.8°C preheated water
The exact temperature achieved in an installed system depends on factors such as actual flow rates, water temperatures, installation configuration and heat losses.
