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Experience with drain water heat recovery
Domestic wastewater is generally warmer than 20 °C, while wastewater from hot showers can reach as much as 35 °C. Considering the large amounts of water that need to be processed in buildings and how much water can contribute to heating, it makes perfect sense to use this heat source in some way.
One standard approach is heat pumps that use wastewater flowing through large intercepting sewer ducts as a heat source. Passive heat recovery or a heat pump can also be used to process gray water. Even the very first Passive House building in Darmstadt-Kranichstein was built with a cooling tank from which wastewater sends its heat to the rooms.
This paper discusses direct, passive heat recovery from flowing wastewater from showers as a way to reduce hot water demand.
This in-depth article is available exclusively to iPHA members!
Drain water heat recovery offers a number of advantages:
- These simple, passive systems can be planned and installed with little chance of mistakes.
- Total hot process water consumption can be decreased by 25 to 35 percent.
- Not much space is needed, and a variety of systems is available for different installation situations.
- The systems are already economically competitive with conventional hot water systems and will become even more so as they become more common.
- In bigger buildings, the nominal heat production output and storage tank size can be reduced.
- Data measured in accordance with standardized procedures are available.
- There is already a calculation method for easily determining expected savings in specific cases (see AkkP49).
The following points are particularly important:
- Savings are highest when the heat exchangers supply both cold and hot shower water.
- If cold water connections from multiple showers are connected to the same heat exchanger, cold showers may not be possible.
- If the heat exchanger is only connected to the hot water connection, high hot water temperatures can significantly reduce the utilization rate.
- If the heat exchanger is only connected to the cold water connection, hardly any heat will be recovered in the case of low hot water temperatures.
- Heat exchangers that replace part of a downpipe must be installed in a completely vertical position.
- Pressure losses can be relatively high in some systems because thin pipes improve heat transfer and down times.
- A thermostatic mixing valve is recommended to prevent pressure losses and fluctuating cold water temperatures.
[AKKP49] Volume 49 “Energy-efficient hot water systems” of the proceedings of the Working Group on Affordable Passive Houses