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Heat recycling with treated greywater

In modern, well-insulated buildings, more energy leaves the building with the warm wastewater than is needed for heating. Most of this energy is contained in the greywater. If a greywater recycling system is installed in a building anyway, it makes sense to extract this energy from the treated greywater shortly after the point of origin via a heat exchanger using a heat pump and to feed it back into the hot water system. With early planning, heat pumps can be used to which this additional heat source can be connected. This results in the lowest additional costs. The heat recovery from the gray water can be up to 15 kWh/m³ (assumption: 33° inlet - 18° drawdown, ca. 1 kWh/°/m³). At energy prices of 30 cents/kWh, this would correspond to a saving of 4,50 € /m³ grey water. 

An example calculation:

Outlet temperature of the grey water

33° C

Heat recovery from grey water per m³15 kWh
Energy price per kWh

0,30 €

Savings per m³ grey water

4,50 €

* (approx. 1 kWh/ °C/ m³)

It does not make sense to lower the temperature further, as otherwise cold water would again absorb heat from the building via the service water pipes. In addition, a temperature below 15°C would worsen the biological degradation.

Note: We are planning to offer tubular heat exchangers for our series of graywater systems from about the beginning of 2023. Currently the heat exchangers are installed by heating installers.