Hot wastewater is channelled into a recuperative heat exchanger from housing properties, boarding houses, hotels and selected technological operation properties such as convalescent homes and fitness centres. There the heat is transferred to a support medium, i.e. cold water that is channelled into the properties’ hot water sources. Water pre-heated in this way consumes much less energy when being heated to the final required temperature. The volume of a double-surface thermally insulated receptacle containing the heat transfer surface of the exchanger only serves to the balancing of discontinuous drain of hot wastewater. Having transferred its temperature potential back to properties, cooled wastewater flows into sewer lines. The whole system working with the so-called grey water, i.e. without pumps thus requires no self-consumption of energy as it works on the principle of hydraulic parameters.




 The recuperative heat exchanger receptacle is a round double-surface container consisting of an inner and outer container centred in the vertical axis, a double bottom and a lid. The interspace is filled with thermal insulation material. The receptacle construction complies with the static conditions for the placement in the field.

1. Material, weight, internal volume, thermal insulation


2. Basic parameters of recuperative heat exchanger


3. Dimension for the connection of heat exchanger

a)  grey water input ØDN 110 grey water output

b) pressure drinking water connection R ¾

4. Hydraulic parameters:

Determination of pressure conditions and flow rate is very important when designing a heat exchanger for housing properties with a high number of flats or properties requiring a greater amount of technological water for increased hot water abstraction. In the case of greater amounts of hot water heating, two recuperative heat exchangers are collaterally united, thus reaching a high transfer performance of 166 kW and a high flow rate.


  1. Recuperative heat exchanger efficiency
    The measurement was performer by the Research Institute of Pozemní stavby – Certifikační společnost: Z-18-002 Report. Based on the measurement, the efficiency was determined to 78.8 %.
  2. Transfer performance of the WR16 DN23 stainless steel coil-wound heat exchanger is determined under the following parameters:
    Corrugated pipe length 70 m, heat transfer surface 7.8 m2, temperature at the entry into the heat exchanger 20 °C, temperature at the exit from the heat exchanger 50 °C. Heat transfer performance at these parameters is 83 kW.
  3. Hot wastewater storage unit of the volume of 370 l is a superstructure making use of the heat from hot wastewater. Its function is to retain heat in the period of interrupted hot wastewater discharge (balancing discontinuous operation). In the period of permanent discharges, the heat exchanger works on the principle of flow-through system with transfer performance according to ad. III. 2. The given solution greatly expands the use options.

  1. Self-consumption
    The recuperative heat exchanger works on the principle of hydraulic and non-hydraulic liquids and different density of liquids depending on temperature changes. Self-consumption = 0.
  2. Service life
    The heat exchanger is made of plastic and stainless-steel components. Contains no drives, nor rotating parts, it makes a passive element in the system of hot wastewaters. Estimated service life is at least 30 years.
  3. Maintenance
  1. Short-term maintenance: 1x per 3 months ½ kg of sodium hydroxide is poured into the recuperative heat exchanger inspection opening in order to clean the heat transfer surfaces of corrugated pipe from sediments. Sodium hydroxide can also be poured into any drain within the housing property (washbasin, bath, shower) and washed away with a sufficient amount of water that subsequently transports the sodium hydroxide into the heat exchanger.
  2. Long-term maintenance: the volume of the recuperative heat exchanger is pumped out through the inspection opening using a small sludge pump. Subsequently the inner space is pressure washed. Unless performed by the used, the cleaning is provided by a contracting company. The washing cycle is 1x per 2 years. Technological water operation and increased operation require a shorter cleaning cycle.



Two potential cases of contamination need to be taken into consideration in connection with the recuperative heat exchanger:

  1. Water that flows through the stainless-steel heat exchanger is cold drinking water (with an exception of hot water for technological purposes). This cold drinking water takes over the heat from the buffer storage unit filled with incoming hot wastewater (grey water). In case the surface of the stainless-steel heat exchanger is disturbed (e.g. in the transition positions between the corrugated stainless-steel hose and the threaded couplings), pressure drinking water may potentially leak into no-pressure grey water environment inside the storage unit. Drinking water contamination is inconceivable.
  2. Bacterial growth (e.g. of legionella pneumophila). General Technical Specifications of the ČSN 060320 norm set out how to work with hot water temperature levels. This concerns especially the cases when hot water operation is shut down and the water temperature ranging from 30 °C to 35 °C is convenient for legionella pneumophilia multiplying. With respect to the recuperative heat exchanger itself, it concerns the volume of 32 l of drinking water resting in stainless steel corrugated tube the temperature of which is approximately 30 °C. Threat to the water users posed by legionella overgrowth is inconceivable for the following reasons:

    a) The volume of drinking water is in direct contact with stainless steel material of the total inner surface of 7.8 m. It is generally known that stainless steel material acts as a disinfectant (see health facilities).

    b) After hot water operation has been re-established, pre-heated drinking water passes into further water heating environment where the temperature destroys bacteria (as in compliance with the ČSN standards). In no event, pre-heated drinking water directly reaches any consumer.

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