Two factors are decisive for achieving the goal of low heating network temperatures:
A panel heating circuit (underfloor heating) is not possible due to the renovation in inhabited condition. Due to the energetic renovation of the building envelope, the realization of low design temperatures of, for example, a maximum of 50 °C is also possible with radiators without any problems.
Hot water generation
There are two basic options for hot water generation: centralised or decentralised.
For central hot water generation - whether with hot water tank or fresh water station - a hot water temperature of 60 °C must be ensured for hygienic reasons. This requires permanent temperatures of about 65 °C from the heat generator. This temperature can be reached with heat pumps. However, this is not efficient in terms of achieving a high annual performance factor: Despite a favourable heat source with temperatures above 12 °C throughout the year, the annual coefficient of performance can hardly exceed 3.0 if 65 °C has to be delivered permanently (Fig. 2).
Decentralised water heating solves this dilemma. The key question here is whether the hot water is generated electrically or via the heating network. The operating costs for the tenant are highest with electric hot water preparation. Also, despite the fact that 40 % of the electricity generated is renewable, the environmental balance is not optimal. Decentralised hot water preparation can also be carried out via the heating network, which is then operated all year round. Here the high unit costs for the dwelling stations are often a deterrent. In addition, there are manufacturers who demand heating network temperatures of 55 °C and more, which is not far away from the 65 °C of the central hot water supply. However, both handicaps can be overcome: Specialised manufacturers supply dwelling stations that reliably deliver hot water temperatures of 40 to 42 °C even with 48 to 50 °C heating flow. In practice, there are no complaints from tenants with these temperatures.
The question of costs is also not critical on closer inspection. Tenants have the greatest possible advantage when it comes to operating costs: The energy comes from the heating system and is much cheaper than electricity. In addition, the amount of energy is 30 to 50 % lower, as three quarters of the circulation losses are eliminated. Finally, the costs for billing the hot water meter are eliminated.
The investment costs require a careful comparison in order to arrive at reliable and realistic statements. The costs for dwelling stations are offset by investments for central hot water preparation:
- Hot water network and circulation network incl. fittings
- Hot water tank / fresh water station
- Circulation pump
- Fittings for hydraulic balancing of the heating network (this also takes place in the dwelling station)
- Additional heat pump costs for 65 °C instead of 55 °C maximum flow temperature
- Investments / rents for hot water meters and heat cost allocators
The dwelling station makes it possible to measure the tenant's total energy consumption for heating and hot water preparation via a single heat meter. The water consumption for cold and hot water is also only recorded by a single cold water meter (Fig. 3).
In addition, there are no costs for regular sampling of the hot water network. From a hygienic point of view, it is always advantageous to dispense with the storage of heated potable water.
In Wiesental the following figures were obtained as examples for one of the four blocks of flats: