Guidelines for the specification and design of surface heating/cooling systems

Detailed guidelines and the methodology for the calculation and design of surface radiators are established in EN 1264, “Water based surface embedded heating and cooling systems”.

Surface heating and cooling are low-temperature systems. For heating systems, PN-EN 1264 states that the maximum heating water supply temperature shall be 60°C (for the design outdoor temperature) and the optimum temperature drop in the heating coils is 10°C (the permitted range is 5-15°C).

For surface cooling systems, PN-EN 1264 states that the minimum chilled water supply temperature is the result of the design water temperature increase by 5°C (the permitted range is 5-10°C) and the permitted temperature of the cooling surface, shall not be more than 6°C lower than the indoor air temperature (for protection against condensation).

Therefore, the medium supply parameters of an underfloor heating system can be:

55°C/45°C; 50°C/40°C; 45°C/35°C; 40°C/30°C; 35°C/30°C.

The typical parameters for a surface cooling system oscillate around these values: 22°C/17°C; 20°C/15°C; 17°C/12°C.

The ever-increasing thermal insulation performance of buildings and the use of ventilation systems that recover heat from the exhaust air mean that the thermal loads of buildings are becoming ever lower. As a result, heating systems can operate at increasingly lower parameters. For this reason, the supply parameters of the system must be determined case by case, by the system designer who follows the details of the structure of the building in question, as well as the type of the system and its heat source. It is important to note that the supply and return temperatures for the entire system are specified for the room with the highest specific heating or cooling demand.

A very important issue with surface heating systems is the maximum attainable temperature that allows them to be operated for thermal comfort. It has been determined that the most physiologically beneficial surface temperature for underfloor heating is approximately 26°C. When the thermal performance of an underfloor heating system is insufficient at this surface temperature, EN 1264 provides these maximum usable temperatures:

Underfloor heating	Wall heating system	Ceiling heating system
29°C for occupied zones	40°C	35°C
33°C for bathroom zones
35°C for boundary zones (ϑi=20°C)

Compliance with these maximum surface temperature values restricts the thermal performance of the flooring to the limit values qmax 100 W/m² for occupied zones and bathroom zones, and 175 W/m² for boundary zones (assuming the design indoor temperatures). For walls and ceilings, the limits are q_max160 W/m² and 98 W/m², respectively.

For surface cooling, the minimum surface temperature must be determined case by case, in accordance with the climatic conditions assumed to prevent condensation. A Mollier chart should be used for this purpose.

To avoid high flow resistance from excessively long thermal medium piping loops and to eliminate incorrect performance of the entire system, the maximum heating loop lengths must be followed, which depend on the diameter of the piping used:

8 x 1 mm – 40 m, 12 × 2 mm – 80 m, 14 × 2 mm – 100 m, 16 × 2 mm – 120 m, 18 × 2 mm – 150 m,

20 × 2 mm – 180 m, 25 × 2,5 mm – 200 m.