Secondary systems used in Europe

Lower pressure

Secondary systems are the parts of the heating systems with a lower pressure and temperature level, installed in buildings. A lower pressure and a lower temperature can be obtained with a shunt connection and a differential pressure control, (direct connected systems). The most commonly used system is, however, the connection through a heat exchanger, completely separating the two systems from each other, (indirect connected systems).

The secondary systems consist of three parts:

• production, boiler or heat exchanger
• distribution
• consumption

When speaking of district heating, the production unit is in fact only atransformation from one temperature- and pressure level to another, but regarding function, it is a production unit.

Comfort

The purpose of the heating system is to create environmental conditions in the building, comfortable for people to live in. Generally an air temperature of 20-23 ºC is considered acceptable, but
there are also other factors influencing the comfort:

• the temperature of surrounding surfaces
• air movements, convection
• activity level
• clothing

The heat transfers which we can influence, towards and from a person in a room, are from radiation, convection and/or conduction. A minor share comes from breathing. Heat transfer by radiation has the biggest influence. We are receiving heat from surfaces with a higher temperature than our skin, and we are emitting heat to surfaces with a lower temperature.

The greater difference the larger the heat transfer. Air with a lower temperature that flows over a surface removes heat from the surface. The higher velocity of the air-flow the more heat is removed. The greater the temperature difference the larger the heat flow. Heat conduction requires direct contact, for instance when you are sitting on a cold chair, but it is normally short-lived as the chair is quickly warmed up by your body heat. The result of the factors mentioned above and the temperature of the room air at a given point in a room, can be calculated.

It is thus possible to determine in advance if a heating system will provide an acceptable comfort in a given room. Surface temperatures close to 20 ºC on all surfaces in a room and air-flow velocities lower than 20 cm/s provides very good comfort.