Heat output
The heat output describes the amount of energy transferred per unit of time and is the key parameter in the design of hot water and heating systems. In order to transfer the heat output generated in a boiler or the heat output required by a consumer, a corresponding mass flow or volume flow of water must be transported. This causes a change in temperature at the heat generator and heat consumer. While the temperature is increased at the generator, it is lowered at the consumer.
The specific thermal capacity for water at 20 °C and 5 baro is 4.183 [kJ/kgK] and increases minimally with increasing temperature. This is, for example, 4.194 [kJ/kgK] for 80 °C and 4.285 [kJ/kgK] for 140 °C. When designing hot water systems, an average specific thermal capacity of 4.240 [kJ/kgK] can be used without any significant error.
The density of water decreases slightly as the temperature rises. At 5 baro between 943.3 [kg/m³] for 120 °C and 926.2 [kg/m³] for 140 °C.
For optimised and efficient operation, there must be a balance between the heat output of the generator and the heat consumption of the consumer over time. In practice, however, differences in volumetric flow rate and temperature difference between the generator and consumer sides occur due to varying requirements.
In order to compensate for these differences, various hydraulic separations and intelligent control systems must be used.