To properly define the capacity of a heating and ventilation system, requirements to indoor climate parameters must be considered.
An important parameter is the indoor temperature which represents the room air temperature in a given heated space.
A BMS (Building Management System) controls heat sources (and thereby temperature) by using a temperature sensor to measure the temperature in a room. The sensor is typically placed on a wall, which is not affected by other sources of heat (sunlight, etc.).
In a heating system where the radiators are controlled by radiator thermostats, the room temperature sensors can be built into the radiator thermostats.