Internal conditions include room gains from lights, equipment and occupants, as well as infiltration rates and plant operation specifications. They are grouped together in profiles which are applied to different zones in the building. Profiles may be stored in a database for later retrieval.
Gains are modelled by resolving them into radiant and convective portions. The convective portion is injected into the zone air, whilst the radiant gains are distributed amongst the zone's surfaces.
Infiltration, ventilation and air movement between the various zones will cause a transfer of heat between the air masses which is represented by terms involving the mass flow, the temperature difference and the heat capacity of air. Tas offers the capability to calculate natural ventilation air flows arising from wind and stack pressures.
The heating and cooling plant is represented by plant capacities, setpoints and control bands. Like gains, plant inputs may have both radiant and convective portions.
Tas solves the sensible heat balance for a zone by setting up equations representing the individual energy balances for the air and each of the surrounding surfaces. These equations are then combined with further equations representing the energy balances at the external surfaces, and the whole equation set is solved simultaneously to generate air temperatures, surface temperatures and room loads. This procedure is repeated for each hour of the simulation.
A latent balance is also performed for each zone which takes account of latent gains, moisture transfer by air movement and the operation of humidification and dehumidifcation plant.