Fresh air
The control of fresh air is a critical component of achieving good performance in any air conditioning system. There are two basic issues to deal with, as discussed below:
Minimum fresh air control
Building occupants require a minimum amount of fresh air to breathe. This figure is typically 8-10 litres per second per person, which is rarely more than 10-20% of total airflow in a ducted air system for an office. If you provide much more than this, energy use will go up, as you will be heating or cooling fresh air that you don't need. If the occupancy pattern of your building is highly variable, you may benefit from controlling fresh air on the basis of carbon dioxide levels. This adjusts the fresh air level on the basis of the amount of air being breathed out, in effect. A simpler alternative is to make sure that nil or reduced fresh air is provided in the start up period in the morning, when no-one is in the building.
Economy cycle operation
If it's 24C inside your building and 18C outside, then you can get "free" cooling just from the fresh air. Setting up your fresh air controls to capture this benefit is not always simple, however, and the consequences of a mistake can be quite serious in terms of energy use. You don't want to be drawing full fresh air if you are trying to heat your building up for instance, or if it's really hot outside. Air quality in particular is affected by modern buildings, which are tightly sealed. Whilst exact amounts of fresh air are required per person in any air conditioned environment in some cases air will need to be introduced directly by other means
