Method and system for controlling fresh air ventilation of an air handling unit of an hvac system

An Air Handling Unit (AHU) for servicing a building space of a building, the AHU comprising:

• a heating and/or cooling unit;
• a fresh air intake damper for admitting a fresh air ventilation air flow;
• a return air duct for receiving return air from the building space;
• a mixed air duct for mixing the fresh air ventilation air flow from the fresh air intake damper and return air from the return air duct and providing a mixed air flow to the heating and/or cooling unit which supplies a supply air flow to the building space;
• a fan for providing a motive force to move the return air, the fresh air ventilation air flow, the mixed air flow and the supply air flow through the AHU;
• the AHU having a load capacity;
• a controller, wherein the controller is configured to:
• determine a current load on the AHU that is used to maintain one or more comfort conditions in the building space;
• determine a remaining load capacity of the AHU, wherein the remaining load capacity is the load capacity that is currently not being used to maintain the one or more comfort conditions in the building space;
• determine a maximum additional fresh air ventilation air flow that could be admitted and conditioned using the remaining load capacity of the AHU such that the AHU could still maintain the one or more comfort conditions in the building space;
• determine a fresh air intake damper position for the fresh air intake damper that increases the fresh air ventilation air flow through the fresh air intake damper by a fraction of the maximum additional fresh air ventilation air flow, wherein the fraction is based at least in part on one or more of a temperature factor and an air quality factor;
• and
• setting the fresh air intake damper to the determined fresh air intake damper position.

A method for controlling a fresh air intake of an Air Handling Unit (AHU) of an HVAC (Heating, Ventilating and Air Conditioning) system servicing a building space of a building, the method comprising:

• estimating a minimum volume of fresh air that will need to be admitted by the AHU to maintain a predetermined air quality parameter below a threshold level in the building space over a predetermined time period into the future;
• forecasting one or more outdoor conditions and one or more indoor conditions including temperature and air quality conditions over the predetermined time period into the future;
• based on the forecasted one or more outdoor conditions and one or more indoor conditions, determining for each of a plurality of time intervals over the predetermined time period:
• a dilution factor that represents an estimated change in the predetermined air quality parameter in response to replacing a predetermined fraction of the air in the building space;
• an energy factor that represents an estimated thermal energy required to condition the predetermined fraction of the air in the building space;
• a cost/benefit factor that represents a ratio of the dilution factor and the energy factor;
• assigning a portion of the estimated minimum volume of fresh air to each of the plurality of time intervals of the predetermined time period based at least in part on the cost/benefit factors so as to minimize a total energy consumption of the AHU over the predetermined time period into the future;
• and
• during a first one of the plurality of time intervals, setting the fresh air intake of the Air Handling Unit (AHU) to admit the portion of the minimum volume of fresh air assigned to the first one of the plurality of time intervals.