A phenomenon seen at ONC from the logged BMS data and meter readings was a substantial amount of simultaneous heating and cooling by the office FCUs in the building. As we were building a CEM this sub-optimal HVAC system behaviour had to be accounted for to compare old and new strategies, both before and after the Energy Conservation Measure (ECM) is implemented. Being able to model simultaneous FCU heating and cooling was an important part of the project.
Simultaneous heating and cooling can happen for a number of reasons. After extensive study of the FCU BMS logged data at ONC, there seemed to be two primary causes for this. Firstly, the control system in the FCUs was not sophisticated enough to control temperatures to the current settings, which were very exacting (effectively trying to control to within 1C, with only a 0.5C deadband). This was termed ‘hunting’ and had been confirmed and explained by the N G Bailey team, who put forward a proposal to cure this issue by widening the deadband.
Secondly, FCU setpoints were varying significantly within zones of the building. This means neighbouring spaces were at different temperatures and this leads to air mixing between these areas due to stack pressure effects. For example a typical FCU on level 4 had the following varying setpoint signal for the year (from 1st January to 11th June 2016).