Hi everyone,

I'm using Design Builder with detailed HVAC mode and I'm having trouble getting the control I would like to have on a VAV AHU.

The model is a very common setting: I have a large room (for example auditorium or cinema) served by only one AHU with water cooling coil and heating coil.

I would like the system to operate this way:

• outdoor air is proportionnal to the number of people in the room
• when there is a cooling need, temperature of the air supplied by the AHU drops to a fixed minimum value (16°C for example). Then the air flow rises if necessary to meet cooling needs
• when there is a heating need, temperature of the air supplied by the AHU rises to a fixed maximum value (35°C for example). Then the air flow rises if necessary to meet heating needs.

In the VAV ADU I chose control on outdoor air flow. In the AHU I enabled recirculation, with minimum air flow rate = 0. No outdoor air schedule. Demand controlled ventilation with the Zone sum method and maximum outdoor air fraction to 1. I have 2 setpoints manager on the AHU outlet, one is single zone heating with T° between 16°C and 35°C, the other is single zone cooling with T° between 16°C and 35°C.

In the results the outdoor air inlet is consistent with the occupation rate. However the total air supplied by the AHU is not consistent at all: it is constant at 0,53 kg/s outside occupation hours, and at 2,12 kg/s during occupation hours (even when there is no heating or cooling demand), whereas outdoor air is only 1,34 kg/s max .

Did I miss something?