AHU & TFA Modeling in DesignBuilder – Coil Availability and Setpoint Manager Query
For an office project, we have cooling during February–November and heating during December–January. The system uses:
- 4-coil AHU (water-based cooling coil + water-based heating coil connected to heat pump)
- Separate TFA unit with DX cooling coil, HRW, and prefilters
- Water-cooled chillers for cooling
In DesignBuilder, we are modelling this by introducing the TFA components into a single AHU template, along with the water-based coils positioned before the fan. Cooling and heating schedules have been provided as per seasonal operation.
My questions:
Availability Schedules: Should the cooling coil and heating coil availability schedules inside the AHU be kept ON 24/7, or should they follow the same seasonal operation schedule as the cooling/heating mode?
Setpoint Managers: Do we need to switch the setpoint managers between cooling SAT and heating SAT according to the months (Feb–Nov for cooling, Dec–Jan for heating), or does DB handle this automatically?
Hi Anurag, I had to look up TFA (Treated Fresh Air). Just so you know, the latest version of DesignBuilder now allows you to connect an air loop to an air loop using the DOAS AHU feature. So, if you wanted to keep it separate from the AHU, you can. Before answering your questions, we need to know the distribution so we can understand your intention. For example, are you conditioning a single zone, or multiple zones but with a single controlled zone? If so, use a unitary AHU. Is this a Central AHU serving zone level equipment?
There are separate AHUs with individual TFA units for each zone. These AHUs are a 4-pipe system with 2 pipes(CHW) coming out of cooling coil and 2 pipes (HW) coming out of heating coil. I was able to model the exact same scenario in designbuilder. I believe there is some problem with heating as we are not realizing it at the end of simulation. The proportion of cooling:heating has a major difference. Heating is almost negligible in the final result, though we have heating during November and December(winter months). I believe somewhere I need to change schedule, but dont know where?