Proposed Building's HVAC system

The fans should be modeled identical to the baseline system fans. What is the baseline system?

No you don't use the allowance for the System 8 PFP boxes.

You have three options that should be acceptable to the reviewer. In all cases the systems would be auto-sized in both models. Provide a thorough narrative describing your approach including manufacturer data is following option 2.

1. Fan coils with the Appendix G fan power allowance for a constant volume system.

2. You could determine a worst case scenario by evaluating the worst case fan power of fan-coils that would likely be used in the space.

3. Model a System 7 configuration in the tenant spaces with your central plant.

I'm working on a similar project with central hot/chilled water plants and DOAS with no tenant HVAC system. I understand that the unfinished area should be modeled with autosized fan coil units with Baseline fan power allowance, but what about the fan control? Our DOAS will provide ventilation during occupied hours, does it mean that the proposed fan coils can cycle on/off?

Thanks!

I think you could do that. Be sure to provide a narrative to the reviewer explaining what you are doing and why.

Hello everyone,

Following your comments, we have a similar situation in a v4 C&S project for an office building:

- The building is connected a DES heating and cooling network.

- In all tenant office spaces, FCUs connected to DES are designed and are to be installed within the scope of work (including all air diffusion elements).

- All tenant office spaces are served by a DOAS with a central AHU, and all the ductwork, which are designed and are to be installed too.

- In the ground floor, there is an unfinished space (with no specific or foreseen use) which has no terminal units nor air distribution ductwork within the scope of work, but which is provided with appropiate connections to DES piping and to central AHU main ducts. This AHU has been designed to provide the necessary airflow for both the office spaces and the unfinished space (making a reasonable assumption about its future use).

Summarizing, all HVAC systems are designed and are to be installad, except the terminal units and air distribution system for the unifinished space.

The guidance provided in the reference guide (Modeling HVAC, lighting, hot water systems for unfinished spaces, Example 2) states the following:

"Example 2. The same hypothetical two-story office building now includes chilled and hot water connections for the
future unfinished retail space. A portion of the HVAC system has been designed.
The proposed building may be modeled as a system that uses the chilled and heating hot water (e.g., 4-pipe fan coil unit). However, because the air-handling units and terminal distribution have not yet been designed, the cooling and heating capacities, design fan volume, minimum volume, fan power, fan controls, etc., must be modeled identically in both the baseline and proposed model, and equal to the requirements of the baseline model."

Thus, since in our case the air handling unit has been designed, from your comments we have made the following assumptions for modelling the HVAC system in the unfinished space:

- The proposed case terminal units would be modelled with and 4-pipe FCUs (connected to DES), with autosized coils and the fan power allowance of a constant volume system according to App G. The DOAS would be modelled with the projected AHU with outdoor airflow and fan power according to design documents.

- Tha baseline case would be modelled with the appropiate system according to G3.1.1A, same as the rest of the building (our project fits into System 7), heated and cooled with the DES. The outdoor airflow shall match the proposed case (according to design documents) and the system will follow all App G requirements (sizing, fan power allowance...). We are assuming that Exception G3.1.1 b. does not apply to our unfinished space (although its final use is still not clear, in the design criteria we have assumed that will not be sigificantly different from the main office use).

Do you find this approach is correct? Thank you in advance for your comments.

Regards,

This is all correct, with the exception of the outdoor airflow. The Baseline outdoor airflow should be modeled using the minimum requirement as calculated within EQp Minimum Indoor Air Quality Performance, which will likely be less than the Proposed design outdoor airflow. 

Thank you very much for your answer, Tyler

Shall it be like that even if the outdoor air flow (including affectation to general AHU and ductwork sizing) is specified in design documents for the unfinished space? Provided that this outdoor airflow matches the local ventilation code and is not provided in excess of the amount required.

Thanks again and regards,

If the design outdoor airflow matches the local ventilation code and is not provided in excess of the amount required, then the Baseline and Proposed outdoor airflow would be modeled identically.