Operation of Parallel Fan-Powered VAV during cooling

asked 2021-08-08 02:35:33 -0600

Keigo
5654 ●1 ●57 ●36 https://www.linkedin.c...

updated 2021-08-08 02:37:39 -0600

When complying with ASHRAE 90.1 Appendix G, should we turn off Zone PIU Fan during cooling? Or should we keep it running all the time?

Appendix G specifies VAV with PFP boxes as HVAC system of baseline model for Large nonresidential buildings in climate zone 0 to 3A (hot or warm&humid climate), and it also specifies the airflow of the parallel fan in PFP box. The airflow of the parallel fan is constant.

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On the other hand, 90.1-2016 User's Manual shows the following figure and suggests that the parallel fan in PFP box should be turned off during cooling.

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I think this Figure 6-V is correct. But if so, since buildinigs in climate zone 0 to 3A has no or less heating mode, the parallel fan is off almost all year round. There is no point in setting airflow and fan power of the parallel fan in accordance with Appendix G. I don't understand why ASHRAE specifies System 6 or System 8 as HVAC systems in hot climate zones.

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answered 2021-08-24 03:53:03 -0600

Keigo
5654 ●1 ●57 ●36 https://www.linkedin.c...

Self-resolved. ANSI/ASHRAE/IES Standard 90.1-2016 Performance Rating Method Reference Manual provides the following explanation.

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The Parallel Fan should be off throughout cooling.

From now on, I will refer to the Performance Rating Method Reference Manual first.

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Hi Keigo, If the terminal unit fan is kept OFF, I wonder how the software will proceed with further simulation in first place. Since the primary airflow through AHU gets distributed mostly through the terminal units. Can you clarify this? Also, did you find this applicability for both ASHRAE 90.1 2010 & 2016? Appreciate your response.

Anurag_G ( 2025-06-25 01:07:16 -0600 )edit

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This post is about "Parallel" fan-powered VAV terminal unit. The terminal unit fan is for "recirculation" only. The primary air is supplied to each zone even if the terminal unit fan is off. Of course, this applies to all versions of ASHRAE 90.1.

Keigo ( 2025-06-25 02:25:03 -0600 )edit

Hi Keigo, I am sorry I misinterpreted the concept earlier. The minimum volume setpoint of 30% for the parallel fan powered box still remains valid for cooling, right? Also, does the reheat coil remain ON throughout the annual simulation?

Anurag_G ( 2025-06-25 02:59:51 -0600 )edit

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answered 2025-10-24 03:40:40 -0600

Anurag_G
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Hi Keigo, I am sorry to open this thread once again. While working on a baseline case following system 8 (appendix G ASHRAE 90.1 2010), there is a requirement for PFP terminal unit. Now I have a couple of queries regarding that: 1. Whether the PFP terminal units are applicable to all zones or only the zone toward the perimeter of the building? 2. We are working on design builder, so there are couple of parameters in PFP box which I am unsure of to input. Parameters like Fan On flow fraction, Maximum primary air flow rate(m3/s), maximum secondary air flow rate(m3/s), and minimum primary air flow fraction. These are currently autosized. Can you help me out in this. Appreciate your help and way forward.

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@Anurag_G since this is a follow-up question instead of an answer to the original post, please leave this text as a comment (either under Keigo's self-answer or question) and then delete this answer post. There is a 600-character limit for comments, so you will have to split this across multiple comments.

Alternatively, you can also create a new question post for your question, and leave a link to reference this post for others to review.

Aaron Boranian ( 2025-10-24 09:54:16 -0600 )edit

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Operation of Parallel Fan-Powered VAV during cooling

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