Overview

This guidance explains how to determine the appropriate increase in fan static pressure for baseline HVAC systems when proposed buildings use MERV 13 filters. The goal is to ensure the baseline model is equivalent without penalizing the use of MERV 13 filters.

Definitions

PHC = Primary Heating and Cooling system

DOAS = Dedicated Outdoor Air System

Glossary of Terms

Example Variable Names:

• f_pr.phc = Percentage of PHC air served by MERV 13 in proposed building
• SA_bl.doas = Supply air from DOAS in baseline building
• SA_bl.filtered = Total baseline supply air that is filtered

Key Principles

Before calculating filter percentages, understand these fundamental principles:

1. Supply air is the sum of PHC and DOAS design air:

   • SA_net = SA_PHC + SA_DOAS

2. Percentage equivalency between proposed and baseline:

   • f_pr.net = f_bl.net
   • The percentage of supply air served by MERV 13 in the proposed system equals the percentage in the baseline system

3. Priority rule for baseline systems with both PHC and DOAS:

   • DOAS air will be filtered to 100% before any PHC air is filtered

Step 1: Calculate Proposed System Filter Percentage (f_pr.net)

If Proposed System has PHC + DOAS:

If Proposed System has PHC Only:

Step 2: Determine Baseline Filter Percentages

Scenario A: Baseline is PHC Only

Simple case:

Scenario B: Baseline is DOAS + PHC

First, calculate the total filtered supply air:

Then apply one of two conditions:

Condition 1: If SA_bl.filtered ≤ SA_bl.doas

All filtered air is supplied by the DOAS:

Condition 2: If SA_bl.filtered > SA_bl.doas

DOAS is 100% filtered, and some PHC air is also filtered:

Step 3: Calculate Fan Static Pressure Increase

The calculated f_bl.phc and f_bl.doas values are used to determine the static pressure increase in the baseline HVAC systems.

Static Pressure Rise Values

The baseline fan static pressure increase depends on the baseline system type:

Applying the Pressure Rise

The pressure rise values shown above represent the maximum increase when 100% of the air is filtered by MERV 13. The actual pressure rise applied to each system is prorated based on the calculated filter percentages:

For PHC systems:

For DOAS systems:

Example Calculation

If a baseline VAV system (merv13_pressure_rise = 0.5 in w.c.) has:

• f_bl.phc = 0.5 (50% of PHC air filtered)
• f_bl.doas = 1.0 (100% of DOAS air filtered)

Then:

• PHC fan static pressure increase = 0.5 × 0.5 = 0.25 in w.c.
• DOAS fan static pressure increase = 1.0 × 0.5 = 0.5 in w.c.

Multizone Systems

For multizone air loops, follow the same process but with an additional final step:

1. Calculate SA_filtered.doas or SA_filtered.phc for each zone
2. Sum all individual zone values for zones served by the air loop
3. Use these totals to determine f_bl.phc and f_bl.doas at the air loop level based on the ratio with total supply air
4. Apply the prorated pressure rise to the air loop fans as described in Step 3

Summary

The process ensures that:

1. The percentage of filtered air in the baseline matches the proposed system
2. The location of filtration prioritizes DOAS before PHC (when both exist)
3. The fan static pressure increase is proportional to the amount of air being filtered
4. Users are not penalized for specifying MERV 13 filters in their proposed design