What You'll Learn

• How to measure and interpret the pressure drop across each individual component: filter, indoor coil, return ductwork, and supply ductwork
• What typical pressure drop ranges are for each component and how they vary by system configuration
• How filter MERV rating affects pressure drop and when to flag an oversized restriction
• How to identify a dirty coil from pressure drop data and distinguish it from other restriction sources
• When ductwork remediation is needed based on component-level pressure data
• How individual component pressure drops connect to the TESP budget and 140% rule from E11
• How to use these measurements to build targeted recommendations for the customer

What You'll Need

• Device: iPhone (iOS 15+) or Android phone/tablet (Android 10+) with measureQuick installed
• Account: measureQuick account with active subscription
• Manometer probes: A Bluetooth manometer paired with measureQuick (Fieldpiece, UEi, CPS, Testo, or Yellow Jacket)
• Static pressure tips and tubing: Multiple sets for simultaneous or sequential measurement at different points
• Drill and test port plugs: For creating access points in the duct or plenum if no existing ports are present
• Completed E11 understanding: Familiarity with the TESP budget concept and the 140% rule
• Time: 20-30 minutes for measurement and analysis beyond a standard TESP reading

From TESP Budget to Component Diagnosis

E11 introduced the TESP budget concept: breaking total external static pressure into four components (filter, coil, return ductwork, supply ductwork) so you can identify where the restriction lives. This article goes deeper into each component, covering the specific measurement techniques, typical ranges, failure indicators, and diagnostic interpretation for each.

The TESP budget tells you where the pressure is. Component pressure drop analysis tells you why it is there and what to do about it.

Step-by-Step Guide

Step 1: Filter Pressure Drop

The filter is the most accessible and most frequently problematic component. It is also the easiest to test.

How to measure:

1. Take a static pressure reading on the dirty-air side of the filter (between the return grille and the filter).
2. Take a static pressure reading on the clean-air side of the filter (between the filter and the blower inlet - this is your standard return static location).
3. Subtract: Filter drop = |dirty-air side| - |clean-air side|.

Alternatively, measure TESP with the filter installed, then remove the filter and remeasure. The difference is the filter's contribution. This shortcut works well on systems where accessing both sides of the filter with separate test ports is impractical.

Typical ranges by MERV rating:

When to flag the filter:

• Filter drop exceeds 0.20 inWC on a system rated at 0.50 TESP maximum. The filter alone is consuming 40% of the pressure budget.
• A clean, new filter is already producing high pressure drop. This indicates the filter is too restrictive for the system's airflow capacity. A MERV 13 filter in a system designed for MERV 8 will restrict airflow even when brand new.
• Filter drop exceeds the manufacturer's published maximum for that filter size and MERV rating.

As Jim Bergmann demonstrated during the Gas Furnace Workflow: "our total external static pressure is a fail because it's a little high, which means my filter is actually undersized." A component-level measurement confirmed the filter as the restriction source without needing to guess.

Total External Static Pressure measurement on a furnace showing manometer connected via tubing to Before Coil and After Filter measurement points, reading 0.51 inWC

Tips:

• Always record whether the filter is clean, partially loaded, or visibly dirty. A dirty MERV 8 filter can produce the same pressure drop as a clean MERV 13.
• Filter face velocity matters. A 20x20 filter at 400 CFM per ton on a 3-ton system handles 1,200 CFM across 2.78 square feet (432 FPM). The same airflow through a 16x20 filter (2.22 sq ft) produces 540 FPM - higher velocity means higher pressure drop from the same filter media.

Step 2: Indoor Coil Pressure Drop

The indoor coil (evaporator on cooling systems, heat exchanger on furnaces) creates pressure drop from its fin density, face area, and cleanliness.

How to measure:

1. Take a reading between the blower outlet and the coil inlet. On upflow systems with the coil on top of the furnace, this means a test port between the furnace outlet and the bottom of the A-coil casing.
2. Compare to your supply static reading (taken after the coil).
3. Subtract: Coil drop = |before coil| - |after coil|.

On some equipment configurations, direct measurement is difficult. See the Tips section below for alternatives.

Typical ranges:

These ranges assume residential split systems with standard fin density. Manufacturer-published coil data takes precedence when available.

Dirty coil indicators from pressure drop data:

• Coil drop exceeds 0.30 inWC on a system with a standard-density coil.
• Coil drop has increased since the last visit (compare to benchmark if available).
• Coil drop is disproportionately high relative to the other budget components. If filter and duct drops are normal but the coil is consuming 50%+ of the TESP budget, coil cleaning or replacement is indicated.

A dirty evaporator coil does more than restrict airflow. It reduces heat transfer efficiency, which shows up as low temperature split, elevated suction pressure, and potentially low superheat. If you see a high coil pressure drop combined with poor refrigerant-side performance, the coil is the likely root cause.

[Visual Reference] A static pressure test port drilled into the ductwork between the furnace outlet and the A-coil casing. A short length of tubing connects the test port to a digital manometer. This tap location isolates the coil pressure drop from the rest of the duct system. The manometer reads the pressure difference across the coil by comparing this mid-point reading to the supply-side reading downstream of the coil.

Step 3: Duct Pressure Drops

Ductwork pressure drop is typically calculated as the remainder after subtracting filter and coil drops from the TESP budget. Direct measurement requires test ports at the return grille and supply register locations, which is not always practical.

Return ductwork:

Return Duct Drop = Total Return Side Pressure - Filter Drop

This isolates the pressure lost in the return duct system between the return grilles and the air handler. Typical range: 0.05 - 0.20 inWC for a well-designed residential system.

Supply ductwork:

Supply Duct Drop = Total Supply Side Pressure - Coil Drop

This isolates the pressure lost in the supply duct system between the air handler and the supply registers. Typical range: 0.05 - 0.20 inWC.

When ductwork remediation is needed:

• Duct pressure drop exceeds 0.25 inWC on either the return or supply side after accounting for filter and coil.
• The 140% rule from E11 is triggered (TESP exceeds 140% of rated maximum) and filter/coil drops are within normal range. The excess pressure must be in the ductwork.
• Return duct drop is extremely high relative to supply (or vice versa), indicating a specific problem on one side of the system.

Common ductwork issues that produce high pressure drop:

• Undersized trunk lines for the system's airflow capacity
• Excessive fittings (elbows, tees, takeoffs) in the duct run
• Kinked or compressed flexible duct
• Closed or blocked supply registers
• Insufficient return air pathways (common in older homes)
• Duct leakage (leaks in the return side draw in unconditioned air; leaks in the supply side reduce delivered airflow)

Pressure drops diagram showing four measurement points along the airflow path (Return, After Filter, Before Coil, Supply) with component pressure drop values at each point

Step 4: Connecting Components to the TESP Budget

Once you have all four component measurements, verify that they sum to approximately the measured TESP:

Filter + Coil + Return Duct + Supply Duct = TESP (within 0.05 inWC)

If the sum does not match, recheck your measurement locations. Small discrepancies (under 0.05 inWC) are normal due to probe placement variation and turbulence at measurement points.

Example budget analysis:

This budget immediately identifies the supply ductwork as the problem. The filter and coil are fine. The return duct is within range. The entire excess pressure (0.25 inWC above normal) is in the supply ductwork. The recommendation is supply duct evaluation and modification, not a filter change.

Step 5: Building the Customer Recommendation

The component pressure drop analysis gives you the evidence to make a specific, justified recommendation:

• Filter is the problem: Recommend a lower-MERV filter, a larger filter rack, or more frequent filter changes. This is typically the cheapest fix.
• Coil is the problem: Recommend coil cleaning or replacement. Show the customer the before/after pressure drop if you clean it during the visit.
• Ductwork is the problem: Recommend a duct evaluation by a qualified contractor. Specify which side (return, supply, or both) needs work. Common solutions include adding return air pathways, upsizing trunk lines, or replacing restrictive fittings.
• Multiple components are high: Prioritize by magnitude. Address the largest contributor first.

The PDF report from measureQuick includes the TESP measurement and pass/fail result. Add your component analysis notes to the project to document the specific source of the restriction.

Video Walkthrough

• YouTube: (187,771 views, 24 min). TESP measurement procedure with discussion of pressure drop components and how to isolate them

• YouTube: (250,236 views, 94 min). Comprehensive static pressure and airflow session covering budget concepts, component isolation, and field measurement techniques

• YouTube: (3,389 views, 39 min). Static pressure workflow in measureQuick, including using pressure data to build component-level service recommendations

• YouTube: (6,296 views, 7 min). How static pressure patterns in the return side reveal duct leakage

• YouTube: - Covers the relationship between filter area, airflow, and pressure drop

• YouTube: (23,171 views). Jim Bergmann discusses how "most technicians think of when they think of measuring air flow, they immediately think of static pressure," and why understanding each component's contribution is critical

Tips & Common Issues

Coil pressure drop is hard to isolate on some equipment

On packaged units or tightly configured air handlers, there may not be a convenient location to drill a test port between the blower and coil. Options:

• Use the manufacturer's published coil pressure drop at rated airflow as an estimate.
• On systems where you can access the blower compartment, use a flexible static pressure tip inserted through the blower access panel.
• Note the coil drop as "estimated" in your budget. The filter and duct portions can still be measured directly, and the coil contribution can be calculated as the remainder.

The four-point static pressure test

Jim Bergmann references a "four-point static pressure test" in the Cooling Commissioning walkthrough: "you'll test in the return after the filter, before the coil, and then in the supply duct." This four-point test gives you all the data needed for a complete component budget in a single set of measurements.

High filter pressure but the filter looks clean

A filter can produce high pressure drop when new if it is too restrictive for the system. MERV 13 and above filters require larger filter racks or lower face velocities to avoid excessive restriction. If a brand-new filter is already at 0.20+ inWC, the filter is too restrictive for this system's configuration, regardless of its cleanliness.

Budget numbers do not add up exactly

Measuring each component involves multiple probe placements, and small errors accumulate. If your budget total is within 0.05 inWC of the measured TESP, that is acceptable. Larger discrepancies suggest a measurement location error or that the blower speed changed between readings.

90% of new construction callbacks were airflow related

DR Richardson (HVAC company owner) reported that "90% of our callbacks were airflow related. Static pressure was getting overlooked." Component pressure drop analysis is the tool that identifies exactly which airflow component needs attention - preventing the callback before it happens.

Related Articles

Prerequisites (complete these first):

• Total External Static Pressure - TESP measurement and pass/fail evaluation
• TESP Budget & 140% Rule - The budget framework and threshold that this article extends

Follow-up articles (next steps after this one):

• Return Duct Airflow Measurement - Identifying duct leakage from static pressure patterns
• Troubleshooting Low Airflow - Diagnostic approach when CFM is below target

Related in the same domain:

• Target Zones - How component measurements are evaluated against acceptable ranges
• Manometer Placement: Air Handler - Probe placement details for air handler measurements
• Manometer Placement: Furnace - Probe placement details for furnace measurements
• Static Pressure Screening Test - Quick TESP assessment before a full budget analysis

Need Help?

If you have questions about component pressure drop measurement or interpretation:

• Check the Related Articles section above
• Contact measureQuick support: support@measurequick.com