What it does. Enter measured Total External Static Pressure and supply CFM. The decoder classifies the system into one of nine diagnoses (duct leakage, restriction, balanced, etc.) with field action items.
TESP Diagnostic Decoder
Field Measurements
Quick Set System Size + Target CFM
Readout
Enter measurements above
Recommended Actions
- Set the manufacturer-rated TESP from the equipment data plate
- Insert manometer probes in supply plenum & return drop, average the readings
- Measure supply CFM with a TrueFlow grid or capture hood, sum all registers
Detailed Overview
The TESP Diagnostic Decoder is a 30-second decision tool for residential service techs. It takes two field measurements (Total External Static Pressure and measured supply CFM), compares them against the manufacturer’s rated values and the system’s nominal tonnage, and returns one of nine specific diagnoses with a tailored action list.
Purpose
Total External Static Pressure on its own is a misleading metric. A “low” TESP reading can mean a perfectly tight, properly sized duct system. It can also mean a 25%-leaky duct system bypassing the registers. Without measured airflow to triangulate against, the manometer reading does not tell you which of those two systems you are looking at.
The decoder solves that by requiring both inputs at once:
- TESP vs rated. Categorized as Low (<70% of rated), Normal (70-110%), or High (>110%).
- CFM vs target. Categorized as Low (<85% of design 400 CFM/ton), Normal (85-110%), or High (>110%).
The intersection of those two axes produces nine cells. Each cell corresponds to a distinct failure mode (or healthy state), and the decoder shows the relevant verdict and field actions for the cell the system lands in.
The Nine Diagnostic Cells
| TESP / CFM | Low CFM | Normal CFM | High CFM |
|---|---|---|---|
| Low TESP | Duct leakage bypass | Minor leakage or rated TESP off | Supply-side leak or oversized ducts |
| Normal TESP | Short-circuit at the boot | Commissioned correctly | Airflow above design |
| High TESP | Restriction in the duct | ECM compensating | Over-pressurized system |
When and Where to Use It
- Cooling and heating service calls where the customer reports comfort issues (“the back room never gets cold,” “this register feels weak”).
- Post-install commissioning to verify the system is operating in spec before signing off the job.
- Equipment-replacement quotes. A high-static reading on the existing ducts means the new equipment will inherit the same problem unless the duct is also addressed. Document it in the proposal.
- Apprentice training. The decoder turns a high-judgment field call into a structured decision tree that a second-year tech can run.
Limitations
- The decoder targets residential ducted systems. Mini-split and VRF systems have different airflow signatures and are not modeled here.
- The 400 CFM/ton target is a residential default. Heat-pump heating applications may run 350-450 CFM/ton; specialty systems deviate further.
- Static pressure must be measured at the manufacturer-recommended location for the rated TESP value to apply.
Sources
- ACCA Manual D (residential duct design).
- ANSI/ACCA 5 (Quality Installation Standard).
- ASHRAE Standard 152-2014 (residential thermal distribution efficiency).
- Sherman & Walker, LBNL-47214 (2002), DeltaQ field surveys.