AHU Selection & Fault Diagnosis Handbook: A Practical Review Workflow
An AHU selection can show the required cooling capacity and still leave important questions unanswered. Are the airflow and pressure figures on the same basis as the specification? Will the coil control humidity? Can the fan deliver the duty as filters load? A useful review connects those questions before equipment is ordered.
Nexora Design Lab’s new Air Handling Unit (AHU) Selection & Fault Diagnosis Handbook brings selection, submittal review, controls, commissioning and troubleshooting into one practical reference. This guide shows how to use it, with a water-flow check you can follow before opening a manufacturer’s selection program.
What you receive
The first edition, published in 2026, is a 182-page A4 PDF for HVAC and MEP engineers. Its cover title is Air Handling Unit Selection & Troubleshooting Handbook; the product listing calls the same product Selection & Fault Diagnosis Handbook.
16 chapters in four parts, from requirements and components to procurement, commissioning and operation
32 worked examples with stated assumptions, plus 11 hypothetical troubleshooting cases
Three annotated synthetic selection sheets and 16 RFI templates
24 technical figures, chapter review checklists and eight reference appendices
SI units with labelled IP conversions, clickable contents and PDF bookmarks
The baseline is a hot-and-humid climate, including the high dew points, dust and coastal exposure familiar to Gulf projects. Guidance boxes explain where practice changes in other climates. The examples and supplier names are illustrative; they are not claims about completed projects or certified equipment performance.
Start with the review question in front of you
You do not need to read the handbook in order to use it. For a new specification, begin with Chapters 1–4: arrangements, project requirements, design conditions, airflow and loads. For a submitted AHU, use the component chapters alongside Chapter 13’s review and offer-comparison method. For a unit already on site, move to commissioning in Chapter 14 or troubleshooting in Chapter 15, then trace the finding back to its design basis.
This structure helps keep a review specific. “Check coil selection” becomes a request for the entering-air state, leaving dew point, water temperatures, flow and pressure drop at one identified duty. “Check fan” becomes a documented pressure budget and operating point.
Five checks to carry into an AHU submittal review
Define the air state and airflow basis. Record outdoor and return conditions, the mixed-air duty, altitude or pressure where relevant, and whether volume flow is actual or stated at a reference density.
Separate temperature from moisture performance. Review total and sensible capacity alongside leaving humidity ratio or dew point and the intended part-load humidity sequence.
Reconcile the pressure and power boundaries. Identify which internal and external losses are included, the wet-coil condition, filter resistance and whether quoted power is shaft or electrical input.
Check the complete unit. Casing, filtration, condensate drainage, acoustics, service access and controls can all affect whether a technically plausible selection is usable.
Close the evidence trail. Link each acceptance criterion to a drawing, schedule, manufacturer output or agreed test, and retain unresolved questions until answered.
These are review prompts, not universal acceptance limits. The contract, adopted codes, authority requirements and selected manufacturer’s data govern the actual project. For a detailed pressure-ledger example, see wet-coil pressure drop and dirty-filter fan duty.
Worked example: a coil can meet the load at the wrong water flow
The free preview includes the handbook’s illustrative Example WE-5.1 on printed page 51. Consider its 145.3 kW cooling duty and chilled-water temperatures of 4.5°C supply and 13.5°C return. The water temperature rise is 9.0 K. Use the example’s water specific heat of 4.19 kJ/(kg·K).
Water mass flow = cooling duty ÷ (specific heat × water temperature rise).
At 9.0 K: 145.3 ÷ (4.19 × 9.0) = 3.853 kg/s.
For a simple screening conversion using water density of approximately 1,000 kg/m³, this is approximately 3.85 L/s. Use the actual fluid properties for a detailed selection, particularly where glycol is present.
Now suppose an alternative selection meets the same 145.3 kW duty using a 5.5 K water temperature rise.
At 5.5 K: 145.3 ÷ (4.19 × 5.5) = 6.305 kg/s, approximately 6.31 L/s.
The second flow is about 64% higher. Both energy balances can produce the same headline cooling duty, yet they impose different flow requirements. If the project requires 4.5/13.5°C water and limits coil flow accordingly, the 5.5 K selection has not demonstrated compliance merely by matching capacity.
This is a comparison of illustrative design conditions, not a prediction of energy savings. Changing the required water temperature rise can affect coil area, rows, circuiting, water pressure drop and part-load behavior. The supplier must demonstrate the complete selection at the actual project conditions.
The same preview example also checks face area and tube velocity. With finned dimensions of 1.22 m × 1.60 m, the face area is 1.952 m². Its stated reference-basis airflow of 4.684 m³/s gives 4.684 ÷ 1.952 = 2.40 m/s on that same basis. An actual-condition face-velocity check must use the actual volume flow. Carryover limits remain specific to the selected coil and arrangement.
Manufacturer guidance provides a useful companion: Trane’s coil-selection newsletter discusses how water temperature difference interacts with coil selection. Confirm the standards and editions adopted for your project rather than treating a historical worked example as a current universal requirement.
Turn that calculation into a useful RFI
A concise review comment should identify the mismatch and the evidence needed to resolve it. For the example above, an original RFI could read:
“For AHU-[tag], the scheduled coil duty is 145.3 kW at 4.5/13.5°C chilled water. The submitted selection uses a 5.5 K water temperature rise and approximately 6.31 L/s. Please resubmit at the specified water conditions and confirm total and sensible duty, leaving dew point or humidity ratio, water flow, coil pressure drop and part-load performance. Identify any required deviation for formal review.”
Replace the tag, conditions and acceptance criteria with the actual project requirements. The handbook’s Appendix B provides 16 RFI templates covering recurring gaps such as airflow basis, ESP inclusions, coil energy balance, leaving dew point, casing evidence, drainage and fan-array redundancy.
Use the preview to inspect the working style
The ten-page preview is a useful way to decide whether the handbook fits your work. It contains selected pages rather than a miniature version of every chapter. You can inspect:
A psychrometric process chart on preview page 4
The face-area, water-flow and tube-velocity example on preview page 5
Cooling-coil common mistakes and a review checklist on preview page 6
Fan curves and filter-condition discussion on preview page 7
An offer-comparison workflow and synthetic quotations on preview page 8
Troubleshooting examples on preview page 9
Some sample sections continue onto pages outside the preview. The full handbook connects the calculations, component checks, annotated selections and reference material.
After handover: use measurements to narrow the fault
The troubleshooting section addresses problems such as poor cooling, high humidity, airflow shortfalls and condensate issues through hypothetical cases. A sensible starting point is to record the operating mode, actual conditions and trustworthy measurements before changing equipment or control settings.
For example, a temperature complaint and a humidity complaint may require different evidence. A room reaching its temperature setpoint does not establish the required moisture performance. Likewise, a mixed-air sensor can misrepresent the coil-entering condition when the airstream is stratified. The companion article on AHU mixing-box stratification and misleading sensor readings explores that measurement problem.
Use site-approved procedures and qualified personnel for inspections and testing. Do not bypass interlocks, open energized equipment or approach exposed rotating parts to obtain a reading.
Who will find this handbook useful?
Design engineers can use it to build clearer AHU requirements. Consultants and technical reviewers can use its sequence to interrogate a submittal. Contractor engineers and estimators can use the offer-comparison material to identify scope and performance differences. Commissioning and facilities teams can use the checklists and cases to organize evidence from installed equipment.
It is an engineering reference, not selection software, a certification course or a substitute for the engineer of record. Calculations are screening and review tools. Final decisions require project-specific requirements, current adopted standards, certified manufacturer data where applicable and professional judgment.
Explore the AHU handbook
Download the free 10-page AHU handbook preview to inspect actual pages, then get the complete AHU handbook when you are ready for the complete reference.
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