The system is not reaching setpoint. The space is too humid. The equipment is short cycling. There are unusual noises from the air handling unit. The air quality complaints have started.
When an HVAC system is not performing as it should, the problem could be in the refrigeration circuit, the air distribution system, the controls, the duct system, or the equipment itself. Without a systematic diagnostic approach, troubleshooting becomes guesswork - and guesswork leads to replacing parts that are not the cause, wasting money and valuable time.
Why HVAC Troubleshooting Requires More Than Experience
Experienced technicians and engineers develop intuition over time. But intuition without a framework leads to the most common troubleshooting mistake: treating symptoms instead of causes.
A space that is too warm and too humid could mean any of the following:
- Insufficient cooling capacity
- Low refrigerant charge
- Fouled evaporator coil
- Inadequate airflow across the coil
- Overcrowded space or increased internal loads
- Controls fault preventing the unit from running at design conditions
The symptoms are identical. The cause - and therefore the fix - is different in every case. The only way to distinguish them is to measure actual system performance and compare it to what the system should be doing. That comparison happens on the psychrometric chart.
The Diagnostic Framework: Psychrometry as the Foundation
The psychrometric chart is not just a classroom tool. It is the primary diagnostic instrument for HVAC field work. When a system is operating correctly, measured conditions at each point - supply air, return air, mixed air, outdoor air - match the design conditions plotted on the chart. When the system has a problem, the measured conditions deviate. The pattern of deviation points to the cause.
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1Establish design conditions
Obtain original design documents: supply air temperature and humidity, return air conditions, outdoor design conditions, airflow rates. This is your baseline. If design documents are not available, calculate expected conditions for the current outdoor conditions using load calculations.
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2Measure actual field conditions
Measure dry-bulb temperature and relative humidity at supply, return, mixed air, and outdoor air points. Measure airflow rates, duct static pressures, water piping pressures, and refrigerant pressures and temperatures where accessible.
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3Plot on the psychrometric chart
Plot both design conditions and actual measured conditions on the psychrometric chart. The visual comparison immediately shows where the system is deviating and by how much.
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4Diagnose from the deviation pattern
The location and direction of the deviation on the chart points to the cause. Use the table below as a guide.
| Deviation from design | Likely cause |
|---|---|
| Supply air too warm, correct humidity ratio | Insufficient sensible cooling - check refrigerant charge, coil fouling, or airflow |
| Supply air correct temperature, high humidity | Insufficient latent cooling - refrigeration circuit issue or coil bypass |
| Both temperature and humidity elevated | System not running at design capacity - check compressor, refrigerant charge, or controls |
| Short cycling | Oversized equipment, low load, or refrigerant pressure fault |
| Correct conditions at coil, poor space conditions | Distribution problem - check duct leakage, diffuser placement, or airflow balance |
Common HVAC Problems and What Causes Them
- Dirty filters or blocked return
- Failed fan belt, low airflow
- Low refrigerant charge
- Fouled evaporator or condenser coil
- Loads exceeding design
- Chilled water supply temperature too high
- Low refrigerant charge
- Coil bypass - damaged baffles or air leakage
- Low airflow across coil
- Controls setting reheat prematurely
- Oversized equipment for actual load
- Low refrigerant causing pressure trips
- Dirty filters triggering safety cutouts
- Faulty thermostat or sensor placement
- Rattling: loose components or duct resonance
- Squeal: worn fan belt or bearing failure
- Banging on startup: liquid slugging - serious
- Air noise: high velocity, undersized ducts
- Inadequate outdoor air - check economizer
- Contaminated filters or coil
- Pressure imbalance between spaces
- Controls not maintaining ASHRAE 62.1 minimums
Field Testing: What You Measure and Why
- Use calibrated psychrometer or digital data logger
- Measure at: supply air diffuser, return air grille, mixed air plenum, outdoor air intake
- Always measure dry-bulb temperature and relative humidity together - RH alone is meaningless without temperature
- Plot all points on psychrometric chart immediately
- Use anemometer or pitot tube traverse
- Compare measured CFM to design airflow for each zone
- Balanced airflow is the foundation of correct system performance
- Duct static pressure: measure at fan outlet, before and after filters, and at key distribution points; compare to design to identify restrictions, leakage, or fan performance loss
- Water piping pressure: measure at pump inlet and outlet and across coils and heat exchangers; compare to design pressure drops to identify fouling, blockages, pump deterioration, or valve issues
- Suction pressure, discharge pressure, suction superheat, subcooling
- Compare to manufacturer charging charts at current conditions
- Superheat and subcooling values confirm refrigerant charge and expansion valve operation
- Compressor amperage vs. nameplate full load amps
- Fan motor current
- Voltage at equipment terminals
Humidity Control Failure at the McMaster Art Museum
The McMaster Art Museum in Hamilton, Ontario could not maintain the humidity levels required to protect its artifacts. Temperature control was functioning, but humidity remained outside acceptable limits despite the HVAC system appearing to operate normally.
Field measurements were taken at the cooling coil - supply and return air conditions were recorded and plotted on the psychrometric chart alongside the design conditions. The chart immediately revealed the problem: the system was providing sensible cooling (lowering the dry-bulb temperature) but almost no latent cooling (dehumidification).
The cause was traced to the chilled water supply temperature entering the cooling coil. The water was not cold enough to bring the coil surface below the dew point of the air passing over it. For condensation to occur - and therefore for moisture to be removed from the air - the coil surface must be below the dew point. When the chilled water is too warm, the coil cools the air sensibly but does not condense moisture out of it. The psychrometric chart made this visible immediately: the process line was moving horizontally (sensible cooling only) rather than down and to the left (combined sensible and latent cooling) as the design required. Once the chilled water temperature was corrected, humidity control was restored.
Who Faces This Challenge
HVAC troubleshooting affects everyone responsible for building systems:
- Facility managers - managing contractors performing diagnostic work, needing to evaluate whether the proposed cause and fix are credible
- Out-of-discipline engineers - assigned building system responsibility and expected to oversee maintenance and repair
- Mechanical engineers - called in on system performance problems, needing a systematic approach beyond component replacement
- Commissioning engineers - identifying deficiencies during acceptance testing
What the Course Covers
The Design, Operation and Maintenance of HVAC Systems course covers the full troubleshooting framework as its final module - after building the foundation of psychrometry, system types, equipment operation, and maintenance procedures. This sequence matters: you cannot troubleshoot what you do not understand.
- Psychrometry and air conditioning processes - the diagnostic foundation
- Basic subsystems of HVAC systems and how they interact
- Types of HVAC systems and their normal operating characteristics
- HVAC equipment selection and rating - understanding what equipment should do
- Operation and maintenance of air subsystem components
- Operation and maintenance of refrigeration subsystem components
- Measurements and instrumentation used in troubleshooting
- Troubleshooting of HVAC systems - systematic diagnostic methods
Attendees leave able to approach a non-performing system with a methodology - not just experience and intuition.
"Very informative and the instructor was very knowledgeable. I registered specifically to better understand HVAC operation and troubleshooting. Best part of the course: pretty well everything."
"I registered to increase the team's knowledge of HVAC fundamentals and testing procedures. Best part of the course: psychrometric practice."
"Working examples are very well structured and representative. The course content shows the great depth and experience of the instructor."
Frequently Asked Questions
Refrigerant pressures only tell you about the refrigeration circuit. Most HVAC problems involve the interaction between the refrigeration circuit, the air distribution system, the building loads, and the controls. The psychrometric chart integrates all of these into one diagnostic picture. Pressures alone cannot tell you whether a humidity problem is a refrigeration fault or an airflow problem.
Relative humidity is a ratio, not an absolute measure of moisture content. The same amount of moisture in the air produces a high RH reading at low temperatures and a low RH reading at high temperatures. Without the dry-bulb temperature, you cannot determine actual moisture content or locate a point on the psychrometric chart. Both measurements must always be taken together.
At minimum: a calibrated psychrometer or digital temperature and humidity meter, an anemometer or pitot tube for airflow, a manometer for duct static pressure, a pressure gauge for water piping, and refrigerant gauges for suction and discharge pressures. A data logger that records conditions over time is useful for intermittent problems.
Dehumidification requires the cooling coil surface to be below the dew point of the air passing over it. If chilled water supply temperature is too high, or refrigerant charge is low, the coil surface may remain above the dew point - providing sensible cooling but no condensation and therefore no moisture removal. On the psychrometric chart this appears as a process line moving horizontally instead of down and to the left.
Commissioning verifies that a new or upgraded system meets its design intent before or at handover. Troubleshooting diagnoses why an existing system is not performing as it should. Both use similar measurement and diagnostic methods, but commissioning works toward defined acceptance criteria while troubleshooting starts from a complaint or observation of poor performance.
Yes. The Design, Operation and Maintenance of HVAC Systems course provides 28 formal CPD hours, all qualifying as core engineering learning toward PEO PEAK requirements. It is PEO PEAK compliant.
Design, Operation and Maintenance of HVAC Systems
5 days · 28 CPD Hours · PEO PEAK compliant · $2,495 per attendee
Introduction to HVAC Systems for Non-Technical Individuals
1 day · 6 CPD Hours · PEO PEAK compliant · $495 per attendee
Group discount: 10% off per attendee for three or more participants from the same organization.
