Walk through the most common reasons an AC system fails to cool: airflow, charge, electrical, and thermostat issues, using the field readings you already have.
Diagnose low airflow problems from static pressure, temperature rise, or temperature split measurements. Identify restricted filters, dirty coils, or undersized duct issues.
Diagnose blower motor faults from amperage, RPM, and airflow readings. Distinguish between capacitor failure, bearing wear, and control board issues on PSC and ECM motors.
Interpret combustion analyzer readings (O2, CO2, CO, stack temperature) to calculate combustion efficiency and flag unsafe or out-of-spec conditions.
Translate common occupant comfort complaints (too hot, too cold, stuffy, humid) into a prioritized diagnostic checklist based on system type and symptoms.
Assess compressor health from compression ratio, discharge temperature, and amperage readings. Identify worn valves, slugging risk, excessive compression ratio, and motor winding issues.
Evaluate condenser performance from subcooling, condensing temperature split, and airflow to flag fouled coils, refrigerant overcharge, or fan issues.
Enter room-by-room airflow or temperature readings to identify which branches are over- or under-supplied and estimate the damper adjustments needed.
Screen a system for common energy-wasting faults, short cycling, oversized equipment, leaky ducts, and poor controls, from basic operating data.
Determine the cause of evaporator coil freezing from suction pressure, superheat, and airflow data. Distinguish low charge, low airflow, and below-freezing evaporator conditions.
Look up common blink/flash fault codes across major furnace, air handler, and heat pump control boards and get a plain-language explanation of each.
Interpret flue gas temperature, draft, and composition readings to check venting performance and screen for incomplete combustion.
Diagnose gas furnace problems from temperature rise, static pressure, and ignition sequence data. Covers short-cycling, high limit lockout, pressure switch failures, and flame rollout.
Walk through indirect field indicators (flame behaviour, CO readings, temperature rise) that suggest a cracked heat exchanger warrants further inspection.
Diagnose heat pump faults in heating and cooling mode from field measurements. Covers reversing valve issues, low charge in heating mode, defrost problems, and balance point analysis.
Diagnose indoor humidity complaints, too humid or too dry, from equipment sizing, runtime, and ventilation data, and identify the likely root cause.
An interactive startup and commissioning checklist covering airflow verification, refrigerant charge, electrical checks, and controls sequencing.
Match common HVAC noise symptoms (rattling, squealing, banging, humming) to their likely mechanical cause by equipment type and location.
Interpret furnace pressure switch readings against manufacturer specs to determine if a switch failure, venting restriction, or inducer fault is present.
Enter suction pressure, discharge pressure, superheat, and subcooling readings to diagnose overcharge, undercharge, or other refrigerant system faults. Covers R-410A, R-32, R-22, and R-454B.
Estimate likely leak locations and severity from repeated low-charge history, oil residue observations, and system type.
Identify the cause of high static pressure from measured component pressure drops. Determine if the filter, coil, supply duct, or return duct is the restriction.
Compare current operating readings against expected performance to give an overall system health snapshot across refrigerant, airflow, and electrical.
Check thermostat reading accuracy against a reference thermometer and determine if calibration, placement, or sensor issues are affecting comfort.
Diagnose multi-zone damper and bypass issues from zone-by-zone temperature and airflow complaints, and identify the likely faulty zone or control.
How to Use Field Measurements for HVAC Diagnosis
Good HVAC diagnosis starts with measuring the right things, in the right order, before touching anything. The three measurements that tell you the most about a refrigerant system are: suction pressure (converted to saturation temperature), discharge pressure (converted to saturation temperature), and superheat at the suction line. Everything else follows from those three numbers.
The Refrigerant Diagnosis Sequence
Start with static pressure measurements on the air side before touching the refrigerant system. Low airflow causes low suction pressure and looks exactly like low refrigerant charge. Fix airflow problems first: clean the filter, check the coil, measure CFM. Only then diagnose the refrigerant circuit. The airflow diagnosis calculator walks you through this, and the static pressure test isolates the specific restricted component.
Superheat vs. Subcooling: What Each Tells You
Superheat at the suction line tells you about evaporator performance and charge level on TXV systems. Target is typically 8-12°F. Low superheat means overcharge or TXV stuck open. High superheat means undercharge or restricted liquid line. Subcooling at the liquid line tells you about condenser performance and charge level. Target is 10-15°F. Low subcooling usually means undercharge or a condenser problem. The refrigerant fault diagnosis tool and condenser performance check handle these calculations together.
Furnace Temperature Rise: The Quick Health Check
Measure return air temperature, then supply air temperature after 10 minutes of operation. The difference is temperature rise. Most furnaces specify 40-70°F rise on the nameplate. Above that means restricted airflow or oversized furnace. Below that means low gas pressure, dirty burners, or cracked heat exchanger. The furnace not heating tool interprets the reading, and the combustion analysis calculator checks whether the burn itself is clean and safe.
Related Tools You May Need
- For evaporator icing specifically, use the evaporator freeze calculator to distinguish airflow from charge as the cause.
- For compressor health, the compressor diagnosis tool checks compression ratio and discharge temperature against safe thresholds.
- For heat pumps specifically, see the heat pump diagnosis tool, which covers reversing valve and defrost cycle issues.
- For venting and combustion safety on gas appliances, use the flue gas analyzer and fault code lookup.
- For occupant comfort complaints not explained by equipment faults, try the comfort complaint solver and zoning problem solver.
Frequently Asked Questions
Airflow should always be checked before refrigerant charge, since restricted airflow produces symptoms (low suction pressure, coil freezing, poor cooling) that closely mimic low refrigerant charge. Checking the filter, coil, and static pressure first with the airflow diagnosis calculator prevents misdiagnosing an airflow problem as a refrigerant problem.
No. These tools interpret the readings and symptoms you provide to narrow down likely causes, but refrigerant handling, gas appliance repair, and electrical work require licensed certification in Canada. Use these tools to understand what's happening and communicate more effectively with a technician, not as a substitute for professional service.
Most refrigerant diagnostic tools, including the refrigerant fault diagnosis tool, need suction pressure, discharge or liquid line pressure, and suction line or liquid line temperature to calculate superheat and subcooling. These readings require refrigerant gauges and a clamp-on thermometer, and interpreting or acting on them requires refrigerant handling certification in Canada.
Yes — all AskHVAC.ca tools are fully responsive and work on any smartphone or tablet without an app install. They run entirely in your browser, and results can be exported to PDF or CSV directly from your phone. Use them freely at the job site, in the truck, or in the office.