Articles & field guides
Refrigerants & Charging
How to Read a PT Chart: Saturation, Dew Point, Bubble Point and Glide
Use a pressure-temperature chart to turn gauge pressure into saturation temperature, and know when to use dew point vs. bubble point on blends.
R-32 vs. R-454B: The Differences That Matter on a Service Call
Side-by-side comparison of R-32 and R-454B for techs, covering composition, GWP, pressures, glide, discharge temperature, charging and safety.
R-454B for Technicians: Pressures, Charging, Tools and Safety
What changes when you service R-454B, the A2L that replaced R-410A in many new systems, from glide and liquid charging to A2L tools and leak sensors.
Superheat vs. Subcooling: What Each Tells You and When to Charge by Which
What superheat and subcooling measure, how to take them, which one to charge by for TXV and fixed-orifice systems, and how to read them together.
Troubleshooting by Symptom
AC Not Cooling? A Step-by-Step Diagnostic Checklist for Techs
Work an AC no-cool call in the right order, from the thermostat call to airflow, electrical and refrigerant charge, without guessing.
Frozen Evaporator Coil: Causes, Safe Thawing and What to Check
Why AC evaporator coils freeze, how to thaw one safely, and the order to check airflow, charge and metering once it's thawed.
Electrical & Controls
AC Compressor Not Starting: Humming, Clicking or Nothing at All
Diagnose a compressor that won't start by what it does: humming points to capacitor or locked rotor, clicking to overload, silence to the control circuit.
Condenser Fan Not Spinning: Capacitor, Motor or Control?
Find out why an outdoor condenser fan won't run, from the fan capacitor and seized bearings to the contactor, wiring and heat pump defrost.
How to Test an AC Contactor: Coil, Contacts and Voltage Drop
Test a condenser contactor the right way, from 24 V at the coil and coil resistance to pitted contacts and voltage drop under load.
How to Test a Run Capacitor: Meter Method and Under-Load Method
Test HVAC run capacitors two ways, with the meter's capacitance setting and under load with amps × 2652 ÷ volts, and know when to replace one.
Thermostat Wiring: Terminal Letters and Wire Colors Explained
What each thermostat terminal does (R, Rc, Rh, C, W, W2, Y, Y2, G, O/B, E, AUX), the usual wire colors, and how to verify a hookup.
Heating
Furnace Flame Sensor: Microamp Readings, Cleaning and When to Replace
How flame rectification works, how to measure flame sensor microamps, what readings are normal, and how to fix weak flame signal.
Furnace Pressure Switch Troubleshooting: Open, Stuck Closed and Dropping Out
Diagnose furnace pressure switch codes by measuring the inducer's draft with a manometer and checking vent, intake, hose, condensate trap and switch.
Furnace Short Cycling: 7 Causes and How to Pin Down Which One
Diagnose a gas furnace that lights and shuts off too soon, from flame sensing and limit trips to oversizing and thermostat settings.
Heat Pump Blowing Cold Air in Heat Mode: What's Normal and What's Not
Tell normal heat pump behavior (cool-feeling air, defrost) from real faults like reversing valve, O/B setup, low charge and aux heat problems.
Airflow & Ductwork
400 CFM per Ton: Where the Rule Comes From and When It's Wrong
Why 400 CFM per ton is the default cooling airflow, when to set airflow lower or higher, and how to check what a system is actually moving.
Total External Static Pressure: How to Measure It and What's Normal
Measure total external static pressure with a manometer, compare it to the equipment's rating, and use it to find airflow restrictions.