The rules every HVAC tech works by

Pressure is a temperature in disguise. Inside the evaporator and the condenser the refrigerant is boiling or condensing, so the pressure you read converts straight to a saturation temperature. What you need to know is how far the real line temperatures sit from those saturation points. Superheat is how far the suction line has warmed past the evaporator's saturation temperature. Subcooling is how far the liquid line has cooled below the condenser's saturation temperature.

Which reading sets the charge depends on the metering device. A fixed orifice or piston cannot adjust, so superheat moves with the charge, and you charge to a target superheat worked out from indoor wet bulb and outdoor dry bulb. A TXV or EEV holds superheat steady on its own, so on those systems superheat stops telling you about the charge. You charge them to the subcooling on the unit's data plate or in its installation instructions.

Two things come before any charge adjustment. Airflow has to be right, because a dirty filter or a slow blower drops suction pressure and makes a correctly charged system look short. And the system has to run long enough to settle, usually ten to fifteen minutes under a steady load, before any reading means much. Charge a system with low airflow and you will overcharge it.

When a system has been opened, evacuated and is being charged from empty, weigh the charge in: the factory charge plus the line-set adjustment from the installation instructions. Then confirm it with superheat or subcooling once it is running. The scale is exact. The readings are the confirmation.

Inside the evaporator and condenser, refrigerant is a saturated mix of liquid and vapor, and its pressure fixes its temperature. That is why a gauge reading converts to a temperature on a pressure-temperature chart. R-410A boils at 40°F at about 118 psig. R-22 boils at the same 40°F at only about 69 psig.

Outside the coils the rule breaks. The suction line carries superheated vapor and the liquid line carries subcooled liquid, so neither line's temperature can be read off the chart. That is exactly why superheat and subcooling exist: they measure the gap between the real temperature and the saturation temperature the pressure predicts.

Blends add a wrinkle. R-454B has a small temperature glide, so it boils across a narrow range of temperatures instead of at a single point. Use the dew point values for superheat and the bubble point values for subcooling, or let a digital manifold set to the correct refrigerant do the conversion.

A PT chart also tells you something before you touch a tool. A system that has been off long enough to settle should read close to the saturation pressure for the surrounding temperature. Well below it suggests there is little liquid left in the system. Above it points to non-condensables or the wrong refrigerant.

Air and moisture are the two things a sealed system cannot tolerate. Non-condensable gases raise head pressure. Moisture reacts with refrigerant and oil to form acids and sludge, and it can freeze at the metering device. Pulling a deep vacuum lowers the pressure so far that water boils at room temperature and leaves as vapor.

Measure it with a micron gauge connected at the system, as far from the pump as practical, not at the pump itself. A pump can read deep while the system is still wet, because the hoses and valve cores between them choke the flow. Pull the Schrader cores with a core removal tool and use short, large-diameter hoses, and the job goes several times faster.

Many manufacturers call for 500 microns or lower. Then comes the part that matters most: isolate the pump and watch the gauge. A reading that rises a little and levels off is moisture still giving off vapor, so keep pulling. A reading that climbs steadily without stopping is a leak, and no amount of pumping will fix it. Find it first with a standing nitrogen pressure test.

Break the vacuum with the system's refrigerant or with dry nitrogen, never with air. Change the vacuum pump oil often, because oil that has absorbed moisture limits how deep the pump can pull.

A typical cooling system is designed around roughly 350 to 450 cubic feet per minute of air for each ton of capacity, with 400 CFM per ton the usual starting point. Humid climates often run lower to pull more moisture out of the air. The manufacturer's data and the design load set the real number for a given system.

When indoor airflow drops, the evaporator gets less heat to absorb. Suction pressure and coil temperature fall, superheat on a fixed orifice drops, and on a bad enough day the coil freezes solid. A tech who reads that as a low charge and adds refrigerant has now overcharged a system whose real problem was a dirty filter.

The usual causes are simple: a clogged filter, a dirty evaporator coil, closed or blocked registers, a crushed or undersized duct, a blower speed set too low, or a blower wheel caked with dust. Total external static pressure, measured across the air handler with a manometer, shows how hard the blower is working against the duct system and is the quickest way to tell a duct problem from an equipment problem.

The Department of Energy puts a number on the simplest fix: replacing or cleaning a dirty filter can lower an air conditioner's energy use by 5 to 15 percent. Check airflow before you connect a single gauge.

Under Section 608, technicians who maintain, service, repair or dispose of stationary equipment that could release refrigerant must be certified. EPA issues four types: Type I for small appliances, Type II for high- and very high-pressure appliances, Type III for low-pressure appliances, and Universal for all of them. Most residential and light commercial air conditioning falls under Type II.

Intentionally venting refrigerant while servicing, repairing or disposing of equipment is prohibited, and that covers the HFC substitutes as well as the older ozone-depleting refrigerants. The narrow exception is the de minimis release that happens during good-faith recovery, such as connecting and disconnecting hoses. Everything else is recovered with certified recovery equipment before a system is opened or scrapped.

Recovery cylinders are painted gray with a yellow shoulder. Never fill one past 80 percent of its capacity: liquid refrigerant expands as it warms, and a full cylinder has nowhere for it to go. Weigh the cylinder while you fill instead of guessing, keep one refrigerant per cylinder, and check the retest date stamped on the shoulder before you use it.

These rules apply whatever the size of the job. A rooftop unit and a window unit are both covered; only the certification type and the recovery requirements differ.

Since January 1, 2025, new residential and light commercial air conditioners and heat pumps made or imported for the US market have had to use a refrigerant with a global warming potential below 700, under EPA's Technology Transitions rule. That took R-410A out of new equipment, and most manufacturers moved to R-32 or R-454B.

In May 2026 EPA removed the January 1, 2026 installation deadline for systems built or imported before 2025, so existing R-410A equipment can still be installed until that inventory runs out. The rule never stopped anyone from servicing an existing R-410A system.

Both new refrigerants carry the A2L safety class: lower toxicity and lower flammability. They are hard to ignite and burn slowly, but they can burn. Keep ignition sources away from an open circuit, ventilate the work area, and use recovery machines, vacuum pumps and leak detectors rated for A2L refrigerants. A2L cylinders have left-hand threaded valves, so your adapters and hoses have to match.

Equipment designed for A2L refrigerants is listed to UL 60335-2-40, which can require a refrigerant detection system that senses a leak and responds, typically by running the indoor blower to dilute the refrigerant. Never bypass one, and follow the manufacturer's instructions for testing and replacing the sensor.

A step-down transformer supplies about 24 volts AC to the control circuit. The thermostat is a set of switches that connects R, the 24-volt supply, to whichever terminal is calling: G brings on the indoor fan, Y the compressor contactor, W the heat, and on a heat pump O or B drives the reversing valve. C is the common side of the transformer, the return path that also powers the thermostat itself.

Safety switches sit in series on that circuit: high- and low-pressure switches, the condensate float switch, door switches and limits. When one opens, the call never reaches the equipment, which is why so many no-cool calls end at a tripped float switch and a full drain pan.

Troubleshoot it with a meter, one point at a time. Confirm 24 volts between R and C at the control board, then follow the call. With Y calling, you should see 24 volts from Y to C at the outdoor unit. A switch in series that reads 24 volts across its own terminals is open; a closed switch reads close to zero. A blown 3- or 5-amp fuse on the board means something shorted the low-voltage wiring, so find the short before you replace the fuse.

Wire colors are a convention, not a rule. Red, white, green, yellow, blue and orange are common, but the terminal letter is what counts. Label every wire before you disconnect it.

A modern furnace lights in a fixed order. The thermostat calls on W. The control board starts the inducer, the pressure switch proves draft, the hot surface igniter heats, the gas valve opens and the flame sensor proves flame within a few seconds. After a delay the blower starts. If any step fails, the board shuts down and usually flashes a code, and the step where it stopped is where you start troubleshooting.

Flame proving works by flame rectification: the flame conducts a tiny DC current, measured in microamps, from the sensor to ground. A dirty or poorly grounded sensor reads low, and the furnace lights, runs a few seconds and shuts off.

Carbon monoxide is why combustion testing matters. CDC counts more than 400 Americans killed each year by unintentional carbon monoxide poisoning not linked to fires, more than 100,000 emergency department visits and more than 14,000 hospitalizations, and it lists gas- and oil-burning furnaces among the common sources. CO has no color and no smell.

Use a combustion analyzer to check CO, oxygen and flue temperature, and verify manifold gas pressure and temperature rise against the furnace's rating plate. Check the venting for the appliance's category, look for flame rollout and signs of a cracked heat exchanger, and make sure the home has working CO alarms.