Q & A
Got an HVAC question? Leave it in the comments of any AC Service Tech training video on YouTube. We have answered over 12,000 comments so far. Below are the questions we get asked most often.
Charging and Checking the Charge
Why must R-410A be charged as a liquid?
R-410A is a blend, so it has to leave the bottle as a liquid to keep the mix consistent. R-22 is a single component refrigerant and can be charged as either liquid or vapor.
Why can't I charge liquid into the liquid port while the system runs?
Pressure differences would push refrigerant back into the bottle. Charge into the low side instead.
How do I check the refrigerant level?
Use Total Superheat for fixed orifice systems and Subcooling for TXV systems, after a short run time, unless the manufacturer specifies otherwise. See Charging Procedures.
Why isn't there one target pressure for every system?
Pressures vary with indoor and outdoor temperatures, airflow, SEER rating and metering device. Always verify with subcooling, superheat, or the manufacturer's method.
How can I tell if a unit is low on refrigerant?
Identify the metering device first, then apply the superheat or subcooling method for that type.
How do I determine the refrigerant type?
Compare the equalized pressure, after the system has been off for an hour or more, with the saturation values for known refrigerants on the P/T Chart.
Is ambient plus 30 a charging method?
It is a rough rule of thumb carried over from R-22 condensers. It cannot set a charge accurately or diagnose the system.
Why use Total Superheat rather than superheat?
Total Superheat is easy to measure at the outdoor service port and it confirms that only vapor is reaching the compressor.
What is the total weight charging method?
Charge the factory specified amount, plus any extra for a longer line set, after breaking the vacuum. See Refrigerant Weights.
Why can't R-410A be added as a vapor?
It has to exit the bottle as a liquid to keep its blend correct.
Why isn't feeling the suction line reliable?
It cannot confirm a full vapor state, efficiency or airflow, and saturated vapor reaching the compressor can damage it.
Why is superheated vapor still at low pressure?
In cooling mode it sits between the metering device and the compressor, so the pressure stays low while the temperature rises.
Refrigerant Leaks
How do I find a refrigerant leak?
Check for oil stains, use bubble solution, an ultrasonic detector, an electronic sniffer or UV dye, or pump down and pressure test each side. See Tools.
Should I top off R-410A after a leak, or recover and recharge?
For a small leak while the system runs, topping off is acceptable. For a severe leak while it is off, recover and weigh in new refrigerant.
Why avoid dish detergent as a bubble solution?
Modern detergents contain corrosive chemicals that can damage joints and tubing. Use a non-corrosive leak detector solution.
What pressure should I pressure test to?
Stay below the low side maximum design pressure, which is listed on the evaporator coil box. Use a lower pressure on older systems.
Brazing and Soldering
Why braze instead of silver solder?
Manufacturers braze, so we follow suit. Silver solder is used only on heat sensitive components.
Can silver solder handle R-410A pressures?
Yes, with 5% or higher silver solder. The drawbacks are acid flux and the need for tight fit-up. A 15% braze rod suits looser joints.
Why flow nitrogen while brazing?
It displaces air and prevents internal copper oxidation, which can clog the filter drier and reduce compressor lubrication.
Troubleshooting
Why does the evaporator coil freeze?
Low airflow, a low charge, or a liquid line restriction. Use superheat and subcooling once the coil has thawed to narrow it down.
Why does a heat pump frost above 32°F?
The outdoor coil has to be below 32°F to absorb heat, and humidity can freeze on the fins.
The thermostat won't turn the heat on.
Check the W to C signal, then the control board output, inducer capacitor, wheel and motor.
The thermostat is blank.
Check the 24V fuse, the batteries, the power supply, or a tripped condensate safety switch.
The blower won't shut off.
Look for a stuck limit switch, shorted G and R wires, a bad thermostat, a stuck blower relay, or a faulty ECM module.
The blower won't turn on.
Confirm the G to C signal and 24V at the control board, then check the thermostat and wiring.
The AC only blows warm air.
If the condenser fan runs but the compressor does not, check the capacitor, then the wiring, then the compressor.
Miscellaneous
Why remove valve cores for recovery?
Removing them speeds up recovery and improves gauge accuracy.
Why remove valve cores for vacuum?
Vacuuming can take 20 to 40 times longer with the cores in place, and the readings are less accurate. See Recovery Bottle.
How do I identify the metering device?
Check the indoor unit rating plate, then verify visually by removing the coil cover.
Piston or TXV?
A piston delivers a fixed flow. A TXV modulates flow to match the heat load.
What does SEER mean?
Seasonal Energy Efficiency Ratio. A higher SEER generally means more heat transfer surface and lower energy use.
Still have a question? Leave it on a YouTube video, or see About and Support.