HVAC School - For Techs, By Techs

A liquid-line restriction should cause high subcooling, right? Not always. In this short episode of the HVAC School podcast, we look at why normal subcooling does not rule out a restriction and why one familiar diagnostic pattern cannot tell the whole story. 
Subcooling describes the refrigerant's temperature margin below saturation at a particular location. It does not directly measure how much refrigerant is stored in the condenser or prove that the metering device is receiving adequate liquid. A thermostatic expansion valve may compensate for a moderate restriction by opening farther, while a receiver can provide another place for liquid to collect. Incorrect charge, poor airflow, and other faults can also exist alongside a restriction.
The practical lesson is to follow the liquid all the way to the metering device. Pressure losses through tubing, filter-driers, valves, and vertical rises can affect liquid delivery even when the reading at the outdoor unit looks normal. Temperature differences across a filter-drier can be useful clues, but a pressure drop may be difficult to detect with temperature clamps while the refrigerant remains subcooled liquid. The episode also discusses flash-gas clues, sight glasses, and the importance of inspecting the actual refrigerant path before settling on a diagnosis.
The key takeaway is that normal subcooling at one measurement point does not clear the entire liquid line. Follow the refrigerant path and consider the equipment, valve response, receiver, and other operating evidence together.
Topics Covered
Why a liquid-line restriction can occur with normal subcooling
Subcooling as a temperature margin rather than a measure of stored refrigerant
How expansion-valve response can mask a moderate restriction
How receivers change refrigerant storage and the diagnostic picture
Why multiple faults can exist at the same time
Pressure losses between the condenser and the metering device
The limits of temperature checks across a filter-drier
Flash-gas clues, sight glasses, and liquid quality versus pressure
Avoiding kinks, excessive line-set rise, and overlooked filter-driers
This episode contains embedded sponsor messages from Refrigeration Technologies, Carrier, NAVAC, Filtrete, and the 2027 AHR Expo.
Resources
Liquid Line Temperature (video)
HVAC School calculators
Submit a podcast question
Learn about the GRIT Foundation
Purchase your tickets or learn more about the 8th Annual HVACR Training Symposium at https://hvacrschool.com/symposium.

In this episode from the 7th Annual HVACR Training Symposium, Alex Meaney delivers a live conference talk he calls "Airflow Serenity," built around the idea of knowing which duct problems are worth chasing and which changes will actually move the needle for a customer.
He opens by describing and reframing how technicians identify airflow problems, including the "complementary failure" situation where both the ductwork and the equipment are improperly sized. The way we communicate with homeowners plays a massive role in how well we can identify these problems. The time spent learning about a homeowner's comfort complaints is a sales tool in itself.
The middle of the talk is where the technical takeaways come in, including how to measure system airflow with anemometers, manometers with static pressure probes & a blower chart, and the TrueFlow Grid. The main engineering takeaway is that friction, not duct size alone, governs airflow, and that friction increases with the square of velocity — so relatively small increases in duct size or reductions in restriction along the path can produce outsized gains.
There are several practical, low-cost fixes technicians can make: pulling slack out of flex duct, adding return capacity, using proper transitions and radius fittings instead of abrupt ones, avoiding compound fittings, and making sure supply plenums have a proper end cap rather than dead-ending against a wall. These can be bundled into a system replacement.
This episode contains embedded sponsor messages from Refrigeration Technologies, Carrier, Copeland, Santa Fe Dehumidifiers, and TruTech Tools.
Resources
Mean HVAC Consulting & Design (Alex's website)
HVAC School calculators
Submit a podcast question
Learn about the GRIT Foundation
Purchase your tickets or learn more about the 8th Annual HVACR Training Symposium at https://hvacrschool.com/symposium.

In this short podcast episode, Bryan talks about compression ratio and compressors, particularly why you see more suction pressure when the compressor unloads and what that means for the HVAC system's operation.
When the compressor drops into low-stage or unloads, the suction pressure goes up. Regardless of the cause of reduced pumping, the head pressure will also drop as the suction pressure rises. If the conditions stay the same and the compressor stages down, the pumping capacity drops; the compressor moves less vapor while the evaporator still needs to absorb the same amount of heat. 
However, if the indoor airflow also drops along with the compressor stage, the load on the evaporator also changes and drops the suction pressure. Dynamic metering devices (such as TXVs and EEVs) also need to equalize and factor into the compression ratio.
We should understand the sequence before condemning a compressor and then use that sequence in diagnosis. To troubleshoot and test staging, we want to identify the equipment and sequence (including how the manufacturer wants us to test staging) and then record everything at each stage or at least a few different points. We want to see how the equipment responds, such as if one part does not respond when the compressor drops to a lower stage.
This episode contains embedded sponsor messages from Refrigeration Technologies, Carrier, NAVAC, TruTech Tools, and the 2027 AHR Expo.
Resources
Compression Ratio (video)
Why Compression Ratio Matters (tech tip)
HVAC School calculators
Submit a podcast question
Learn about the GRIT Foundation
Purchase your tickets or learn more about the 8th Annual HVACR Training Symposium at https://hvacrschool.com/symposium.

In this episode, Bryan welcomes back Louise Kellar of UEi Test Instruments for a deep dive into getting more out of a digital clamp meter than the basic volts, amps, and ohms most techs rely on every day. The tool in the spotlight is UEi's High Accuracy Clamp Meter (HAC), which is accurate when dealing with the low current draws common on modern condenser fan and blower motors and is critical for reliable wattage and capacitor-under-load testing. 
The podcast also walks through a series of lesser-used but valuable meter functions, including microamp measurement for flame rectification testing, diode testing for diagnosing boards on mini-splits, the Low Z setting for eliminating ghost voltage readings and safely bleeding capacitors, and the safety-locking temperature toggle that prevents technicians from accidentally crossing voltage and thermocouple leads.
Bryan and Louise also cover common mistakes, such as not knowing when to use continuity or resistance and auto-ranging mistakes, as well as some crucial takeaways about true RMS measurements, temperature settings, and CAT safety ratings.
This episode contains embedded sponsor messages from Refrigeration Technologies, Carrier, NAVAC, and Filtrete.
Resources
UEi products, manuals, and resources 
HVAC Multimeter Skills: Continuity vs. Resistance & Diode Testing (video)
CAT Ratings on Electrical Multimeters (tech tip)
HVAC School calculators
Submit a podcast question
Learn about the GRIT Foundation
Purchase your tickets or learn more about the 8th Annual HVACR Training Symposium at https://hvacrschool.com/symposium.

In this short podcast, Bryan and Roman discuss premature coil failure in ductless mini-splits and VRF equipment. 
These coils are susceptible to a few different types of corrosion. Warmer coil saturation temperatures and shorter runtimes result in lower latent capacity; whatever moisture is on the coil dries on the coil instead of draining out. Minerals and chemicals (including chlorides from bleach or cleaners) interact with the coil and may cause corrosion, including formicary ("ant's nest") corrosion. Leaks and corrosion may be environmental rather than entirely attributable to manufacturer defects.
Technicians can use proper cleaners with correct dilution ratios and rinse the coils. In situations where the environment contributes to coil corrosion, such as with substances off-gassing or dealing with coastal air, we will need to dilute the pollutants by ensuring proper ventilation. Dilution needs to be intentional, especially in hot-humid climates, and ventilating dehumidifiers with good filtration may be a good move. Source control and filtration are also good mitigation strategies, and clear, open client communication can help clients understand the problem and make informed decisions about their HVAC system.
This episode contains embedded sponsor messages from Refrigeration Technologies, Carrier, NAVAC, and Fieldpiece.
Resources
Coil Corrosion w/ Kevin Hart and Bryan Orr
Living in a World of Acids (and why HVAC coils are secretly crying for help)
HVAC School calculators
Submit a podcast question
Learn about the GRIT Foundation
Purchase your tickets or learn more about the 8th Annual HVACR Training Symposium at https://hvacrschool.com/symposium.