Pressure's on!
Don't let this number scare you—it tells you the pressure of the refrigerant returning to the compressor and is commonly called the “low side.”
What is suction pressure?
A technician measures 24 volts across a relay coil but the relay isn't pulling in. Before throwing the relay in the trash, this is the next thing he should check.
What is whether the coil has the proper resistance/continuity?
Before a modern gas furnace will open its gas valve, this device must prove that the inducer has established the proper draft.
What is the pressure switch?
A TXV's job is to make sure the evaporator gets enough refrigerant while helping maintain this important measurement at the evaporator outlet.
What is superheat?
This component takes the refrigerant vapor from the evaporator and compresses it before sending it toward the condenser.
What is the compressor?
You measure 118 PSI suction pressure and 40°F evaporator saturation temperature. If your actual suction line temperature is 55°F, this is the superheat.
What is 15°F superheat?
You shut off power and measure resistance across a contactor coil. Your meter displays OL. This means your coil has gone on permanent vacation.
What is an open coil?
The inducer runs, the pressure switch proves, the ignitor glows—but you never get flame. Assuming the ignitor is actually working, this component is next on your suspect list.
What is the gas valve?
Before condemning a TXV for high superheat, this simple condition should be verified because insufficient liquid entering the TXV can make a perfectly good valve look guilty.
What is proper refrigerant charge/liquid supply to the TXV?
This component gets extremely hot during the heating sequence and is responsible for igniting the gas on many modern furnaces.
What is the hot-surface ignitor?
You walk up to an R-410A system and find 118 PSI suction and 118 PSI head with the compressor running. Congratulations—you've discovered this condition.
What is an equalized system / compressor not pumping?
A technician keeps blowing the 3-amp fuse on a 24V circuit. He disconnects the thermostat and the fuse still blows. He disconnects the outdoor unit and the fuse stops blowing. This is where he should start looking.
What is a short in the outdoor low-voltage circuit?
The furnace fires up perfectly, but 10 seconds later the burners shut down. You check the flame sensor and find only 0.4 µA DC. The sensor is clean and properly positioned. The furnace isn't losing its mind—it's losing this.
What is flame-rectification signal? 🔥
A technician discovers the TXV sensing bulb is loose, poorly insulated, and mounted incorrectly. The system has erratic superheat and the TXV keeps opening and closing like it can't make up its mind. This is the likely culprit.
What is improper sensing-bulb installation?
It's not a compressor, but it can make a heat pump look like it's lost its mind. This component reverses the direction of refrigerant flow, allowing the same system to heat in winter and cool in summer.
What is the reversing valve? 🔄
You have low suction, high head, high superheat, and high subcooling. The refrigerant is in the system—but it isn't getting where it needs to go.
What is a restriction?
A 24V transformer reads 28V with the control circuit disconnected. Once the circuit is connected, the voltage drops to 4V and the transformer begins overheating. The technician says, “Must be another bad transformer.” The senior tech says, “OHM MY GOD… stop replacing transformers and find this.”
What is a shorted or excessive load on the 24V control circuit? ⚡
A furnace has a 35°F–65°F rated temperature rise. You measure 82°F across the heat exchanger. The customer says, “But it's heating!” The furnace is heating alright—and this safety is probably not happy about it.
What is the high-limit switch?
Superheat repeatedly swings from 6°F to 28°F while suction pressure rises and falls with it. Airflow, charge, and bulb installation are confirmed correct. The TXV is opening and closing repeatedly instead of maintaining stable superheat. This is the condition.
What is TXV hunting?
This component doesn't switch the refrigerant itself. Instead, it uses an electrical signal to control where refrigerant flows in a heat pump by moving a slide inside the valve body.
What is the solenoid coil on a reversing valve?
An R-410A system starts at 120 PSI suction, 300 PSI head, 12°F superheat and 8°F subcooling. As the system runs, suction slowly drops to 90 PSI, head rises to 325 PSI, superheat climbs to 35°F, and subcooling rises to 18°F. Airflow is confirmed good. The refrigerant charge is correct. This component has most likely gone from “doing its job” to “holding up the show.”
What is the TXV?
A condenser won't run. You have 28V R-to-C at the air handler. At the condenser, you have 28V R-to-C, but only 4V Y-to-C while the system is calling. You measure 24V from Y-to-C at the air handler. The contactor coil tests good. The wire from the air handler to the condenser tests good for continuity with power off. Yet the voltage collapses when the contactor is connected.
What is a high-resistance connection in the Y circuit?
A furnace repeatedly opens its high-limit after several minutes of operation. The filter is clean, blower speed is correct, and the evaporator coil is clean. Temperature rise is 85°F on a furnace rated for 40–70°F. Gas pressure is above the manufacturer's specification.
The airflow checks out, but the furnace is still overheating. Before replacing the limit switch, this is what you should investigate.
What is excessive gas input / overfiring?
A technician finds 40°F superheat and 20°F subcooling. He immediately declares, “Bad TXV!” But before replacing it, he discovers the evaporator has extremely low airflow.
The TXV may actually be functioning correctly. The real problem is this.
What is insufficient evaporator airflow causing a low evaporator load?
This component sits in the refrigerant circuit and contains no moving motor, no electrical coil, and no sensing bulb. Its job is to separate liquid from vapor and prevent liquid refrigerant from returning to the compressor.
When it becomes saturated, it can also help return oil to the compressor.
What is an accumulator? ❄️