A warm house, a unit that runs all afternoon, low suction pressure and high superheat. Two faults fit that description: the system is short of refrigerant, or refrigerant is being held back somewhere between the condenser and the evaporator. They need opposite fixes, so guessing wrong wastes a call.
Why do they look the same at first?
In both cases the evaporator is not getting enough liquid for the heat it is receiving. The refrigerant that does arrive boils off early, so superheat climbs and suction pressure drops. From the suction side alone, a leak and a plugged drier are indistinguishable.
What does subcooling add?
Look at the other end of the system. Subcooling measures how much liquid is sitting in the condenser.
- Undercharge: there is little refrigerant anywhere, so very little liquid backs up. Subcooling is low.
- Restriction: refrigerant piles up on the high side behind the blockage. Subcooling is normal or high, even while the evaporator starves.
That single comparison, starved evaporator plus low subcooling versus starved evaporator plus high subcooling, is the most useful habit in charge diagnosis.
How do you find a restriction?
A restriction drops pressure, and liquid flashes to vapour at the drop, which cools it. So a restriction shows up as a temperature drop across a component that should have none.
- Measure both sides of the filter drier with the same instrument. A clear drop, or sweating and frost at the outlet, locates it.
- Check any other liquid line components (solenoid valve, sight glass, kinked tubing) the same way.
- If the liquid line is clear to the metering device, suspect the device itself: a TXV that has lost its bulb charge, a blocked strainer, or debris in a piston.
Brazing without a nitrogen purge is a common source of the debris that causes restrictions months later.
If it is a leak, why not just top it up?
Refrigerant does not get used up. A system that is low has leaked, and adding refrigerant without finding the leak means the customer calls back and more refrigerant is released. For systems covered by the Federal Halocarbon Regulations, 2022, charging requires a prior leak test and repair (s. 10), and a detected leak has to be repaired, isolated or recovered within seven days (s. 18). The sequence is: find the leak, repair it, pressure test with nitrogen, evacuate, weigh in the charge, then confirm subcooling.
What else can starve an evaporator?
Low load looks different: low suction with low superheat usually means too little air across the evaporator (filter, blower, iced or dirty coil). A compressor that is not pumping properly shows high suction and low head together. Always confirm airflow before you touch the refrigerant.
Sources
- Federal Halocarbon Regulations, 2022 (SOR/2022-110), ss. 10 and 18
- Red Seal task matrix: F-16.01 Troubleshoots HVAC/R systems
Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.