Cassette Not Cooling: Stuck Gas-Flow Valve, Not Compressor

A cassette that will not cool, on a system that keeps being topped up. When repeated gas is treating a symptom rather than a leak.

By Team Snowflake | Updated 24 Aug 2026

Teaching case based on recurring Snowflake service patterns; identifying details are combined.

How this presents

Weak cooling from a unit with a correct refrigerant charge, having had gas added previously to no effect whatsoever. Airflow is normal and the coil is clean. Cooling is present but well short of what the space it serves actually needs.

Setup: Daikin Cassette4-6 yearsIndustrialPioneer
Misread as: A leak that nobody can find, with the system needing gas again and again.
Check first: Whether a previous top-up produced any improvement at all

What else produces this

A correct charge alongside poor cooling separates the quantity of refrigerant from the rate at which it moves. Both produce weak cooling and only one involves a shortage. Where the charge measures correct and airflow is fine on both sides, what remains is something restricting flow within the circuit. Restricting flow is precisely what the metering valve does on purpose, and one stuck part-closed does exactly what this symptom describes.

A previous top-up producing no improvement rules out a shortage of refrigerant.
A correct charge on measurement confirms the quantity in the system is not the problem.
Normal airflow and a clean coil remove the air side from consideration on both halves.
Pressures consistent with a restriction rather than a shortage place the fault at the flow control itself.

Why it happens

A flow-control valve meters refrigerant into the indoor coil, opening and closing to match demand. Deposit, moisture or mechanical wear can leave it stuck part-closed, and from then on the coil receives less than it needs however much refrigerant the system holds. The result is weak cooling on a full charge, which is precisely the situation a top-up cannot help and will often make worse than it found it.

What resolution involves

The valve is freed or replaced and the system returns to full output without any change to the charge it already held. Worth saying plainly for anyone in this position: where a top-up made no difference, do not authorise a second one. That result is evidence about the fault, and repeating it costs money while loading the system with refrigerant it never needed.

How to tell it worked

Output measured after the valve work with the original charge in place. A system that performs correctly on the charge it already had confirms the quantity was never the issue.

Before you approve a quote

No improvement after a top-up is itself evidence

Weak cooling comes from too little refrigerant or from refrigerant moving too slowly. Only one is a top-up.
A top-up with no improvement rules out a shortage. Treat that as evidence rather than a wasted visit.
A flow-control valve stuck part-closed starves the coil while the system stays completely full.

Questions this case answers

Aircon fan running but the air stays warm: what does that mean?

It rules out a shortage of refrigerant, which is the most commonly assumed cause. That is genuinely useful, and it directs the search toward the faults that reduce flow rather than quantity.

Can a compressor be fine and the room still not cool?

It meters refrigerant into the indoor coil, opening and closing to match how much cooling is being asked for. When it sticks part-closed, the coil is starved regardless of how much refrigerant the system contains.

What does an expansion valve do, and why does it stop a cassette cooling?

No, and it usually makes matters worse. Putting more into a system that already holds the correct amount raises pressures, reduces capacity further and puts avoidable load on the compressor.

Before approving a compressor replacement, what should I ask?

Sometimes. A valve stuck by deposit can occasionally be freed, while one that has failed mechanically or electrically needs replacing. Either way the charge is left alone, because it was never the problem.

Ready to get started?

Tell us what’s going on. Symptoms, setup, photos, anything we should know. We’ll assess and come back with the right next step.

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