Aircon EEV: Why Inverter Systems Need Active Metering
Something has to stand between the high-pressure side and the indoor coil, deciding how much refrigerant gets through. Older equipment settled that at the factory and never revisited it. An inverter cannot work that way, and the part that replaced it fails looking like low gas.
By Team Snowflake | Updated 16 Sept 2026
What the metering stage is there to do
Every cooling circuit has one point where high-pressure liquid is let down to low pressure: the metering stage. Refrigerant leaves the outdoor coil warm and under pressure and has to reach the indoor coil cold enough to boil against room air.
Pressure sets the temperature at which refrigerant boils, so the drop is the whole trick. Hold a liquid under pressure and it boils hot; release the pressure and it boils cold. Refrigerant pushed through a narrow gap arrives on the far side at a much lower pressure.
The same gap also settles quantity, and that half gets far less attention. Too little through, and the coil runs out of liquid before the air has finished crossing it, so the remaining fin area does nothing. Too much through, and liquid is still present at the outlet, travelling back toward a compressor built to handle gas.
Two jobs therefore sit on one restriction. Pressure drop and flow rate are set by the same gap, so neither can be adjusted without moving the other. Every metering design answers a single question: how wide that gap ought to be at this moment.
Where the metering stage sits
On a household split system the metering usually sits at the indoor unit, on the thin liquid line arriving from outside; some designs place it in the outdoor unit instead. Either way it marks the border between the high side of the circuit and the low side.
Systems with more than one indoor unit meter each branch on its own. A single outdoor unit feeds several rooms, and each needs its own answer because each carries its own heat load. That detail matters later, since a metering fault on one branch leaves every other branch untouched.
Three ways to set the gap, in order of control
The oldest answer is a fixed length of very narrow tubing with nothing moving inside it. Bore and length are chosen for one model at one design condition, and that choice is made at the factory. The restriction offered on a mild night is the restriction offered at the peak of a hot afternoon.
A thermostatic valve adds feedback without electronics. A sensing bulb clamped to the pipe leaving the coil is filled with a small charge of its own. Pipe temperature moves the pressure inside that bulb, and the bulb pushes on a diaphragm linked to the valve: warm gas opens the valve wider, cold gas closes it down.
The electronic version replaces that mechanical link with an instruction from the control board. A small stepper motor turns a threaded shaft in counted increments, and each increment sets the size of the opening. Thermistors on the pipework report what the refrigerant is doing, and the board sends a new step count.
The three differ in authority rather than accuracy. A fixed tube holds no information and can act on none. A thermostatic valve senses one condition and answers it mechanically. An electronic valve is directed by a board that can weigh coil readings, compressor speed and room demand together, and hold a chosen position deliberately.
| How the gap is set | What it can respond to | Where it runs out of answers |
|---|---|---|
| A fixed bore sized once at the factory | Nothing. The restriction it offers never changes | Correct at the condition it was sized for, drifting wrong on either side of it |
| A mechanical valve driven by a sensing bulb | Temperature at the coil outlet, and nothing beyond that | Answers after the fact, and only inside a narrow band of movement |
| A stepper-driven opening commanded by the board | Coil readings, compressor speed and room demand at once | Depends on its sensors and its firmware being right about the circuit |
- How the gap is set
- A fixed bore sized once at the factory
- What it can respond to
- Nothing. The restriction it offers never changes
- Where it runs out of answers
- Correct at the condition it was sized for, drifting wrong on either side of it
- How the gap is set
- A mechanical valve driven by a sensing bulb
- What it can respond to
- Temperature at the coil outlet, and nothing beyond that
- Where it runs out of answers
- Answers after the fact, and only inside a narrow band of movement
- How the gap is set
- A stepper-driven opening commanded by the board
- What it can respond to
- Coil readings, compressor speed and room demand at once
- Where it runs out of answers
- Depends on its sensors and its firmware being right about the circuit
Why fixed metering lasted as long as it did
Its weakness only turns expensive once the rest of the system starts moving. Fixed metering with fixed output is a matched pair; fixed metering with variable output is a mismatch, and the mismatch shows up as capacity quietly not delivered.
A compressor that changes speed changes the sum
An inverter compressor has a range of outputs rather than one. Speed climbs while a flat is still hot and drops back once the room has settled, and each speed sends a different amount of refrigerant round the loop. The metering stage has to size its gap for whatever amount is arriving.
A fixed restriction can only be correct once. Size the bore for full output and it throttles the circuit at low speed; size it for low speed and it floods the coil as soon as the compressor winds up. It is right at one operating point.
So the valve is not an accessory bolted onto an inverter system; it is what makes the modulation usable at all. Compressor drive, control board and valve form one control loop with three parts, and the loop was designed as a whole. Losing sight of the third part is why a metering fault so often gets attributed to the compressor or the charge.
- Compressor speed, which the board changes continuously as the room approaches its setpoint
- Heat load in the room, which rises with afternoon sun on the wall and with people in it
- Conditions around the outdoor unit, which decide how much heat the circuit can shed
- Indoor fan speed, which sets how much warm air actually crosses the coil
- On a multi-split, how many indoor units happen to be calling at the same time
What active metering buys across the load range
The figure the board is holding steady is superheat, and superheat measures how completely the coil was used. Liquid enters the indoor coil, boils along its length, and should finish boiling a little before the outlet; how far the gas warms past that point is the reading.
Holding that figure steady across the load range is what a fixed bore cannot do. Compressor speed, room temperature and outdoor conditions all shift where boiling finishes inside the coil. A valve that can reopen or close down keeps the finishing point roughly where it belongs whatever else moves.
The penalty at each end of the range is worth naming. Starve the coil and its last stretch carries gas that has already boiled, warming up and doing nothing useful, so capacity the equipment was paid for is not delivered. Flood the coil and liquid reaches the outlet, then travels down the return line into a machine built to compress vapour.
Constant movement is the design, not a symptom
An electronic valve rarely sits still on a running system, because the conditions it answers to keep shifting. Small corrections, made continuously, are what a steady superheat looks like from the inside. An owner who hears that the valve is always adjusting has been told the machine is working.
The abnormal state is a valve that has stopped moving. A shaft seized by debris or corrosion, a stepper that has lost its drive signal, and a board that has stopped issuing steps all end in the same place. The opening freezes at whatever size it last held, and the system reverts to fixed metering.
A valve stuck part closed reads as low gas
A partly closed valve and a partly empty circuit make the same complaint, because both starve the coil. Less refrigerant reaches the evaporator than the design assumed. Low-side pressure falls, cooling weakens, the flat takes longer to settle, and in the worse cases part of the coil frosts over.
The gap then gets filled by the likelier cause, and the likelier cause is refrigerant. Loss through a joint is common and valve failure is not, so a weak-cooling call gets read as a leak before anything is measured. The hose comes out and the charge goes up.
The billing pattern that follows is the reason to know any of this. A top-up billed against a jammed valve gets paid for and leaves nothing behind. The complaint returns, and the second visit reads that return as proof of a larger leak, so a leak search is quoted for an opening that never existed.
Three things separate the two before the circuit is opened. Charging history comes first: a system that has taken gas more than once with no leak point named is describing a flow problem rather than a loss. Room selectivity comes second, because branch metering means a fault at one valve leaves the other rooms cooling normally, while a circuit short of refrigerant pulls every room down together.
| What the household reports | What a refrigerant shortage would need | What a jammed valve explains directly |
|---|---|---|
| Gas has gone in more than once and no leak point was ever named | An opening nobody has managed to locate across repeat visits | Nothing left the circuit, so a search was always going to come back clean |
| One room underperforming while every other room holds its setting | Awkward, since a shared circuit loses its charge for everybody | Direct, because that branch carries a valve of its own |
| Cooling that fell away with no run-up to it | Possible only on a fast loss through a sizeable opening | Ordinary. A shaft that seizes gives no notice beforehand |
| A charge added against a gauge rather than weighed in | Unproven. The circuit was never confirmed short of anything | Consistent, since a restriction produces gauge readings of the same shape |
- What the household reports
- Gas has gone in more than once and no leak point was ever named
- What a refrigerant shortage would need
- An opening nobody has managed to locate across repeat visits
- What a jammed valve explains directly
- Nothing left the circuit, so a search was always going to come back clean
- What the household reports
- One room underperforming while every other room holds its setting
- What a refrigerant shortage would need
- Awkward, since a shared circuit loses its charge for everybody
- What a jammed valve explains directly
- Direct, because that branch carries a valve of its own
- What the household reports
- Cooling that fell away with no run-up to it
- What a refrigerant shortage would need
- Possible only on a fast loss through a sizeable opening
- What a jammed valve explains directly
- Ordinary. A shaft that seizes gives no notice beforehand
- What the household reports
- A charge added against a gauge rather than weighed in
- What a refrigerant shortage would need
- Unproven. The circuit was never confirmed short of anything
- What a jammed valve explains directly
- Consistent, since a restriction produces gauge readings of the same shape
The sensor gets blamed last and should be cleared first
A valve does what it is told. If the thermistor reporting coil temperature has drifted, the board issues a wrong instruction and the valve carries it out faithfully. The circuit then behaves as though the metering has failed, while the metering is doing exactly what it was asked.
That matters because the two repairs sit in different categories of job. A sensor sits outside the sealed circuit and comes off with a screwdriver. A valve sits inside it, and reaching one means recovering the refrigerant first. Clearing the outside-the-loop suspect before committing to the inside-the-loop one is the order any sound sequence follows.
Metering is diagnosed by reading, not by looking
What can be observed is the effect. Pressure and pipe temperature taken either side of the metering stage describe what is passing through it. A restriction shows as too large a drop across that stage while the circuit still holds a full charge. Pressure on its own cannot tell a starved coil from an empty circuit.
The electronic version carries one advantage the fixed kinds lack: it can be commanded. Service software can drive the valve open and closed while readings are watched, and a valve that will not move gives a flat response no refrigerant shortage would produce.
The order of the work is what protects the customer. Charge gets confirmed by weight before the valve is blamed, because a circuit genuinely short of refrigerant imitates a restriction in every reading. Airflow gets confirmed before either, because a coil starved of air and a coil starved of liquid look alike from the room.
- Whether the charge was established on a scale rather than judged off a gauge
- Which readings were taken either side of the metering stage, and under what running condition
- Whether the valve was driven open and closed, and what it did when it was
- Whether the sensors feeding the board were checked before the valve was condemned
- Whether the branch showing the fault is the only branch affected
What an owner can settle without tools
Two facts decide whether any of this applies, and both are available in the flat. The first is whether the system modulates at all, which the model number on the outdoor unit will confirm. Fixed metering on a single-speed unit fails in its own way and is a separate conversation.
The second is the charging record. How many gas visits this system has had, and whether any ended with a leak point actually named, is worth more to a technician than anything else you can supply. Writing it down takes nothing and narrows the search before anyone has arrived.
Common questions
What does an electronic expansion valve do in an aircon?
Why can an inverter aircon not run on a fixed capillary tube?
How is a stuck expansion valve mistaken for low refrigerant?
Does an electronic expansion valve stay still on a running system?
Sources
- Electronic Expansion valves on Emerson XJ condensing units
Copeland · Checked
EXV opening is recalculated from thermistor readings as conditions change.
- Daikin VRV X RXQ-A(N)R Service Manual SiME341909EA
Daikin · Checked
Service manual labels the Electronic Expansion Valve as a circuit component.
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