How the two differ in kind
A split system with a long line set needs two separate corrections, and because both are described as 'allowing for the line length' they get conflated.
DERATING is about performance. Refrigerant moving through a long line loses pressure to friction, and the system has to work harder for the same effect — so the capacity delivered falls below the nameplate and the energy used per unit of cooling or heating rises. The VERTICAL separation between the indoor and outdoor units matters separately and often more: lifting refrigerant costs pressure, and returning oil up a riser depends on the gas velocity being high enough to carry it, which is why manufacturers state a maximum lift as well as a maximum length and why the direction of the lift — indoor unit above or below — changes the figures. The correction is published as a factor against equivalent length and lift.
CHARGE ADJUSTMENT is about quantity. A system is factory-charged with enough refrigerant for a standard line length; a longer run has more internal pipe volume to fill, and the additional charge is stated as an amount per unit of extra length for that line diameter. It is not optional and it is not approximate.
The consequences of getting the charge wrong are not marginal. UNDERCHARGE starves the evaporator, lowers suction pressure, returns insufficient oil, and runs the compressor hot — which shortens its life. OVERCHARGE raises discharge pressure and can return liquid refrigerant to the compressor, which does not compress and causes immediate mechanical damage. Both are compressor failures by different routes, and both are installation errors rather than product faults.
Both corrections use EQUIVALENT length rather than measured length: every bend, every fitting and every trap adds a pressure loss expressed as a length of straight pipe, so a run with many bends is hydraulically longer than the tape says.
The factors that actually differ
Show
| Capacity derating | Charge adjustment | |
|---|---|---|
| What it corrects | The capacity and efficiency actually delivered. | The quantity of refrigerant in the system. |
| What drives it | Equivalent length AND vertical lift, with the direction of the lift mattering. | Equivalent length beyond the factory-charged standard, and the line diameter. |
| Consequence of ignoring it | The system underperforms against its nameplate and costs more to run — a quiet, permanent shortfall. | Undercharge or overcharge, both of which damage the compressor. |
| When it is done | At selection, so the unit chosen delivers the required capacity after derating. | At commissioning, by weighing the additional charge in. |
| Vertical lift | A separate limit with its own maximum, and oil return up a riser is why. | Not directly — the charge follows length and diameter. |
| Fittings | Counted as equivalent length; a run with many bends derates more than its measured length suggests. | The same equivalent length is used. |
| How the quantity is added | Not applicable. | By WEIGHT, with scales — not by pressure, and not by feel. |
| Who it falls to | The designer, at selection. | The installer, at commissioning — and it is where most of the errors happen. |
| Recoverable later | Only by changing the unit or shortening the run. | Yes — the charge can be recovered, weighed and corrected. |
| Both apply | Yes, to the same installation, and neither substitutes for the other. | Yes. |
Which one, and when
Choose capacity derating when…
- Selecting equipment for a run longer than the manufacturer's standard length.
- Any installation with significant vertical separation between the units.
- Where the capacity has to be delivered rather than nominally specified — which is always.
- Diagnosing a system that has never quite kept up, where the shortfall may be in the pipe run rather than the unit.
Choose charge adjustment when…
- Commissioning any system whose line set exceeds the factory-charged length.
- After any work that opens the refrigerant circuit — a repair, a relocation, a leak.
- Diagnosing poor performance, where charge is the first thing to verify by weight.
- Wherever the line diameter differs from the standard, since the per-length figure depends on it.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why does vertical lift matter more than horizontal length?
- Because it costs pressure directly and because of OIL RETURN. Lifting refrigerant against gravity requires pressure the compressor has to supply, so the same equivalent length arranged as a vertical rise costs more than as a horizontal run. The oil problem is the one that limits the design: compressor oil circulates with the refrigerant, and on a vertical riser it has to be carried upward by the gas — which requires the gas velocity to stay above a threshold. Too little velocity, typically at part load on a variable-capacity system, and the oil falls back and accumulates at the bottom of the riser instead of returning to the compressor, which eventually runs short of it. That is why manufacturers state a maximum lift separately from a maximum length, why the figure differs depending on which unit is higher, and why oil traps are specified on tall risers.
- Why must charge be added by weight?
- Because weight is the only accurate measure of how much refrigerant is in the system, and the alternatives are not measurements at all. The manufacturer states the additional charge as a mass per unit of extra line length for a given line diameter, so the correct method is to weigh it in with scales — evacuate the line set, weigh the cylinder, charge the stated amount, confirm by the weight change. Charging by pressure alone is unreliable because pressure varies with ambient and load; charging by feel or by superheat alone without knowing the starting quantity compounds errors. Modern variable-capacity systems are particularly sensitive, since their operating envelope assumes a correct charge, and a small percentage error has a disproportionate effect on performance and on compressor life.
- What does undercharging actually do?
- Starves the evaporator and overheats the compressor, and the damage accumulates rather than appearing at once. With insufficient refrigerant the evaporator is not fully wetted, so suction pressure and suction temperature fall, capacity drops, and the compressor's discharge temperature rises because the returning gas is no longer cooling it adequately. Less refrigerant also means less oil circulating back to the compressor. The symptoms — the system running constantly, poor cooling, ice forming on the suction line — look like a unit that is too small or a fault in the equipment, which is why undercharge is so often misdiagnosed. It is also the more common of the two errors, since the usual cause is simply not adding the line-length charge at commissioning.
- And overcharging?
- Raises pressures and risks returning liquid to the compressor, which is the faster route to damage. Excess refrigerant backs up into the condenser, reducing the area available for condensing and raising discharge pressure, which raises the compressor's work and its temperature. Worse, if liquid refrigerant reaches the compressor's suction, it does not compress — liquid is essentially incompressible — and the result can be immediate mechanical damage to valves and connecting rods, a condition known as slugging. Excess refrigerant also dilutes the oil. The tell-tales are high discharge pressure and poor efficiency, and the remedy is to recover the charge and re-weigh it in correctly rather than to bleed some off, since bleeding leaves the quantity unknown.
- How is equivalent length calculated?
- By adding, to the measured pipe run, an allowance for every fitting expressed as the length of straight pipe that would cause the same pressure loss. Bends, elbows, traps, and any service valve each contribute, and on a run with many direction changes the fittings can add a substantial fraction to the effective length. Manufacturers publish the equivalent lengths for their fittings, and both the derating and the charge calculation use the resulting figure rather than the tape measurement. The practical consequence is that two installations with the same straight-line distance between the units can need different corrections, and that a route chosen for convenience — up, along, down, around an obstruction — is hydraulically longer than a direct one even though the walk between the units is the same.
- What are the maximum lengths for?
- They are hard limits rather than points at which performance merely degrades, and exceeding them puts the installation outside the manufacturer's design envelope and its warranty. The maximum total equivalent length is set by pressure loss and by the system's ability to distribute refrigerant and return oil; the maximum lift is set by oil return and by the pressure required. Beyond either, the manufacturer cannot predict the behaviour, oil return may not occur at all at part load, and the compressor is at risk. There are usually also limits on the number of bends and on the length between branches in a multi-split arrangement. When a route cannot be kept within them, the answer is to relocate a unit, to choose a system rated for longer runs, or to use a different configuration — not to exceed the limit and hope.
- Does any of this apply to a single-split domestic unit?
- Yes, and it is where the charge adjustment is most often skipped — because the unit arrives with a factory charge and the installer assumes it is complete. Domestic split systems are charged for a standard line length that is short, and a typical installation where the outdoor unit is round the side of the house comfortably exceeds it. Without the additional charge the system runs undercharged for its life, delivering less than it should and shortening the compressor's. The check is simple and it is documented: the manufacturer's data plate or manual states the standard length, the additional charge per unit length, and the line diameters, and a commissioning record should show the actual line length and the charge added. Asking for that record is the way a homeowner can verify it.
- How do I tell whether an existing system was charged correctly?
- By measurement rather than inference, and the definitive method is to recover the charge, weigh it, and compare with what the line length requires. That is invasive, so the usual diagnostic route is indirect: measuring superheat and subcooling against the manufacturer's target values at the current conditions, which together indicate whether the charge is high, low or correct. Those readings have to be taken at stable operating conditions and interpreted against the manufacturer's data for that model, not against general rules of thumb. The supporting evidence is documentary: the commissioning record showing the line length and charge added. Where no record exists and the readings suggest a problem, recovering and re-weighing is the honest fix, because it replaces an estimate with a known quantity.