Aircon Compressor Oil: The Lubricant That Has to Travel

Compressor oil holds three jobs at once, and all three happen inside the shell. It keeps a film between bearing surfaces that would otherwise touch, seals the small gaps the machine compresses against, and draws heat away from the parts that generate it. Oil sitting anywhere else is doing none of those things.

Keeping it in is not an option, because the gas being pumped passes through the same space the oil occupies. A fine mist leaves on every discharge, too slight to notice and too steady to ignore. Allied Air Enterprises' refrigerant piping design guidelines state that small amounts leave constantly and circulate with the refrigerant throughout the system. What leaves has to be brought back, or the bearings lose the film they run on.

That turns the pipework into two circuits sharing one set of pipes. One moves heat, the job the system was bought to do. The other carries lubricant out to whichever room sits furthest away and brings it home, which nobody raises at the quotation stage, and it is what the design rules in an installation manual quietly protect.

Oil does not find its own way to the compressor. It gets dragged there, pushed along the inside of the pipe by refrigerant vapour moving over it. The trade word is oil entrainment, describing a film clinging to the tube wall that the gas has to peel forward.

Speed is the whole mechanism, which is why published piping guidance gives a velocity rather than a pipe size. Allied Air Enterprises' guidelines set a minimum of roughly 800 feet per minute through horizontal suction lines. Vertical risers need 1,200 feet per minute, with 1,500 preferred, whatever the height of the riser.

Gravity is the whole of the gap between those two figures. On a flat stretch the gas only fights the film's grip on the copper; on a climb it fights the grip and the weight together. The same oil therefore wants half again as much speed, and a vertical rise is its own design problem rather than extra length.

A trap belongs at the foot of a vertical rise, where oil gathers until the gas has enough behind it to shift the load up. Taller lifts get further traps spaced along the riser. The run should also carry no sags or low points, since anything that dips collects oil. That is one reason hard copper is preferred to soft coil on long horizontal stretches.

Return outranks efficiency wherever the two disagree. Those guidelines note that the suction line's most important function is bringing oil home, and that on very long runs a higher pressure drop may have to be accepted to hold the velocity up. A designer will knowingly give away some capacity rather than let the film stop moving, a trade that never appears on a quotation.

A larger suction line carries less oil, not more: the same mass of gas through a wider bore travels slower, and the film that depends on that travel stops being carried. Allied Air Enterprises' guidelines state directly that an oversized suction line can drop refrigerant velocity below what is needed to bring oil back. The pipe wall is where this bites, because the wall is the slowest part of any pipe. Oil is stuck to the surface, exactly where the flow is weakest.

Oversizing on a real job is seldom a decision. It is the size that was on the van, a round-up because bigger sounded safer, or copper reused from the system that came out. The reuse case is common in a flat being refitted and looks most like thrift, but pipe sized for the old machine is wrong for the new one whenever the new one moves less gas.

Nothing about a wide line gives itself away on handover day: it holds pressure, passes a vacuum test and cools the room. The material is right, the joints are sound, and the only thing wrong is a dimension nobody will measure again. Total run and vertical drop carry published ceilings for closely related reasons, written down as the maximum pipe length for a given model.

Liquid lines can be oversized too, with a different consequence. A wider liquid line holds more refrigerant, and the same guidelines connect a larger system charge to slugging and to oil dilution. Dilution is a question about a machine that has been sitting, and that argument is made elsewhere.

A correctly sized run brings oil back at the flow it was sized for, and not below it. Anything that lowers the flow lowers the carrying force with it, and the pipe cannot make up the difference.

Part load is the first and largest of those conditions. Allied Air Enterprises' guidelines work through an example where a line chosen for full capacity falls to roughly 850 feet per minute at minimum output. That is enough along horizontal runs and not enough up the vertical risers.

The double riser exists to answer this. Two risers run side by side with a trap between them. At low output the trap fills until it seals the second riser off, the gas is forced up one pipe instead of two, and the speed returns. An installation will absorb extra fittings, joints and cost to keep a film of lubricant moving, and nothing else in the pipework earns that.

Inverter equipment sits at part load by design, which changes what the sizing has to satisfy. A machine bought because it modulates runs under its rated output for most of the time it is on, so the bottom of that range is the operating point the pipe has to serve. Sizing that only holds at full capacity holds during the hour nobody chose the unit for.

Stopping and starting repeatedly empties the sump by a different route. Copeland's application bulletin for its R-410A scroll compressors describes minimum run time as the span oil needs to find its way home. Oil thrown out at startup must return and restore the level the crankshaft draws from. Cycling for less than that brings progressive oil loss and damage, and the bulletin sets the span on a system piped to the longest lines approved for it.

A charge below the specified weight thins the mechanism everywhere at once. Fewer kilograms of refrigerant crossing the same bore means a weaker stream doing identical carrying work, so an undercharge is a lubrication problem before it is a cooling one. That ordering is the part most people have backwards, and it is why topping up by feel is worse than it looks.

Idle branches on a shared condenser are the case nobody thinks to check. Copeland notes that circuits left inactive through part of the normal cycle can hold a meaningful quantity of oil. Where the piping lets it fall out of the refrigerant flow, it collects in a branch nothing is passing through. A bedroom head switched off for a season does exactly that: the oil is not lost, it is parked, and the compressor is short by however much is parked.

A compressor short of lubricant gives nothing away while it is happening. Bearing surfaces wear a fraction on every rotation that runs thin, the wear does not undo itself, and the room keeps cooling throughout. No remote reports it and no display codes a film thinner than it should be.

The first outward sign points somewhere else entirely. Oil settling in the evaporator coats the inside of the tubing and gets in the way of heat transfer, which HVAC School names among the costs of a poor return. What that produces is a system cooling slightly less well, and weak cooling has a dozen likelier explanations that get examined first.

The delay finishes the job: the route was fixed on install day, the wear gathers across years, and the failure lands far enough downstream that nobody connects the two. The invoice says compressor; the explanation offered is age. A second failure on the same pipework is the signal: the replacement goes back onto an identical route, size, climb and charge, so whatever starved the first one has not gone anywhere.

What that does not settle is whether the machine in place now is healthy. Symptoms belong elsewhere: the observable side of signs compressor failing is its own page, and what a compressor burnout leaves in the pipework is separate again. Electrical testing reads the windings rather than the bearings, so a clean result rules out less than it appears to.

The questions below get answered from the job rather than from the machine, and that is deliberate. A route leaves a paper trail and a worn bearing does not: whoever sized the run has figures ready, while whoever fitted what was to hand has an opinion.

• •What suction line size this model calls for, and whether that figure came out of its installation manual.
• •Whether the copper was reused from an earlier system, and what capacity that earlier system had.
• •Whether the route includes a vertical rise, and whether a trap was fitted at the foot of it.
• •Whether any long horizontal stretch sags or dips anywhere along its length.
• •What weight of refrigerant went in, set against what the routed length calls for.
• •How the unit behaves under light demand, whether it holds a steady low output or keeps stopping and restarting.
• •Whether any indoor head on a shared condenser stays switched off for long stretches.