HVAC
Refrigerant oil return field guide for long and vertical line sets
Keep the velocity high enough to sweep the compressor's oil home, trap and double-riser the verticals, and design for minimum load, not just the nameplate day.
Direct answer
Oil return is keeping the compressor's lubricating oil moving with the refrigerant and back home, because the oil leaves with the discharge gas and a system that does not bring it back runs the compressor dry until it fails. On long runs and tall risers, velocity carries the oil. The equipment manufacturer sets the limits.
Key takeaways
- Oil leaves the compressor with the discharge gas, and piping that fails to return it runs the crankcase dry until the compressor seizes.
- Vertical suction risers need roughly 1000 to 1500 fpm gas velocity to carry oil up; horizontal lines need around 700 fpm plus pitch toward flow.
- Size the suction riser to minimum expected load, not full load, because oversized pipe stalls the oil at low flow.
- Fit a double riser (parallel small and large pipes joined by a trap) on capacity-unloading or VFD systems with tall risers.
- The equipment manufacturer's line length, lift, trap, and charge-per-foot limits are the boundary; exceed them and no field fix returns the oil.
What oil return is, and why a compressor starves on bad piping
Oil return is the part of the design that keeps the compressor's lubricating oil circulating with the refrigerant and finding its way back to the crankcase. A compressor does not hold its oil neatly inside itself. A small fraction of it leaves with every stroke, swept out of the discharge port as a fine mist mixed into the hot gas, and from there it rides the refrigerant all the way around the loop: through the condenser, down the liquid line, through the metering device, across the evaporator, and back up the suction line. If the piping carries it home as fast as the compressor sheds it, the level in the crankcase stays steady and nobody thinks about it. If the piping does not, the oil pools somewhere out in the system and the crankcase level falls.
That is the whole problem in one sentence: the oil that leaves the compressor has to come back, or the bearings run dry. On a short, well-pitched lineset it returns on its own and the subject never comes up. On long runs, tall vertical risers, and any system that spends real time at low load, the gas slows down, the oil drops out of the stream, and it stays where it dropped. This is the hidden reason line sizing and traps matter, and it is why a system that cooled fine on startup day can kill its compressor a year later with nobody having touched it.
This guide is the oil-return side of refrigerant piping. Picking the diameter for pressure drop and velocity lives in the line sizing guide, and reading superheat and subcooling to confirm the charge lives in the charging guide. Here the single question is whether the oil the compressor sheds actually comes back.
Why oil return decides whether the compressor lives
The oil leaves the compressor with the gas and it must return. There is no second supply. A reciprocating or scroll compressor lubricates its bearings, its scroll flanks, its wrist pins from the charge in the crankcase, and that charge is a fixed quantity that the system has to keep recirculating. Lose enough of it out in the piping and the level drops below the pickup, the bearings run on metal, and the failure is not gradual once it starts. It is a seized rotor or a spun bearing, and it is not a warranty conversation you win.
There are two ways the loss shows up, and the worse one is silent. A no-return condition starves the crankcase and ends in a dead compressor. The quieter one is oil logging in the evaporator, where oil that will not return collects in the coil, films over the tubes, and insulates the very surface that is supposed to be boiling refrigerant. The system loses capacity, the suction superheat reads strange, and the tech chases a charge problem that is really a slug of oil sitting in the coil where the velocity could not lift it out.
So the priority order is not subtle. No oil return kills the compressor. Everything else in this guide, the velocities, the traps, the double risers, exists to make sure the oil that left the crankcase this minute is on its way back.
Where the oil drops out and pools
Oil drops out wherever the refrigerant slows below the speed it takes to carry it. Velocity is the only thing holding the oil in the stream, so the oil collects in exactly the spots where velocity falls: the bottom of a vertical suction riser, the low points of a long horizontal run, a sagging line that was never pitched, and the evaporator itself when the load drops off. The gas keeps moving as vapor with no trouble. The oil, heavier and stickier, falls behind and stays.
The suction riser is the hardest place in the system, because the gas has to drag the oil straight up against gravity and it is doing it at the coldest, lowest-pressure, lowest-density point in the loop. The liquid line rarely has an oil-return problem, since the refrigerant there is a dense liquid the oil mixes into and rides along. The discharge line carries oil too, but the gas is hot and fast and usually moves it without complaint. The trouble concentrates on the suction side, on the way up.
Low load is the other place the oil drops, and it overlaps with everything above. A system sized to keep velocity up at full load can fall well under the threshold the moment a compressor unloads, a VFD slows down, or mild weather drops the demand. The pipe is the same diameter, but the gas crawling through it at part load no longer has the speed to lift the oil, and the riser that worked all summer stalls in the shoulder season.
How much velocity does it take to carry oil up a riser?
Carrying oil up a vertical suction riser takes a minimum gas velocity, and the figures commonly cited for halocarbon systems land around 1000 to 1500 feet per minute, with the upper end treated as the safe design target for risers. Below that, the gas no longer sweeps the oil up the tube wall and the oil stalls and falls back. The exact number depends on the refrigerant, the saturated suction temperature, the pipe diameter, and the oil, so treat the range as a starting point and size to the manufacturer's tables and the refrigerant's own data, not to a number off a forum.
The mechanism is worth picturing, because it explains why bigger pipe is not safer. The oil does not ride up the center of the tube. It crawls up the inside wall as a film, and the gas has to keep that film moving. In a larger pipe the same mass of gas spreads over a bigger bore, the center velocity has to be higher to keep the wall film moving, and a pipe that is too large simply cannot push the wall fast enough at low flow. That is why the riser is sized to the minimum expected load, not the maximum.
Horizontal lines are easier and the threshold is lower, commonly cited in the rough neighborhood of 700 feet per minute, because gravity is not fighting the flow. Horizontal suction lines still need help, though, and the help is pitch: slope them in the direction of flow, toward the compressor on the suction side, so any oil that does drop out drains the way you want it to instead of pooling in a belly. Velocity does the lifting on the verticals. Pitch does the draining on the flats.
| Line and orientation | Commonly cited minimum velocity | What it depends on |
|---|---|---|
| Suction riser (vertical, up) | ~1000 to 1500 fpm | Refrigerant, suction temp, diameter, oil |
| Suction line (horizontal) | ~700 fpm | Pitch toward flow still required |
| Discharge riser (vertical, up) | Lower, gas is dense and fast | Verify per manufacturer |
| Design basis | Minimum expected load, not full load | Where the velocity is lowest |
The line-sizing tradeoff: oversized stalls the oil
Suction line sizing pulls in two directions and oil return is the reason. Make the pipe larger and you drop less pressure, which protects capacity and efficiency. Make it larger still and the velocity falls below what it takes to carry oil up the riser, the oil stalls, and you have traded a small efficiency gain for a compressor that slowly starves. Bigger is not better on a suction line. There is a window, and the top of the window is set by the lowest velocity the system will ever see, not the highest.
Undersizing has the opposite failure and it is the one people guard against by reflex, so it gets oversized into the oil-return failure instead. Too small a suction line drops too much pressure, the compressor sees lower suction pressure than it should, capacity falls, and the run gets hot and expensive. The instinct to avoid that is to go up a size, and on a long riser that instinct is exactly how the oil-return problem gets built in.
The takeoff and the install are where this goes wrong quietly. Someone sizes the suction line for pressure drop at design load, the riser checks out, and nobody runs the velocity at minimum load. The full sizing method, the pressure-drop budget, and the velocity check across the load range live in the line sizing guide. The point to carry here is that the suction riser is the one line where the larger size can be the wrong size.
Oil traps: the P-trap at the bottom and the inverted trap at the top
An oil trap is a deliberate bend in the line that manages where the oil collects and how it gets moving again. The classic one is a P-trap at the bottom of a long suction riser. It is a short U formed at the base of the vertical, and its job is to collect the oil that drains back down the riser into a small pool. As the pool fills it narrows the gas path, the gas speeds up over the restriction, and it picks the oil up in slugs and carries it up the riser in batches rather than asking the gas to lift a continuous film it cannot manage at low flow.
The other trap is an inverted trap, a loop that rises above the top of the riser before the line turns and runs horizontal. Its job is different: it stops oil and liquid refrigerant from draining back down the riser into the evaporator or the idle compressor during the off cycle. On a system where the evaporator sits below the compressor, that drain-back on every shutdown is its own headache, and the inverted loop at the top is what holds it.
Traps are not free and not always wanted. Every trap adds pressure drop, and on a properly sized line some manufacturers would rather you delete the trap than pay the penalty. Tall risers are often broken into sections with a trap at the base of each, on the order of one trap per story of lift as a rough convention, but the convention is not universal. Follow the equipment manufacturer's piping detail. If their drawing shows traps at a spacing, use it; if it shows none and the line is sized right, do not add them out of habit.
What is a double riser, and when do you need one?
A double suction riser is two parallel vertical pipes, one small and one large, joined at the bottom by a trap, used on systems whose capacity unloads. It exists to solve a problem a single riser cannot: a compressor with capacity control swings between full load and a fraction of it, and no single pipe diameter holds the right velocity across that whole range. Size the single riser for full load and the velocity collapses at minimum load and the oil stalls. Size it for minimum load and the pressure drop at full load is unacceptable. The double riser gives the gas two different pipes to use depending on how much of it there is.
The way it works is the part worth understanding. The small riser is sized to keep oil-return velocity at the lowest load the system will run. The large riser, in parallel, gives the extra flow area so the pair together drops acceptable pressure at full load. A trap sits at the base between them. At low load the gas does not have the velocity to carry oil up both pipes, so oil gradually fills the trap until it seals off the large riser, and from then on all the gas funnels up the small one with enough speed to bring the oil with it. When the load comes back up, the rising gas blows the oil out of the trap, the large riser opens again, and both carry the flow.
If a system unloads and the suction riser is tall, this is not optional. The exact sizing of each leg and the trap geometry come from the manufacturer's tables and the refrigerant, so design it to their numbers rather than eyeballing the pair. Get it right and the compressor sees oil return at 25 percent load the same as at 100. Skip it on an unloading system and you have built the low-load oil stall into the job.
VRF and ductless long line sets
VRF and ductless systems push line lengths and elevation changes far past anything a conventional split deals with, and oil return is the constraint that sets the limits. Total piping can run into the hundreds of feet and the elevation between the outdoor unit and the farthest indoor head can be tens of feet, and across all of that the inverter compressor still has to get its oil back. The manufacturers know this better than anyone, which is why their design guides spell out maximum total length, maximum length to the farthest unit, and maximum lift, along with periodic oil-return logic in the control that runs the compressor up to sweep the lines clean.
Follow the manufacturer's line-length and lift limits as hard numbers, not guidance. Exceed the maximum lift and no field trick brings the oil back reliably. The piping practice on VRF also differs from older refrigeration habits: several manufacturers want no standard P-traps in the lines at all, because the equipment runs its own oil-return cycle and a trap just adds pressure drop and a place for oil to sit, while some still call for inverted traps at specific points. Build the traps their detail shows, and only those.
Charge is the other long-line item. A VRF or a long split ships with enough refrigerant for a baseline length, often only the first stretch of lineset, and beyond that you add refrigerant by the manufacturer's table, typically an amount per additional foot keyed to the liquid line diameter. Getting that addition right matters for oil return too, because an undercharged or overcharged long system shifts where the oil and liquid sit. Weigh in the added charge by their table and record it. The charging guide covers reading the system to confirm it once it runs.
Oil type and the refrigerant it has to live with
The oil and the refrigerant are matched, and the match decides how well the oil returns. Older R-22 systems ran mineral oil, which mixes well with that refrigerant and travels with it without much fuss. The HFC refrigerants like R-410A and the newer A2L refrigerants like R-32 and R-454B, and their relatives, are not miscible with mineral oil, so they run on polyolester oil, POE, which mixes with them well enough to circulate. Put the wrong oil in and the refrigerant and oil separate, the oil stays where it lands, and return falls apart no matter how well the pipe is sized.
Miscibility is the property that actually governs return. When the oil and refrigerant mix freely, the oil moves as part of the stream and comes home with it. When miscibility is poor, and even a matched POE pair can lose miscibility at the temperature extremes, very cold evaporators or very hot liquid lines, the oil drops out of solution and tends to stay in the evaporator. That is the chemistry behind oil logging, and it is why low-temperature work is harder on oil return than comfort cooling.
POE oil carries a handling cost the older mineral oil did not. POE is hygroscopic: it pulls moisture straight out of the air, and the water it grabs does not leave easily under vacuum and goes on to form acids in the system. That is the real reason an A2L or HFC system gets capped tight, evacuated to a deep vacuum, and not left open to the air on the truck. The oil's appetite for water is a system-killer separate from oil return, and the two problems ride in on the same drum of oil. Use the oil the compressor manufacturer specifies and the refrigerant it is charged with, and do not mix types to top off.
The low-load problem: design for minimum, not nameplate
Most oil-return failures are low-load failures, because velocity falls with flow and the lowest flow is where the oil stalls. A VFD-driven compressor turned down, a digital scroll or a reciprocating compressor with unloaders cutting cylinders, a mild shoulder-season day pulling a fraction of the design load: all of them drop the mass flow through a pipe that was sized for more. The pipe diameter does not change. The gas just moves slower, and at some point it stops carrying the oil.
This is the trap people fall into when they size only for the design day. The nameplate cooling load is the easy case for oil return, because the velocity is highest there. The hard case is the minimum the system will ever run, and that is the velocity the suction riser has to be sized against. A riser that sails through July can stall every October if the only check that was ever run was the full-load number.
Design for the minimum load, and where the turndown is wide, that is exactly what the double riser is for. Know the lowest capacity the equipment can run at, run the velocity at that point, and if a single riser cannot hold velocity there without choking the full-load flow, the system needs the two-pipe arrangement. The question to ask at design is not whether the oil returns at full load. It is whether it returns at the worst load.
When an oil separator earns its place
An oil separator sits in the discharge line right off the compressor and pulls most of the oil out of the gas before it ever enters the system, dropping it into a reservoir that feeds it straight back to the crankcase. Instead of asking the whole loop to return every drop, the separator returns the bulk of it at the source, within a few feet of where it left, and only the small fraction that slips past has to make the full trip and come back up the suction riser.
On a short, conventional split it is not needed and nobody fits one. The case for it is the hard system: a long line set, a tall lift, a low-temperature application, a wide-turndown compressor, or any layout where the suction-side velocity is going to be marginal at low load no matter how the pipe is sized. On those, a separator takes most of the oil out of the equation and turns a borderline oil-return situation into a manageable one.
It is not a license to ignore the piping. A separator is good but not perfect, some oil always passes, and that passed oil still has to return through lines that hold velocity and traps that work. Treat the separator as insurance on top of correct piping, not a substitute for it, and follow the manufacturer's call on whether the application needs one.
Pitch the horizontal lines in the direction of flow
Horizontal suction lines get pitched downward in the direction the gas flows, which on the suction side means sloping toward the compressor, on the order of a fraction of an inch per several feet as a common convention. The slope means any oil that drops out of the stream on a flat run drains the way the refrigerant is already going instead of collecting in a low spot and waiting for the velocity to come pick it up.
The discharge line gets pitched the same way, toward the flow, so oil drains downstream toward the condenser rather than running back into the compressor head on the off cycle. A line that sags or runs uphill against the flow builds a belly where oil and liquid collect, and that belly becomes the spot a slug forms and the compressor swallows it at the next start.
Pitch is cheap on the day you hang the line and expensive to fix after the ceiling is closed. Hang it right the first time.
Oil in the discharge line
The discharge line carries oil too, since the gas leaving the compressor is where the oil enters the system in the first place. It is usually the easy line for return: the gas is hot, dense, and fast, and it moves the oil along without much help. The discharge side earns attention only when it has to climb.
A tall discharge riser is the case to watch, especially on systems that cycle or unload, where the velocity in the riser can fall enough that oil drops back down toward the compressor on the off cycle or stalls at low load. The fix is the same logic as the suction side: size the riser to keep velocity at minimum load, and where the manufacturer's detail calls for it, trap the base of a tall discharge riser so oil does not drain back into the head. The discharge riser velocity and any required traps come from the manufacturer's piping tables; do not assume the suction-side numbers carry over.
Why does my compressor keep losing oil?
A compressor that keeps losing oil is almost always a system that cannot return it, not a compressor that is burning it, and the symptoms point at the piping if you read them together. The crankcase sight glass reads low or empty. The compressor trips on its oil-pressure safety, sometimes intermittently and worse at low load or in mild weather. Capacity falls off for no charge reason that adds up, which is oil logging in the evaporator filming the tubes. Put those together and the answer is upstream in the lines, not inside the compressor.
Work it in the order the failure happens. Check the velocity first: was the suction riser sized for minimum load, or only for the design day, and does the system unload below where a single riser holds velocity? Then the traps and the slope: is there a trap at the base of a tall riser if the detail calls for one, is the horizontal run pitched toward flow, is there a belly the line sags into. Then the lift and length against the manufacturer's limits, because an exceeded maximum lift no field fix overcomes. Then the oil and refrigerant match and the charge, since the wrong oil or a badly off charge moves where the oil sits.
The trap people fall into is topping off the oil and calling it solved. Adding oil to a system that cannot return it raises the level for a while and then loses it again, and now there is too much oil out in the lines logging the evaporator on top of the original problem. Find why it is not coming back. Then fix the piping, not the level.
Verifying oil return at startup
Verifying oil return is not a startup-day reading. It is watching the system hold its oil over time and across the load range, because the failure shows up at low load and over weeks, not in the first hour at full output on a hot afternoon. The day-one check tells you the system runs. It does not tell you the oil comes back when the compressor unloads in October.
The verification that matters is at low load. Run the system down at the lowest capacity it operates at, the worst case for velocity, and watch the crankcase level hold over time rather than drift down. On equipment with an oil-return cycle in the controls, confirm it is enabled and running. On a long or high-lift job, the level over the first weeks of part-load operation is the real test, so leave a way to read it and check back rather than signing off on the startup reading alone.
Charge ties into this directly. Weigh in the charge for the actual line length by the manufacturer's table, since an undercharged or overcharged long line shifts where the oil and liquid sit and skews the oil-return picture. Confirm the running charge by superheat and subcooling, which the charging guide covers, and record the added charge and the line length so the next tech knows what the system was set up to carry.
Follow the manufacturer's line length, lift, and trap tables
The equipment manufacturer specifies the piping for a reason: they tested the compressor with its oil and its refrigerant and they know the limits the lab found. Their design guide gives the maximum total line length, the maximum length to the farthest unit, the maximum vertical lift up and down, the required line diameters, where to put traps and what kind, and how much refrigerant to add per foot beyond the baseline charge. Those are not suggestions to refine with judgment. They are the boundary the equipment was qualified inside.
Velocities, lengths, and charge all hedge to the manufacturer and the refrigerant, because every one of them depends on the specific compressor, oil, and refrigerant in front of you. A velocity that returns oil on R-410A with one POE oil is not automatically the number for R-454B with another. A length limit on one VRF line is not the limit on the next. Use the table that came with the box you are installing, in the edition that matches it, and when the field condition is not on their table, call their application engineering rather than guessing.
Exceed the limits and you own the failure. The most common version is a lift or a total length pushed past the maximum because the building made the easy route impossible, and the system that ran fine on the bench loses oil in the field because it was installed outside what the manufacturer qualified. If the building cannot be piped inside the limits, that is a design conversation before the lines go in, not a problem to discover at commissioning.
Common oil-return failures and the fix
The failures repeat from job to job, and they trace back to a short list of decisions made at design or install. The pattern is almost always velocity, traps, or an exceeded limit, and the fix follows from which one it is.
| Condition | Oil-return risk | Fix |
|---|---|---|
| Oversized suction line | Velocity drops, oil stalls in the riser | Size to minimum-load velocity, not just pressure drop |
| No trap at base of a tall riser | Oil cannot restart up the riser at low flow | Add the trap the manufacturer detail shows |
| No double riser on an unloading system | Velocity collapses at part load | Fit a double riser sized to min and full load |
| Exceeded max lift or length | No field fix returns the oil | Re-route within limits or change equipment |
| Line not pitched toward flow | Oil pools in the belly, slugs at start | Re-hang with slope in the flow direction |
| Sized for design load only | Stalls at part load and mild weather | Run velocity at the lowest expected load |
| Wrong or mixed oil for the refrigerant | Oil separates, will not circulate | Use the specified oil; do not mix types |
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Common mistakes
- Oversizing the suction line so the velocity drops and the oil stalls in the riser.
- Leaving out a trap at the base of a tall suction riser the manufacturer detail calls for.
- Skipping the double riser on a capacity-unloading or VFD system with a tall riser.
- Exceeding the manufacturer's maximum line length or vertical lift because the building route was easier.
- Hanging lines flat or against the flow so oil pools in a belly and slugs at the next start.
- Sizing for the design load and never checking velocity at the minimum load the system runs.
- Topping off the oil instead of finding why it is not returning, which logs more oil in the evaporator.
Standards and references
The equipment manufacturer's installation and piping design guide is the controlling document for any specific job. It sets the line diameters, the maximum total length and length to the farthest unit, the maximum vertical lift up and down, the trap locations and types, and the added charge per foot. When a velocity, a length, or a charge in this guide differs from that document, the manufacturer's document wins, because it was written for the compressor, oil, and refrigerant actually in the system.
The ASHRAE Refrigeration Handbook is the reference behind the design principles, covering refrigerant line sizing, the velocities that carry oil up risers, double-riser construction, and traps. The minimum riser velocities commonly cited, in the range around 1000 to 1500 feet per minute for suction risers and lower for horizontal lines, come out of that body of work, and they vary with the refrigerant, the saturated suction temperature, and the oil. Treat them as the design framework and confirm the specific numbers against the refrigerant and the equipment.
The refrigerant and oil pairing governs whether the oil returns at all. POE oil with the HFC and A2L refrigerants, mineral oil with the older R-22, and the miscibility between them decide whether the oil travels with the refrigerant or drops out and logs. The refrigerant manufacturer's data and the compressor manufacturer's oil specification are the authority on which oil, how much, and the temperature limits of its miscibility. The three rules under all of it: velocity carries the oil, traps and double risers handle the verticals, and the manufacturer's limits are the boundary you stay inside.
Units and terms
Oil-return work crosses imperial and metric units and a handful of terms that mean specific things, so the same idea reads differently across a manufacturer guide, a handbook, and a spec.
Velocity is given in feet per minute (fpm) in most North American piping work and in meters per second in metric sources, where roughly 1000 fpm is about 5 m/s. Line length and lift are in feet or meters, and the manufacturer's limit is whichever the guide uses. Charge addition is in ounces or grams per foot or per meter of liquid line. The terms below are the ones that carry the meaning.
- Oil return
- Keeping the compressor's oil circulating with the refrigerant and back to the crankcase
- Oil logging
- Oil that will not return collecting in the evaporator, filming the tubes and cutting capacity
- Suction riser
- A vertical suction line the gas must drag oil up, the hardest spot for oil return
- Double riser
- Parallel small and large risers with a trap, used on unloading systems to hold velocity across loads
- Oil trap (P-trap)
- A U at the base of a riser that pools oil and slugs it up; inverted at the top to stop drain-back
- POE oil
- Polyolester oil for HFC and A2L refrigerants, hygroscopic, replaces mineral oil from the R-22 era
- Miscibility
- How well the oil mixes with the refrigerant; poor miscibility means the oil drops out and stays
- Oil separator
- A discharge-line device that returns most oil to the crankcase near the source
FAQ
What is oil return in refrigeration?
In refrigeration, oil return is the system bringing the compressor's lubricant back to the crankcase. A little oil leaves with every discharge stroke and rides the refrigerant around the loop, so the piping has to carry it home. If it does not, the crankcase runs low and the bearings fail.
Why does oversizing the suction line cause problems?
Oversizing the suction line drops the gas velocity, and velocity is the only thing carrying oil up a riser. Below roughly 1000 to 1500 fpm the oil stalls and falls back, pooling in the riser and starving the compressor. Bigger pipe lowers pressure drop but a too-large suction riser cannot return oil at low load.
What is a double riser?
A double riser is two parallel vertical suction pipes, one small and one large, joined by a trap, used on compressors that unload. At low load oil seals the trap and all the gas goes up the small riser fast enough to carry oil. At full load both risers open to hold down pressure drop.
What is a suction line oil trap?
A suction line oil trap is a U-shaped bend, usually at the base of a tall riser, that collects draining oil in a small pool. As the pool narrows the gas path, the gas speeds up and lifts the oil up the riser in slugs. An inverted trap at the top stops oil draining back on the off cycle.
How much velocity does it take to carry oil up a suction riser?
Carrying oil up a vertical suction riser commonly takes around 1000 to 1500 fpm of gas velocity, with the upper end used as the design target. Horizontal lines need less, roughly 700 fpm, plus pitch toward flow. The exact number depends on the refrigerant, suction temperature, and oil, so size to the manufacturer's tables.
Why does my compressor keep losing oil?
A compressor that keeps losing oil usually cannot return it. Look for an oversized or untrapped suction riser, a missing double riser on an unloading system, a lift or length past the manufacturer's limit, or a line not pitched toward flow. Topping off the oil hides it briefly and logs more oil in the evaporator.
What oil goes with R-410A and the A2L refrigerants?
R-410A and the newer A2L refrigerants R-32 and R-454B, like the other HFC and HFO-blend refrigerants, run on POE (polyolester) oil, because they are not miscible with the mineral oil used on older R-22. POE is hygroscopic and pulls in moisture, so cap and deep-vacuum the system. Use the oil the compressor manufacturer specifies, and never mix types.
Do VRF systems need oil traps?
It depends on the manufacturer. Several VRF makers want no standard P-traps in the lines, since the equipment runs its own oil-return cycle and a trap just adds pressure drop, while some call for inverted traps at specific points. Build exactly the traps their piping detail shows and follow their line length, lift, and charge limits.
Why does oil return get worse at low load?
Velocity falls with flow, so when a compressor unloads, a VFD slows, or mild weather cuts demand, the gas through the same pipe slows below the speed that carries oil. A riser that returns oil at full load can stall at part load. Design and check velocity at the lowest expected load, not the nameplate.