HVAC and refrigeration FAQ
109 questions technicians actually ask on site — superheat and subcooling targets, refrigerant glide, contactor terminals, defrost timing, cascade systems — plus how R-Pro works. Written by a working refrigeration technician.
About R-Pro
How much does R-Pro cost?
USD 9.99 per month, or USD 8.99 if you opt in to anonymous data sharing. Six reference tools stay free forever with no time limit and no login: symptom search, manufacturer error codes, refrigerant pressure-temperature, manual finder, field tips and inspection checklists. You pay only when you want to keep records — customer and equipment history, work logs, estimates, invoicing, accounting and cloud backup.
Does R-Pro work offline?
Yes. R-Pro is offline-first because machine rooms usually have no signal. Error codes, refrigerant tables, manuals, field tips, checklists and every record you saved open with no connection, and sync when signal returns. The AI features — photo scan, AI diagnosis and schematic reading — need a connection because they call a server.
What can I use for free?
Symptom-to-cause search, manufacturer error-code lookup, refrigerant P-T calculator, manual finder, field tips and inspection checklists. No account needed. These stay free permanently — they are not a trial.
How many languages does R-Pro support?
Fourteen: English, Korean, Chinese, Japanese, Spanish, Hindi, Vietnamese, Thai, Indonesian, Arabic, Portuguese, French, Italian and German. The technical data is translated, not only the menus.
How many manufacturer error codes does R-Pro have?
1,879 codes across 62 brands, each linked to that maker's service manual. Brands include Samsung, LG, Daikin, Mitsubishi, Carrier, Gree, Midea, Hitachi, Panasonic, Toshiba, Bosch and York, plus smaller regional makers most apps leave out.
How many refrigerants does the P-T calculator cover?
108 refrigerants, generated from CoolProp (NIST REFPROP-grade equations of state). Bubble and dew pressures are stored separately so glide on zeotropic blends such as R-407C is handled correctly: superheat is calculated from dew, subcooling from bubble.
What is real-measurement diagnosis?
You enter what your gauges and thermometer actually read — high side, low side, suction line and liquid line temperatures. R-Pro calculates superheat and subcooling and ranks the likely causes: undercharge, overcharge, metering restriction, flooding, dirty condenser, starved evaporator, non-condensables or compressor wear. It compares readings against reference values rather than guessing from a description.
Can R-Pro read a wiring diagram or control panel photo?
Yes, as a reading aid. Photograph a schematic, a wiring guide, a control panel or a terminal close-up. R-Pro lists the parts it can see and — most usefully — the terminal markings to put your meter on, such as coil A1-A2, main contacts 1/L1-2/T1 and overload trip contacts 95-96, with what to measure and what a good reading looks like. Part names can be wrong, so each part carries a confidence level, and it says plainly when a picture is not electrical at all. Always confirm against the original diagram.
How is R-Pro different from ColdSnap?
ColdSnap is USD 29 per month, English only, and built around US field-service management: dispatch, team management and EPA refrigerant logging. R-Pro is USD 9.99, runs in 14 languages, and carries reference depth ColdSnap does not — 1,879 error codes across 62 brands, 108 refrigerants from CoolProp data, 176 field tips and control-panel reading. ColdSnap suits a US shop running several trucks; R-Pro is built for the technician doing the work, anywhere.
Where can I get R-Pro?
On Google Play as "HVAC Refrigeration Repair App" (package app.rpro), or open www.r-pro.app in a phone browser and install it as a web app. Both are the same product. It runs on tablets too, and a companion PC-side ERP handles the office.
Does R-Pro work on tablets?
Yes. The app runs on phones and tablets, and there is a companion PC-side ERP for the office — customer records, billing and accounting sync between them under one subscription.
What happens to my records if I change phones?
Records are backed up and restored on the new device. R-Pro also keeps a rolling local backup on the phone itself and can share a backup file out, so a lost or replaced phone does not mean starting over.
Can I record a job by voice?
Yes. Speak on the drive home and R-Pro fills in the work log — what you did, parts used and amounts — into structured fields rather than a wall of text. It handles field speech including corrections, cancellations and spoken refrigerant names.
Does R-Pro do estimates and invoicing?
Yes. Estimates, invoicing, expenses and accounting are included in the subscription, with revenue and net profit rather than just paperwork. The PC-side ERP covers the office end of the same data.
Is there a free trial?
There is no time-limited trial. Instead, the six reference tools are free permanently so you can judge the app before paying. The subscription covers the record-keeping and AI features.
Who built R-Pro?
Market Free, a Korean business run by a working refrigeration technician. The diagnostic content comes from field work rather than a manual rewrite, which is why cases like cascade systems and defrost timing are covered in depth.
Does R-Pro store my customer data safely?
Records are held on your device first and synced to your own cloud account when you are subscribed. Data belongs to the account that created it, and you can export or delete it.
Can several technicians share one account?
R-Pro is built around the individual technician. One active session per account is enforced, so a shared login will sign the other device out. Team dispatch is not a feature of R-Pro.
What is the R-Pro ERP?
A PC-side companion for the office side of the same business: customers, billing, expenses and accounting on a bigger screen. It is included with the subscription rather than sold separately.
Does R-Pro replace the manufacturer service manual?
No. R-Pro links to the maker's manual for every error code and tells you to confirm against manufacturer specification. It is a field reference and record tool, not an official service document.
Superheat, subcooling and charging
What is a good superheat on R-410A?
For a fixed-orifice system, target roughly 10–20°F (5–11°C) of superheat measured at the outdoor unit service port, with the exact target depending on indoor wet bulb and outdoor dry bulb. On a TXV system, superheat is controlled by the valve and typically sits near 8–14°F; on a TXV you charge to subcooling, not superheat. Always confirm against the manufacturer charging chart for that model.
What is a good subcooling on R-410A?
Most R-410A systems target roughly 8–12°F (4–7°C) of subcooling, with some designs specified up to 15°F. Subcooling is the charging target on TXV systems. Check the nameplate or charging chart, because the figure is set by the manufacturer, not by the refrigerant.
How do you calculate superheat?
Superheat = suction line temperature − suction saturation temperature. Read the low-side pressure, convert it to saturation temperature using the dew column of the P-T table for that refrigerant, then subtract that from the actual suction line temperature measured near the service port.
How do you calculate subcooling?
Subcooling = liquid saturation temperature − liquid line temperature. Read the high-side pressure, convert it to saturation temperature using the bubble column, then subtract the measured liquid line temperature from it.
Why do zeotropic blends need bubble and dew pressures?
A zeotropic blend such as R-407C boils and condenses across a temperature range rather than at one point — the glide, which can be several degrees. Superheat must be calculated from the dew point and subcooling from the bubble point. Using a single saturation figure for both produces an error the size of the glide, which is enough to misdiagnose a charge.
Low subcooling and high superheat — what does it mean?
That combination is the classic undercharge signature: not enough refrigerant to fill the condenser or to feed the evaporator fully. Confirm there is no leak before adding charge — topping up a leaking system just delays the same call.
High subcooling and low superheat — what does it mean?
Usually overcharge, or a restriction downstream that backs liquid up in the condenser. Check the metering device and the liquid line filter drier before removing charge; a partially blocked drier produces the same pattern as an overcharge.
Both pressures low — what should I check?
Low on both sides usually points to undercharge, a restriction ahead of the evaporator, or poor evaporator airflow. Check filter and coil first, then the metering device, then charge. Adding refrigerant to a system with a dirty coil hides the real fault.
Both pressures high — what should I check?
Look at the condenser first: dirty coil, failed condenser fan, recirculating hot air, or blocked airflow. Then consider overcharge and non-condensables. High head with high subcooling suggests overcharge; high head with normal subcooling points at heat rejection.
How long should the system run before taking readings?
Let the system reach steady state — typically 10–15 minutes on a residential split, longer on larger equipment or after a defrost. Readings taken during the first minutes after start-up are meaningless because pressures are still equalising.
Where do I measure superheat and subcooling?
At the charging access ports on the outdoor unit for air conditioning work: suction line temperature near the suction port for superheat, liquid line temperature near the liquid port for subcooling. Clamp the temperature probe on clean bare pipe and insulate it, otherwise ambient air pulls the reading off.
What causes high superheat with normal subcooling?
A starved evaporator: a restriction at the metering device, a partly blocked drier, or a TXV bulb that has lost charge or is badly mounted. The condenser is filling normally but the evaporator is not being fed.
Can non-condensables be diagnosed from pressures?
Yes, indirectly. Air or nitrogen in the system raises head pressure above what the condensing temperature would explain, so measured condensing temperature reads higher than the ambient and coil condition justify. Compare the saturated condensing temperature to ambient — an unexplained gap points at non-condensables.
Should I charge by weight or by superheat and subcooling?
Charge by weight when the manufacturer gives a charge and the line set length is known — it is the most accurate method. Use superheat or subcooling to verify afterwards, or when the exact charge is unknown. On a TXV system verify with subcooling; on a fixed orifice, with superheat.
Why is my superheat reading unstable?
A hunting TXV, an incorrectly mounted or loose sensing bulb, or a system not yet at steady state. Also check that the temperature probe is on bare pipe and insulated. If the valve hunts, verify bulb contact and insulation before replacing anything.
Refrigerants
What replaces R-22?
Common retrofit and replacement options include R-407C, R-422D, R-427A and R-421A for existing equipment, while new equipment moved to R-410A and now to lower-GWP options such as R-32 and R-454B. The right choice depends on oil compatibility, the metering device and local regulation — a drop-in that suits one system can be wrong for another.
What is the difference between R-410A and R-32?
R-32 is a single-component refrigerant with no glide, higher volumetric capacity and roughly a third of the GWP of R-410A. It runs at similar pressures but with higher discharge temperature, and it is A2L — mildly flammable — so charge limits, leak detection and service practice differ. R-410A is a blend of R-32 and R-125.
What is an A2L refrigerant?
An ASHRAE safety classification meaning lower toxicity and mild flammability: refrigerants such as R-32, R-454B and R-1234yf. They require different service practice — spark-free tools where mandated, charge limits by room volume, leak detection, and ventilation before brazing. Rules differ by country, so check local regulation.
What is refrigerant glide?
The temperature difference between the bubble point and dew point of a zeotropic blend at a given pressure. R-407C glides roughly 7°C. Glide matters when calculating superheat and subcooling, and it means such blends must be charged as liquid, since vapour charging changes the composition of what is left in the cylinder.
Why must blends be charged as liquid?
A zeotropic blend does not evaporate as one substance. If you charge vapour, the more volatile component leaves the cylinder first and the mixture in both the cylinder and the system drifts away from the specified composition, changing capacity and pressures. Charge liquid, metering it so the compressor is not slugged.
Can I mix refrigerants in one system?
No. Mixed refrigerant has no valid P-T relationship, so superheat and subcooling readings become meaningless and capacity is unpredictable. It also cannot be recovered cleanly. If contamination is suspected, recover the charge, evacuate and recharge with a known refrigerant.
What is R-290 and where is it used?
R-290 is propane, a natural refrigerant with very low GWP and excellent thermodynamic performance, used widely in self-contained commercial refrigeration and small appliances. It is A3 — highly flammable — so charge sizes are limited by standards, and service requires ignition-source control.
What is R-744 and why are its pressures so high?
R-744 is carbon dioxide. Its critical point is 31.1°C, so above that temperature it cannot condense and the system runs transcritical, with high-side pressures often above 80 bar. That is why CO2 systems use different components, relief settings and service equipment from HFC systems.
What is R-717 and where is it used?
R-717 is ammonia, used in industrial refrigeration for its efficiency and near-zero GWP. It is toxic and flammable at higher concentrations, it attacks copper, and it requires steel pipework and trained handling with dedicated leak detection.
Why is R-454B replacing R-410A?
R-454B has roughly a quarter of the GWP of R-410A with similar capacity and pressures, which is why manufacturers adopted it as regulation tightened. It is A2L, so installation and service practice change: charge limits, leak detection and different service tooling in some jurisdictions.
What is R-1234yf used for?
R-1234yf is an HFO with very low GWP used mainly in automotive air conditioning and some chillers. It is A2L, and its P-T behaviour is close enough to R-134a that pressures look familiar, but service requirements differ.
What refrigerants are used in ultra-low temperature systems?
Ultra-low temperature cascade systems typically use an HFC or natural refrigerant on the high stage and a very low boiling refrigerant on the low stage — R-23, R-508B, R-14 or R-170 (ethane). The low stage cannot reject heat to ambient on its own, which is why the high stage exists.
Does R-Pro cover obsolete refrigerants like R-12 and R-502?
Yes. The P-T data includes legacy CFCs and blends such as R-11, R-12, R-13, R-113, R-114, R-115, R-500, R-502 and R-503, because technicians still meet them on old plant that has never been converted.
How accurate is the R-Pro P-T data?
It is generated from CoolProp, which implements NIST REFPROP-grade equations of state, rather than transcribed from printed tables. Bubble and dew pressures are separated for blends, and the data was regenerated in July 2026 with monotonicity and structure checks producing zero errors.
Electrical and starting faults
Air conditioner does not run at all — where do I start?
Start at the electrical side, not the refrigerant side. Check the breaker or NFB for a trip caused by overload or a surge, then supply voltage at the unit, then the contactor coil, then the start capacitor. A tripped breaker that resets and holds points somewhere different from one that trips again immediately.
How do I test a start or run capacitor?
Discharge it first, then measure capacitance with a meter that has a capacitance function and compare against the printed rating — a common service limit is ±6% of the marked value. A failing capacitor typically shows a humming compressor or fan that will not turn, and often a bulged top or leaked oil.
The compressor hums but will not start — what is it?
Most often a weak or failed start capacitor, a failed start relay or PTC, or a mechanically seized compressor. Check the capacitor and relay before condemning the compressor. A hard-start kit may confirm the diagnosis but does not repair the underlying cause.
What are C, R and S on a compressor?
Common, Run and Start. Measure resistance between the three pins: the largest reading is between Run and Start, and Common to Run is the smallest. If any winding reads open, or any pin reads to the shell, the compressor is electrically failed.
How do I check a compressor for a grounded winding?
With power off and leads disconnected, measure from each terminal to the compressor shell with a megohmmeter or the highest resistance range available. A healthy compressor reads effectively open to ground; a low reading means winding insulation has broken down and the compressor must be replaced.
Breaker trips as soon as the unit starts — what does that mean?
An immediate trip usually means a short — a grounded compressor winding, a shorted fan motor or damaged wiring — rather than an overload. A trip after several minutes of running points instead at high current draw: failing capacitor, high head pressure, low voltage or a seizing motor.
Why does the overload relay keep tripping?
The load is drawing more current than the setting allows. Check actual running amps against nameplate FLA, then look for high head pressure, low supply voltage, loose or pitted contacts, or a motor beginning to fail. Only raise the setting if the measured current is genuinely within the motor rating.
How do I check supply voltage correctly?
Measure under load, not at rest. Voltage that looks fine with the unit off can sag badly when the compressor starts. Check phase-to-phase and phase-to-neutral, and on three-phase check voltage imbalance — a few percent imbalance produces disproportionate current imbalance and motor heating.
What causes single phasing on a three-phase unit?
A blown fuse, a failed contactor pole, or a broken conductor leaves the motor running on two phases. Current on the remaining phases rises sharply and the motor overheats quickly. Check all three phases at the load side of the contactor while running.
Why do I look before I touch?
Because most of what has failed is visible before you put a meter on anything: scorch marks, oil traces, ice, loose wires, insects. Opening a panel and scanning it first costs seconds and often finds the fault outright.
Is most field failure electrical or refrigerant?
The majority of field faults are electrical, which is why the multimeter is the most-used diagnostic tool. A practical rule: if it does not run, start with electrical; if it runs but performance is wrong, start with the refrigeration cycle.
What is a PTC starter and how does it fail?
A PTC is a solid-state start device used mainly on domestic refrigeration compressors: it conducts briefly at start-up then heats and blocks current. It fails by going open or by staying conductive. Check resistance cold against the marked value, and let it cool fully before retesting.
Compressors
What causes compressor overheating?
High discharge temperature is driven by high compression ratio, low suction pressure, high head pressure, insufficient return gas cooling, or a low charge. Sustained high discharge temperature carbonises the oil, which then loses viscosity and damages bearings.
What discharge temperature is too high?
Discharge line temperature above roughly 107°C (225°F) measured near the compressor is generally treated as the limit for continuous running, because oil begins to break down beyond it. Manufacturer limits vary, so check the specification for that compressor.
What is liquid slugging and what does it sound like?
Liquid refrigerant entering the cylinder, which cannot be compressed. It produces a sharp knocking or hammering, and can break valves and rods. Causes include a flooded evaporator, a failed metering device, migration during off-cycle, or an over-charged system.
What is refrigerant migration?
During a long off-cycle, refrigerant migrates to the coldest part of the system — usually the compressor — and condenses into the oil. On start-up the oil foams and lubrication is lost. Crankcase heaters exist to prevent this; verify the heater works before blaming the compressor.
Why does a crankcase heater need to be on before start-up?
To drive refrigerant out of the oil before the compressor runs. Standard practice after a long shutdown is to energise the crankcase heater for at least two hours, then confirm pressures have equalised, before starting.
How do I know if a compressor has lost capacity?
Both suction and discharge pressures drift toward each other, the pressure differential falls, and the system cannot reach temperature despite correct charge and clean coils. Confirm charge, airflow and metering first — worn valves are diagnosed by elimination, not by a single reading.
What causes semi-hermetic motor burnout?
Sustained overheating, single phasing, contactor problems, low charge causing poor motor cooling, or acid formation after a previous burn. After a burnout the system must be cleaned properly — acid and carbon left in the pipework will destroy the replacement.
What oil does a system take?
Mineral oil is used with CFC and HCFC refrigerants such as R-22; POE ester oil is required with HFC and HFO refrigerants including R-410A and R-32. POE absorbs moisture rapidly, so containers stay sealed and the system must be evacuated properly. Mixing oil types causes return and miscibility problems.
Defrost
Is it normal for the fan to stop during defrost?
Yes. Evaporator fans are stopped during a defrost cycle so heat is not blown into the room. What matters is that the fan restarts after the defrost terminates. If it does not restart, look at the fan delay and the termination sensor.
How quickly should temperature recover after defrost?
A practical field standard is that room temperature should return to within about 2°C of setpoint within 30 minutes of defrost termination. If it takes longer, the defrost is running too long or the fan delay is set too long — shorten the cycle or adjust the delay.
Why is my defrost cycle running too long?
Either the termination sensor is faulty and never signals completion, or the timer is set for a longer period than needed. Excessive defrost drives room temperature up and wastes energy. Check the termination sensor before changing the timer.
Why does ice keep building on the evaporator?
Common causes are too few defrost cycles, a failed defrost heater, a failed termination sensor, a blocked drain refreezing the coil, or door seals letting warm humid air in. Work through door and seal, then coil, then defrost, then drain.
What is a defrost timer and how do I test it?
A defrost timer initiates and ends the defrost cycle on a schedule. Test by advancing it manually into defrost and confirming the heater energises and the fan stops, then that it terminates and the fan restarts. Terminals are typically numbered 1 to 4 — confirm against the wiring label on the timer.
What is hot gas defrost?
Instead of electric heaters, hot discharge gas is routed through the evaporator to melt frost. It is faster and more efficient on larger systems but requires additional valves and controls, and a fault in those valves shows up as failed or partial defrost.
Why is the drain line freezing?
Either the drain heater has failed, the line is not trapped or sloped correctly, or the line runs through a space below freezing without protection. A frozen drain backs water into the coil and refreezes it, which looks like a defrost failure but is not.
How often should a cold room defrost?
It depends on humidity, door traffic and product load rather than on a universal figure. The correct number of cycles is the fewest that keeps the coil clear. Frequent door openings and wet product mean more defrosts; a stable low-traffic freezer needs fewer.
Walk-in and cold storage
Walk-in cooler is not holding temperature — what do I check first?
Work in this order: door and seals, then coils (condenser and evaporator), then defrost, then drain. Most walk-in cooling complaints turn out to be a combination of dirty coil, damaged seal and failed defrost rather than a refrigerant problem.
How much does a bad door seal matter?
A great deal. A door left open or a split gasket admits warm humid air continuously, which raises room temperature and frosts the evaporator. It also makes the defrost cycle fight a load it was never sized for, so the symptom looks like a defrost fault.
Why is there frost on the evaporator in a cooler above freezing?
Humid air entering through doors or seals, an evaporator running colder than intended because of low charge or restricted airflow, or defrost not clearing the coil. Check infiltration first, then airflow, then charge.
What suction pressure should a walk-in freezer run at?
It depends on the refrigerant and the intended coil temperature rather than being a fixed number. Convert the target evaporator temperature to pressure using the P-T table for that refrigerant — the coil should run roughly 5–8°C below room setpoint for typical designs. Always check the equipment specification.
Why does the condensing unit short cycle?
Short cycling usually points at a control problem — pressure switch differential set too tight, thermostat differential too small — or at high head pressure tripping the high-pressure switch. Check the switch settings and the condenser before touching the charge.
What causes high room humidity in a cold room?
Infiltration through doors and seals, an evaporator running at too small a temperature difference, or product being loaded warm and wet. Persistent humidity shows up as ice on the coil and on the room ceiling near the door.
How do I find out why a cold room slowly warmed over weeks?
Slow drift usually means gradual loss of capacity: a dirty condenser, a slow leak, or a coil progressively icing because defrost is marginal. Compare current pressures and superheat against the last recorded service reading — this is exactly why keeping equipment history matters.
Should the condensing unit or the evaporator be sized first?
The load determines both. Sizing the evaporator to the room load and then matching the condensing unit at the design evaporating temperature is the normal order. A mismatched pair produces symptoms that look like faults — short cycling, poor pull-down or excessive coil frost.
Contactors and control panels
What are terminals A1 and A2 on a contactor?
A1 and A2 are the coil terminals — the control circuit that pulls the contactor in. Measure voltage across A1-A2 to confirm the control circuit is calling; measure the coil resistance with power off to confirm the coil is not open or shorted.
What are terminals 1-3-5 and 2-4-6 on a contactor?
1/L1, 3/L2 and 5/L3 are the line-side main contacts and 2/T1, 4/T2 and 6/T3 are the load side. With the contactor pulled in, measure voltage drop across each pole — a significant drop across a closed contact means pitted or burnt contacts.
What are terminals 13-14 and 21-22?
Auxiliary contacts. 13-14 is normally open and 21-22 is normally closed. They are used for interlocks, indication and holding circuits rather than for carrying motor current.
What are terminals 95-96 and 97-98 on an overload relay?
95-96 is the normally closed trip contact wired into the control circuit, so it opens and drops the contactor out when the relay trips. 97-98 is the normally open indication contact, used for alarms or lamps. Checking continuity across 95-96 tells you immediately whether the overload has tripped.
How do I tell if a contactor coil has failed?
With power off, measure resistance across A1-A2 — an open reading means a burnt coil. If the coil reads correct and rated voltage is present at A1-A2 but the contactor does not pull in, the armature is mechanically stuck. Coil burnout is one of the most common panel failures.
What causes contactor contacts to weld shut?
Repeated high inrush current, short cycling, or an undersized contactor. A welded contactor keeps the compressor energised even when the control circuit drops out, which is dangerous — it must be replaced, not filed.
Why should I never leave a jumper across a pressure switch?
A jumper across a safety switch removes the protection that stops the compressor before damage occurs. Using one briefly to prove whether the switch or the control circuit is at fault is a recognised diagnostic step, but it must be removed immediately afterwards. Leaving it in place is how compressors are destroyed.
What do the labels on a control panel tell me?
On a typical refrigeration panel the contactors are labelled by load — compressor, defrost heater, evaporator fan, condenser fan — often with a hand-written or printed sticker. Reading those labels first tells you which contactor belongs to the complaint before you measure anything.
Leaks and moisture
How do I find a small refrigerant leak?
Work from most to least likely: visible oil traces at joints, then electronic detector along the whole circuit, then bubble solution on suspect joints, then a nitrogen pressure test with the system isolated. UV dye helps on intermittent leaks but takes a return visit.
What pressure should I use for a nitrogen leak test?
Test pressure depends on the equipment design pressure and local regulation — never simply pressurise to cylinder pressure. Use dry nitrogen with a regulator, hold the test long enough for temperature to stabilise, and record start and end pressure so a slow leak is visible.
How deep should I evacuate a system?
Common practice is to evacuate below 500 microns and then confirm the vacuum holds with the pump isolated. A rising vacuum means either moisture still boiling off or a leak; a rapid rise to atmospheric means a leak.
Why does moisture matter so much with POE oil?
POE ester oil absorbs moisture from the air quickly and does not release it easily under vacuum. Water in the system forms acid, damages windings and freezes at the metering device. Keep POE containers sealed and do not leave a system open longer than necessary.
Both sides low on a cascade system — is it a leak?
If both circuits read low, treat it as a leak rather than a charging problem. Check soak pressure with the plant off, then pressure test, then evacuate. On ultra-low temperature systems even a small loss shows up immediately in performance, so topping up without finding the leak means a repeat call within weeks.
Cascade and two-stage systems
How does a cascade refrigeration system work?
Two separate refrigeration circuits are linked by a cascade heat exchanger. The high stage rejects heat to ambient and cools the cascade exchanger; the low stage rejects its heat into that exchanger rather than to ambient, which is how temperatures around −80°C are reached with normal components.
The high stage runs but the low stage does not start — what is wrong?
That is the designed sequence: the high stage must run first and cool the cascade heat exchanger before the low stage is allowed to start. Check that the high stage is running correctly before looking at low-stage parts. The most common actual fault is a failed low-stage compressor relay; the pressure switch should be suspected alongside it.
Why must the low stage never run without the high stage?
Because the low stage has nowhere to reject its heat. Discharge pressure rises immediately and the compressor is damaged. That is why the interlock exists, and why bypassing it to "test" the low stage destroys compressors.
The freezer only reaches −55°C instead of −80°C — where do I look?
It looks like a low-stage problem but is often high-stage charge loss: if the high stage cannot cool the cascade exchanger enough, the low stage cannot reach its design temperature no matter how well it runs. Check high-stage charge and capillary restriction as well as the low stage.
Why do two-stage compressors need different oil handling?
On a two-stage compressor the oil sump sits at intermediate pressure rather than suction pressure, so oil level and return behave differently from a single-stage machine. Judging the oil by single-stage habits leads to wrong conclusions. Two-stage exists because a single compressor pulling the whole compression ratio would drive discharge temperature high enough to carbonise the oil.
Manufacturer error codes
How do I look up a manufacturer error code?
Enter the brand and the code in R-Pro and you get the meaning, the likely causes and a link to the maker's service manual for that model. The database covers 1,879 codes across 62 brands, including regional makers that general search engines return poorly.
Why does the same code mean different things on different brands?
Error codes are not standardised across manufacturers. E5 on one brand may indicate a communication fault and on another a pressure problem. Always look the code up against the specific brand and, where it matters, the specific series.
What does a communication error between indoor and outdoor units usually mean?
Most often a wiring fault on the communication line, incorrect polarity, damaged cable, or a failed control board. Check the physical connection and voltage on the communication line before ordering a board.
Does an error code tell me the failed part?
It tells you what the control board detected, which is not always the failed part. A sensor fault code can mean a failed sensor, a damaged harness or a board input problem. The code narrows the branch; measurement identifies the part.
What if the unit shows no code but still fails?
Many faults never produce a code — mechanical, refrigerant and airflow problems in particular. That is why symptom-based diagnosis and real-measurement diagnosis exist alongside the error code database.
Can I read error codes offline?
Yes. The whole error-code database, including the manual links, is stored in the app and opens with no connection, which is the point — machine rooms rarely have signal.