Condensing pressure too low

Troubleshooting · Applies to: Direct expansion refrigeration system · Heat pump and air conditioning · Liquid chiller (water or brine) · Cold rooms with several evaporators

Condensing pressure too low: cold air and fan control, low suction pressure, a cold receiver, leaking valves and settings.

Last checked: October 25, 2026 Field mode: large buttons, questions first and the screen kept on.

A condensing pressure that is too low gives too small a pressure difference across the expansion valve, so the evaporator gets too little refrigerant and bubbles appear in the sight glass. Danfoss mentions an oversized condenser, low load, low suction pressure, leaking compressor valves, a pressure regulator set too low and an uninsulated receiver standing colder than the condenser ^[1]. The problem shows mostly in winter on air-cooled condensers.

Work on the refrigerant circuit may only be carried out by certified personnel. Isolate the power before working on electrical parts and take particular care with high pressures (CO2) and flammable refrigerants (A2L and A3). Always follow the manufacturer's instructions.

Confirm the fault first

Measure the condensing pressure with a gauge and convert it to temperature. Compare with the outdoor temperature and with the head pressure setpoint. Check whether there are also bubbles in the sight glass and whether the suction pressure is low.

📏 Measurement guide: calculate superheat and subcooling 🔢 Alarm and fault codes

Flowchart

Causes, most likely first

The order is approximate and based on common faults in the field. Always measure before replacing anything.

1
Flowchart A
The fan control does not hold up the pressure in cold outdoor air.
Remedy: Check the fan control, speed control and the condensing pressure setpoint. With a large condenser, a head pressure control valve may be needed.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check head pressure control
Too low condensing pressure in winter starves the expansion valve; too high in summer costs energy and trips the HP switch. Check fan control, head pressure valves and water regulating valves against the setpoint.
1. Read the condensing pressure and setpoint. Connect a calibrated gauge to the discharge or liquid line port and note the condensing pressure setpoint and fan stages in the controller or speed controller. Never exceed the system PS.
  ✓ Check: You know the actual condensing pressure and the setpoint.
2. Check fan staging and speed control. Watch the fans as the pressure rises and falls: stages cut in and out, or the speed changes, around the setpoint. Measure the speed controller output if a fan does not respond.
  ✓ Check: Fans start, stop or change speed at the set pressures.
3. Check the fans with power off. If a fan does not run, isolate and lock off, check the fan motor and blades and that the coil is clean before blaming the control. Fans can start automatically.
  ✓ Check: Fans turn freely and the condenser is clean.
4. Check head pressure valves in cold weather. On systems with head pressure valves (e.g. Sporlan ORI/ORD or Danfoss KVR/NRD), check during low ambient that the bypass to the receiver gets warm when the valve regulates, and that there is enough refrigerant for the flooded condenser (winter charge, sight glass full).
  ✓ Check: The condensing pressure is held at the valve setting and the sight glass stays full in cold weather.
5. Check the water regulating valve. On water-cooled condensers, check that the water regulating valve opens more as the condensing pressure rises and closes at standstill. Compare water in and out temperatures.
  ✓ Check: The valve modulates and the condensing pressure is steady; no water flows at standstill.
6. CO2 gas coolers. On transcritical CO2 (R744) the high pressure is set by the high pressure valve and the gas cooler fans together; check it as described in the CO2 high pressure valve guide.
  ✓ Check: Gas cooler outlet temperature and high pressure follow the controller setpoints.
2
The fan control pressure or temperature sensor reads wrong, so the fans run too fast.
Remedy: Compare the sensor with a calibrated gauge or thermometer and replace a faulty sensor.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check a sensor against a reference
A temperature or pressure sensor that reads wrong makes the controller regulate wrong: wrong superheat, wrong defrost, false alarms. Compare it with a calibrated reference before changing settings or parts.
1. Fit the reference next to the sensor. Fasten a calibrated thermometer probe beside the temperature sensor on the same pipe or in the same air stream, with heat-conducting paste and insulation. For a pressure sensor, connect a calibrated gauge to a service port at the same point in the circuit. Wear goggles and gloves when connecting to the refrigerant circuit.
  ✓ Check: The reference reading is steady for a few minutes.
2. Compare with the controller reading. Read the live value of the sensor in the controller display or service tool at the same time as the reference.
  ✓ Check: The difference is within the tolerance given by the sensor or controller manufacturer (for a temperature sensor typically within about 1 K).
3. Check offsets and sensor type. In the controller parameter list, check that the sensor type (e.g. NTC, PT1000, 0–10 V, 4–20 mA), the pressure range and any calibration offset match the fitted sensor. Record the original values before changing anything.
  ✓ Check: Sensor type, range and offset in the controller match the sensor data sheet.
4. Measure the sensor resistance. Isolate and lock off the controller, disconnect the sensor and measure its resistance with a multimeter while the reference shows the temperature at the sensor. Compare with the manufacturer's resistance table. A PT1000 reads 1000 Ω at 0 °C; an ice-water bath gives a simple 0 °C check point.
  ✓ Check: The resistance matches the table value for the reference temperature; OL or 0 Ω means a broken or shorted sensor or cable.
5. Check the sensor location and mounting. Inspect the pipe clamp or pocket: firm metal-to-metal contact, heat-conducting paste, insulation against the surrounding air and the correct position (e.g. evaporator outlet, not in the air stream of a fan).
  ✓ Check: The sensor sits firmly at the correct point and is insulated.
6. Reconnect and confirm. Reconnect the sensor, restore power and compare the reading with the reference once more. Replace a sensor that is out of tolerance with one of the same type.
  ✓ Check: Controller and reference agree within the tolerance.
3
Flowchart B
Too little refrigerant: both the suction and the condensing pressure fall.
Remedy: Leak test and repair the leak before adding refrigerant; follow the low-pressure troubleshooting.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Leak test a refrigeration system
Find leaks systematically, confirm the repair and record the work. Work that opens the refrigerant circuit requires F-gas certification (Regulation (EU) 2024/573 or national equivalent, EPA 608 in the USA).
1. Prepare the system and the detector. The system must be under pressure (running, or at standstill with warm ambient). Stop fans in the area to get calm air and ventilate low or enclosed spaces. With A2L/A3 refrigerants use no ignition sources and only detectors rated for them; with CO2 (R744) remember the very high pressure. Check the electronic detector (EN 14624) against its reference leak or as the manufacturer describes.
  ✓ Check: The detector reacts to the reference leak, and the air around the system is calm.
2. Search with the electronic detector. Move the probe slowly, a few centimetres per second, close to every brazed joint, flare, valve stem, Schrader core, gauge and coil bend. Refrigerants are heavier than air, so hold the probe at and below each joint. Follow the circuit in a fixed order so that nothing is missed.
  ✓ Check: Every joint along the circuit has been passed, and each alarm has been repeated at the same spot.
3. Confirm with bubble solution. Spray bubble leak solution on each suspected point and watch it for at least a minute; small leaks form bubbles or foam slowly.
  ✓ Check: Bubbles or foam pinpoint the leak, or the point is confirmed tight.
4. Use UV dye only where it is approved. If the system already contains a UV dye approved by the compressor manufacturer, light the joints with a UV lamp and wear UV glasses.
  ✓ Check: A fluorescent trace marks the leak, or no trace is visible.
5. Repair correctly. Recover the refrigerant (or isolate the section), repair with nitrogen flowing when brazing, pressure test with nitrogen, evacuate and charge by weight. Never top up refrigerant 'to test' without finding and repairing the leak.
  ✓ Check: The repaired joint holds the nitrogen test pressure and the system is charged with the specified amount.
6. Record and re-check. Write the leak location, cause, repair, refrigerant amount and your certificate number in the equipment logbook. Check the repair again within one month, as the F-gas regulation requires.
  ✓ Check: The logbook entry is complete and a follow-up check is booked.
Continue with: System trips on low pressure →
4
Flowchart C
The receiver acts as a condenser and pulls the pressure down.
Remedy: Insulate the receiver or move it to a warmer place.
5
Flowchart E
A head pressure valve or water valve (water-cooled condenser) is set too low or stuck open.
Remedy: Set the regulator as designed. Clean or replace a water valve that does not close ^[1].
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check a pressure regulator
Evaporator, crankcase and condensing pressure regulators keep a pressure within limits. A wrong setting or a sticking valve gives wrong evaporating temperature, an overloaded compressor or low head pressure in winter.
1. Identify the regulator and its setting. Find out which type it is (evaporator pressure regulator holds the upstream pressure, crankcase pressure regulator limits the downstream pressure, condensing pressure regulator holds the upstream pressure), and the set value in the plant documentation or on the valve label.
  ✓ Check: You know which side the regulator controls and what pressure it should hold.
2. Connect gauges before and after. Connect calibrated gauges to the service ports before and after the regulator, with minimal refrigerant loss. Wear goggles and gloves.
  ✓ Check: Both gauges show stable readings that agree with the controller's transducers.
3. Compare with the setting in stable operation. Let the plant run at steady load for at least 10–15 minutes. Read the controlled pressure and the pressure drop across the valve and compare with the set value.
  ✓ Check: The controlled pressure is close to the set value; a large difference or no pressure drop at all means a wrong setting or a valve that sticks.
4. Adjust in small steps. Remove the cap and turn the adjusting spindle with the valve key a little at a time, in the direction the manufacturer gives. Wait for stable pressures after each step.
  ✓ Check: The pressure follows each adjustment smoothly and settles at the set value.
5. Check that it closes and does not hunt. Watch the gauges over a full load change (or a compressor stage switching). An evaporator pressure regulator must close when the pressure falls below its setting; a crankcase pressure regulator must throttle at start-up.
  ✓ Check: The pressure stays at the limit without creeping or swinging; the compressor current stays within the nameplate value at start-up.
6. Replace a faulty valve. If the valve does not respond or leaks through, it must be replaced. This opens the refrigerant circuit: only certified personnel (F-gas / EPA 608), recover the refrigerant, braze with nitrogen, pressure test, evacuate and record the work in the logbook.
  ✓ Check: The new valve holds the set value and the work is recorded.
Check head pressure control
Too low condensing pressure in winter starves the expansion valve; too high in summer costs energy and trips the HP switch. Check fan control, head pressure valves and water regulating valves against the setpoint.
1. Read the condensing pressure and setpoint. Connect a calibrated gauge to the discharge or liquid line port and note the condensing pressure setpoint and fan stages in the controller or speed controller. Never exceed the system PS.
  ✓ Check: You know the actual condensing pressure and the setpoint.
2. Check fan staging and speed control. Watch the fans as the pressure rises and falls: stages cut in and out, or the speed changes, around the setpoint. Measure the speed controller output if a fan does not respond.
  ✓ Check: Fans start, stop or change speed at the set pressures.
3. Check the fans with power off. If a fan does not run, isolate and lock off, check the fan motor and blades and that the coil is clean before blaming the control. Fans can start automatically.
  ✓ Check: Fans turn freely and the condenser is clean.
4. Check head pressure valves in cold weather. On systems with head pressure valves (e.g. Sporlan ORI/ORD or Danfoss KVR/NRD), check during low ambient that the bypass to the receiver gets warm when the valve regulates, and that there is enough refrigerant for the flooded condenser (winter charge, sight glass full).
  ✓ Check: The condensing pressure is held at the valve setting and the sight glass stays full in cold weather.
5. Check the water regulating valve. On water-cooled condensers, check that the water regulating valve opens more as the condensing pressure rises and closes at standstill. Compare water in and out temperatures.
  ✓ Check: The valve modulates and the condensing pressure is steady; no water flows at standstill.
6. CO2 gas coolers. On transcritical CO2 (R744) the high pressure is set by the high pressure valve and the gas cooler fans together; check it as described in the CO2 high pressure valve guide.
  ✓ Check: Gas cooler outlet temperature and high pressure follow the controller setpoints.
6
Flowchart D
The compressor pumps poorly.
Remedy: Troubleshoot the compressor that runs but does not cool.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Test whether the compressor pumps
Low pressure difference, low current and poor capacity can mean a worn compressor, but often the cause is a bypass elsewhere. Compare measurements with the manufacturer's data and rule out other causes before you condemn the compressor.
1. Measure in stable operation. Connect gauges to suction and discharge, and clamp thermometers on the suction line and on the discharge line close to the compressor, where the manufacturer specifies. Let the system run steadily for at least 15 minutes. Beware of the hot discharge pipe.
  ✓ Check: Pressures and temperatures are steady and noted together with the saturation temperatures.
2. Measure current and voltage. Measure the supply voltage and the current in each phase with a true-RMS clamp meter (CAT III/IV, insulated probes, no loose clothing).
  ✓ Check: You have voltage and current for all phases; the phases are balanced.
3. Compare with the compressor data. Enter the measured evaporating and condensing temperatures in the manufacturer's selection software or data sheet and read the expected current, capacity and discharge temperature.
  ✓ Check: Measured current and pressure difference are close to the data; clearly lower current at a low pressure difference suggests the compressor is not pumping.
4. Rule out bypasses. Check the hot gas bypass valve, unloaders or capacity control solenoids, a digital scroll valve and any reversing valve: a pipe that is hot where it should be cold shows gas leaking back to suction.
  ✓ Check: All bypass paths are closed when they should be; otherwise repair them first and measure again.
5. Check the internal relief valve. Many hermetic and scroll compressors have an internal pressure relief valve (IPR) between discharge and suction. If it has opened, suction pressure is high, discharge low, the shell is hot and the motor protection may trip. Find out why it opened (high pressure, blocked condenser) before restart.
  ✓ Check: The cause of an opened relief valve is found and removed, or the relief valve is ruled out.
6. Check the valves after stop (reciprocating). On a reciprocating compressor, watch the gauges right after it stops. Discharge pressure falling fast while suction rises points to leaking discharge valves. Do not run scroll compressors into deep vacuum to test them; it damages them.
  ✓ Check: Pressures equalise slowly; fast equalisation confirms leaking valves.
7. Decide. If the compressor still pumps too little with all other causes ruled out, plan a compressor replacement (see the guide on replacing a compressor). Record the measurements.
  ✓ Check: The decision is based on measurements compared with the manufacturer's data, and they are written down.
Continue with: Compressor runs but does not cool →

Checks from the manufacturers’ manuals (7)

Values and checks the manufacturers give for their own products; they apply to the product named.

Temperzone air-to-air units, UC8: Heat pump in heating mode: the condensing pressure falls when the indoor fan runs high in a very cold room or when a lot of cold outdoor air is brought in through the fresh air damper.^[2]
Actron Air PKV160T–240T: Inverter units with compressor envelope protection trip on 'low compression ratio' when the condensing pressure stays too low (about 90 s on Actron PKV); an outdoor fan stuck at high speed, for example a welded fan relay on the board, is a typical cause.^[3]
Wessamat MicroCube; True Ice: Water-cooled ice machines and condensing units: a water regulating valve that passes too much water gives low head pressure and high water consumption. Check its setting against the maker's value (Wessamat MicroCube: about +30 °C) and adjust or replace it.^[4]
Hussmann transcritical CO2 racks: CO2 booster in cold weather: with the high-pressure valve almost closed the receiver pressure can fall too low for the case valves and the oil system. Check that the hot gas bypass into the receiver opens (Hussmann typically at about 31.7 bar g, 460 psig) and lifts the pressure.^[5]
Hitachi RUA-NP4ATS…RUA-NP10ATS self-contained air conditioners (R410A): The RUA-NP is designed for condenser air inlet temperatures of 20–52 °C DB and evaporator air inlet from 19.5 °C DB / 14 °C WB up to 35 °C DB / 21.5 °C WB; the manual describes no head pressure control. Running in cooling at outdoor temperatures below 20 °C, or with cold return air, is outside the design range and gives low head pressure, low suction pressure and possible low pressure trips.^[6]
Daikin VRV, VRV-III and VRV-WII systems (RXYQ, REYQ, RWEYQ) with FXxQ indoor units: In cooling at low outdoor temperature a VRV outdoor unit lowers the outdoor fan step as the high pressure falls, so fans that run slowly or stop in cold weather in cooling mode are part of normal control, not a fan fault.^[7]
NTF linear (auger) granular ice makers, G series (G160, G280, G510, G1000): NTF G-series auger machines (air-cooled): the condenser fan pressure switch is factory set to cut in and out between about 14.8 and 16 bar. It can be adjusted with the screw on the switch: one turn moves the setting about 1.2 bar, clockwise raises it, and no more than 3 turns from the original setting.^[8]

📝 Fill in a service report

Terms in this article

Subcooling Liquid receiver Sight glass

Sources

Danfoss Ref Tools – Troubleshooter: condensing pressure too low
Temperzone – UC8 Troubleshooting Guide, issue 2 (27 July 2015), p. 14
Actron Air – Variable Capacity Commercial Troubleshooting Guide PKV160T–240T, Doc. 9590-2013-03 Ver. 2 (2025-10-30), § 03.03 Fault Code Troubleshooting, E9/E11, p. 16–17
Wessamat – MicroCube M120–M800 operating manual DE/EN (2023-04-17) + True Manufacturing – True Ice service manual TEC_TM_198 Rev. A (03/2024), Wessamat technical annex §2 Störungsbeseitigung (PDF p. 32); True p. 64
Hussmann – Transcritical CO2 Rack, Installation & Operation, PN 3182569_G, Liquid Injection, Table 7, p. 61
Hitachi – Self-Contained Air Conditioners RUA-NP4ATS…RUA-NP10ATS (R410A), Installation, Operation and Maintenance Instructions Manual 65MP3197-A2 (Bosch Home Comfort Philippines); undated, PDF file created 2025, Operating range (preface, Condenser and Evaporator Air Inlet Temperature)
Daikin AC (Americas) – VRV Service & Troubleshooting, Participant Guide: VRV Systems Basic Operation Guide, Engineering Data EGVUSE09-07C (09/2007), with VRV III service manual navigation and Service Checker Type III workshop slides, 5.1.1 Cooling Operation, p. 24
NTF – Technical Documentation, NTF ice makers families (SL, IC, CM, FRM/IW/DSS/ID, SLF/SLT, CVC/CV, GM/MGT, IFT, SM), 2019 (sections dated 2006–2018), Linear ice makers series – More info (condenser pressure switch), page 116

Condensing pressure too low

Confirm the fault first

Troubleshoot step by step

Flowchart

Causes, most likely first

Check head pressure control

Check a sensor against a reference

Leak test a refrigeration system

Check a pressure regulator

Check head pressure control

Test whether the compressor pumps

Checks from the manufacturers’ manuals (7)

Service report

Terms in this article

Sources