Heat pump outdoor unit ices up

Causes, most likely first

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

1. 1
   Flowchart A

   The defrost does not start or ends too early: sensor fault, wrong parameters or a four-way valve that does not change over.

   Remedy: Check the defrost sensor and parameters, and that the four-way valve changes over (the pressures and temperatures swap).

   🛠 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 the defrost system

   Incomplete defrost leaves ice on the evaporator, which reduces capacity and can block the airflow completely. Check heaters or hot gas, termination, interval, drip time and fan delay.

   1. Check the defrost settings. Read defrost type, interval, maximum time, termination temperature, drip time and fan delay in the controller and compare with the manufacturer's recommendations. Check the controller clock if defrosts are scheduled.

      ✓ Check: The settings match the application and the clock is right.

   2. Measure the heater resistance. Isolate, lock off and prove dead, then disconnect the heaters and measure each one. The expected resistance is R = U² ÷ P from the heater's voltage and power. Also measure insulation to earth with an insulation tester.

      ✓ Check: Each heater is close to the calculated value and has no earth fault; OL means a broken heater.

   3. Measure the current during defrost. Restore power, start a manual defrost and measure the current in each heater supply with a clamp meter. Watch out for hot surfaces.

      ✓ Check: The current is about P ÷ U for each heater, or matches the nameplate defrost current.

   4. Check hot gas defrost. On hot gas systems, check that the hot gas valve opens (warm pipe after the valve) and that the liquid or suction valves close as intended. Never exceed the system PS.

      ✓ Check: Hot gas reaches the evaporator and the coil warms evenly.

   5. Check termination. Check that the termination sensor sits on the coldest part of the coil, usually where ice remains longest, and that the defrost ends on temperature rather than on maximum time.

      ✓ Check: The defrost ends on temperature and the coil is completely free of ice.

   6. Check drip time and fan delay. After the defrost, watch the drip time and the evaporator fans when refrigeration restarts. Check that the drain and the drip tray heater work.

      ✓ Check: Fans stay off until the coil has reached the fan-delay setpoint, and water drains without refreezing in the tray.

   Check a four-way (reversing) valve

   A heat pump that will not change mode, or loses capacity, often has a reversing valve that does not get a signal, does not shift or leaks internally. Temperatures on the pipes tell you which.

   1. Find out the valve logic. Check in the wiring diagram or manual in which mode the coil is energised. This differs between makers: some energise in cooling, others in heating. Note the mode the controller asks for.

      ✓ Check: You know whether the coil should be on or off in the current mode.

   2. Check the coil and its signal. With a CAT III/IV meter, measure the voltage at the coil when the mode changes. Then isolate, lock off, prove dead, and measure the coil resistance. Check that the coil voltage marking matches the supply and that the coil sits fully on the valve stem.

      ✓ Check: The coil gets its rated voltage in the right mode and its resistance matches the data sheet; OL means a broken coil.

   3. Measure the pipe temperatures. With the compressor running, measure the four main pipes with clamp thermometers: discharge into the valve (hot), suction from the valve to the compressor (cold), and the two coil connections. Beware of the hot discharge pipe. The valve needs enough pressure difference to shift, so check with the compressor running.

      ✓ Check: The hot side follows the mode: to the outdoor coil in cooling, to the indoor coil in heating.

   4. Check the pilot tubes at switch-over. Change the mode and feel or measure the small pilot capillaries on the valve during the switch. Their temperatures should change as the pilot valve moves.

      ✓ Check: The pilot tube temperatures change at switch-over; if they do but the main pipes do not, the main slide is stuck.

   5. Look for internal leakage. Compare the temperature of the suction pipe between valve and compressor with the pipe coming from the coil that is working as evaporator. A clearly warmer pipe after the valve means hot gas is leaking across the slide.

      ✓ Check: Both pipes have nearly the same temperature; a large difference, together with low capacity, points to internal leakage.

   6. Try to free it, otherwise replace. Switch the mode several times with the compressor running and full pressure difference. If the valve still sticks or leaks, it must be replaced by F-gas certified personnel: recover the refrigerant, braze with nitrogen flowing and a heat sink or wet cloth on the valve body, pressure test, evacuate and weigh in the charge. With R32 or R290 follow the rules for flammable refrigerants (no ignition sources, ventilation, gas detector).

      ✓ Check: After repair the valve shifts in both directions and the pipe temperatures follow the mode.

   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.

2. 2
   Flowchart B

   The meltwater freezes in place and builds up ice from below.

   Remedy: Provide free drainage and, if needed, a heating cable in the drip tray and drain. Mount the unit high enough above the ground and snow.

   🛠 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 drain, tray and frame heaters

   Heater cables keep drains, drip trays, door frames and pressure relief ports free of ice. A broken heater gives a blocked drain, water on the floor or a frozen door.

   1. Isolate, lock off and prove dead. Switch off and lock the isolator for the heater circuit, attach a tag and prove dead with a two-pole voltage tester (checked on a known live source before and after) at the heater terminals.

      ✓ Check: The tester shows no voltage between the conductors and between each conductor and earth.

   2. Check fuse, thermostat and control output. Check the fuse or circuit breaker for the heater and the thermostat or controller relay that switches it. A drain heater is often switched only during defrost; a frame heater is often on all the time. Read the wiring diagram.

      ✓ Check: The fuse is intact and you know when the heater should be on.

   3. Measure the resistance. Disconnect the heater leads and measure resistance across the element. Calculate the expected value from the nameplate: R = V² / P (for example 230 V, 100 W gives about 530 Ω). Self-regulating cables cannot be judged this way; check them by current instead.

      ✓ Check: The reading is close to the calculated value; OL (open) means a broken element, near 0 Ω means a short.

   4. Measure insulation resistance to earth. Use an insulation tester at 500 V DC between the joined heater conductors and the earth conductor or metal braid, with the heater disconnected from any electronics.

      ✓ Check: At least 1 MΩ (IEC 60364-6 minimum for 230/400 V circuits); a low or falling value means moisture in the cable or end seal.

   5. Measure voltage and current in operation. Reconnect, refit the covers and switch on. Make the controller call for the heater (or start a defrost), then measure voltage at the heater and current with a clamp meter. Expected current is about P / V.

      ✓ Check: The heater gets full voltage and draws roughly the nameplate current.

   6. Check the heater position. Check that the drain heater runs the full length of the drain inside the cold space and through the wall, that the tray heater lies against the tray, and that the frame heater warms the whole door frame.

      ✓ Check: After the next defrost the drain runs freely and the frame is warm and free of ice.

   Clear the condensate drain

   A blocked or frozen drain lets melt water overflow into the room and freeze on the floor or the coil. Clear it, then fix the cause: fall, trap or a dead heater cable.

   1. Isolate and protect yourself. Isolate, lock off and prove dead the evaporator before working at the tray. Wet cold-room floors are slippery: wear shoes with grip and tell someone you are inside.

      ✓ Check: The meter shows 0 V on the evaporator terminals.

   2. Thaw and clean the tray and drain. Thaw ice in the tray and drain with warm water. Clean out slime and debris with a flexible drain brush.

      ✓ Check: Warm water poured into the tray runs straight out without backing up.

   3. Measure the heater cable resistance. With the power still off, disconnect the drain heater cable and measure its resistance. Calculate the expected value from the rating (R = U² / P) or take it from the data sheet. Check the fuse and the thermostat, if any.

      ✓ Check: The resistance is close to the expected value; OL (open) means a broken cable.

   4. Check the heater in operation. Reconnect, refit covers and switch on. Measure the current with a clamp meter and feel or measure that the drain warms along its full length inside the cold room.

      ✓ Check: The current matches the rating and the drain is warm all the way out of the room.

   5. Check fall and trap. Check with a spirit level that the drain has a continuous fall with no sags. In freezer rooms the trap should be outside the cold room, in the warm area, so it cannot freeze.

      ✓ Check: The drain falls all the way, the trap holds water and is placed where it cannot freeze.

   6. Run a defrost and check. Start a defrost and watch the melt water.

      ✓ Check: All melt water leaves the room and nothing refreezes in the tray or drain.

3. 3

   The heater cable in the base tray or drain has failed.

   Remedy: Measure the heater cable resistance and voltage and check the fuse.

   🛠 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 drain, tray and frame heaters

   Heater cables keep drains, drip trays, door frames and pressure relief ports free of ice. A broken heater gives a blocked drain, water on the floor or a frozen door.

   1. Isolate, lock off and prove dead. Switch off and lock the isolator for the heater circuit, attach a tag and prove dead with a two-pole voltage tester (checked on a known live source before and after) at the heater terminals.

      ✓ Check: The tester shows no voltage between the conductors and between each conductor and earth.

   2. Check fuse, thermostat and control output. Check the fuse or circuit breaker for the heater and the thermostat or controller relay that switches it. A drain heater is often switched only during defrost; a frame heater is often on all the time. Read the wiring diagram.

      ✓ Check: The fuse is intact and you know when the heater should be on.

   3. Measure the resistance. Disconnect the heater leads and measure resistance across the element. Calculate the expected value from the nameplate: R = V² / P (for example 230 V, 100 W gives about 530 Ω). Self-regulating cables cannot be judged this way; check them by current instead.

      ✓ Check: The reading is close to the calculated value; OL (open) means a broken element, near 0 Ω means a short.

   4. Measure insulation resistance to earth. Use an insulation tester at 500 V DC between the joined heater conductors and the earth conductor or metal braid, with the heater disconnected from any electronics.

      ✓ Check: At least 1 MΩ (IEC 60364-6 minimum for 230/400 V circuits); a low or falling value means moisture in the cable or end seal.

   5. Measure voltage and current in operation. Reconnect, refit the covers and switch on. Make the controller call for the heater (or start a defrost), then measure voltage at the heater and current with a clamp meter. Expected current is about P / V.

      ✓ Check: The heater gets full voltage and draws roughly the nameplate current.

   6. Check the heater position. Check that the drain heater runs the full length of the drain inside the cold space and through the wall, that the tray heater lies against the tray, and that the frame heater warms the whole door frame.

      ✓ Check: After the next defrost the drain runs freely and the frame is warm and free of ice.

4. 4
   Flowchart C

   Too little airflow through the coil.

   Remedy: Remove obstructions and snow and check the fan. Follow the manufacturer's clearances.

   🛠 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.

   Clean a condenser or gas cooler

   A dirty condenser or gas cooler raises the condensing pressure, the power consumption and the risk of high pressure trips. Clean it, straighten the fins and check that air can flow freely.

   1. Measure before you clean. With the system running, note the condensing pressure (or gas cooler outlet temperature on CO2) and the air-on temperature, so that you can see the effect afterwards.

      ✓ Check: Pressure and air temperature are recorded.

   2. Isolate and lock off. Isolate the condensing unit and fans, lock and tag the isolator and prove dead at the fan motor terminals. Fans can otherwise start automatically. On roofs, use fall protection.

      ✓ Check: 0 V at the fan terminals and the fans have stopped.

   3. Remove loose dirt. Brush off leaves and fluff from the air-entering side with a soft brush along the fins. Blow or rinse from the air-leaving side, against the normal airflow, so dirt is pushed out rather than in.

      ✓ Check: The surface of the coil is free of loose dirt.

   4. Clean with coil cleaner and water. Protect fan motors and electrical parts from water. Apply a coil cleaner suitable for the fin material, follow its safety data sheet (gloves and goggles) and rinse with low-pressure water against the normal airflow. Never use a high-pressure washer, which flattens the fins.

      ✓ Check: The rinse water runs clear and the fins are undamaged.

   5. Straighten fins and check clearances. Straighten bent fins with a fin comb of the right pitch. Check that nothing blocks the air inlet or outlet and that warm air cannot recirculate to the inlet (walls, snow, other units).

      ✓ Check: Fins are straight and the free distances around the unit match the installation instructions.

   6. Restart and compare. Remove your lock, restore power and let the system stabilise. Compare the condensing pressure and the air-on temperature with the values before cleaning.

      ✓ Check: The condensing temperature is lower at a similar air temperature.

   Check a condenser or evaporator fan motor

   A fan that stops, runs slowly or turns the wrong way gives high pressure on the condenser side, or low pressure and ice on the evaporator side. Check the supply, the control signal, the motor and the fan itself.

   1. Look and listen with the unit running. Watch the fan from a safe distance: does it run, at what speed, and in the right direction (air through the coil the way the arrow on the housing or the manual shows)? Listen for bearing noise and look for ice, dirt or a damaged blade.

      ✓ Check: You know whether the fan is stopped, slow, noisy or turning the wrong way.

   2. Check the supply and the control signal. Measure the voltage at the motor terminals or the EC fan connector with the fan called for. For an EC fan also measure the control signal (0–10 V or PWM) and read any status lamp or fault code on the fan electronics. Check fan delay settings and thermostats in the controller: a fan that is held off on purpose is not faulty.

      ✓ Check: Supply voltage is within the rated range on the nameplate, and the control signal asks for the fan to run; otherwise the fault is upstream (contactor, fuse, controller output, wiring).

   3. Isolate, lock off and make sure the fan cannot start. Switch off and lock the isolator, prove the circuit dead with a tested two-pole tester, and wait for the fan to stop completely. Remember that an EC fan can restart on its own when the signal returns, and that a fan wind-milling in a draught can generate voltage.

      ✓ Check: 0 V on the motor terminals and the blades are blocked or stationary.

   4. Turn the fan by hand and check it mechanically. Turn the impeller by hand: it should spin freely and evenly, without scraping, axial play or a rough bearing. Check that the blades are tight on the shaft and not cracked, and that the guard and the coil are free of ice, dirt and loose objects.

      ✓ Check: The fan turns freely and quietly and the blades are undamaged; a stiff or rough motor must be replaced.

   5. Measure the motor (AC motors). With the motor disconnected, measure the winding resistance between the terminals and compare the windings with each other and with the data sheet; test the run capacitor (see the capacitor guide). Measure the insulation resistance to earth with an insulation tester – never with the fan electronics or an EC motor connected. Check that the motor's internal thermal protection is closed when the motor is cold.

      ✓ Check: Windings are equal where they should be and match the data sheet, insulation resistance is well above the limit your electrical rules require, and the thermal protection is closed when cold.

   6. Restore, run and measure the current. Reconnect from your photo, fit guards and covers, remove the lock and start. Measure the running current on each phase and compare it with the nameplate, and check the direction and the air flow.

      ✓ Check: The fan runs smoothly in the right direction, the current is at or below the nameplate value and the pressures settle (head pressure falls on a condenser, suction pressure rises and ice clears on an evaporator).

5. 5

   The outdoor fan motor or capacitor has failed.

   Remedy: Measure voltage and current on the fan motor and check the capacitor.

   🛠 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 condenser or evaporator fan motor

   A fan that stops, runs slowly or turns the wrong way gives high pressure on the condenser side, or low pressure and ice on the evaporator side. Check the supply, the control signal, the motor and the fan itself.

   1. Look and listen with the unit running. Watch the fan from a safe distance: does it run, at what speed, and in the right direction (air through the coil the way the arrow on the housing or the manual shows)? Listen for bearing noise and look for ice, dirt or a damaged blade.

      ✓ Check: You know whether the fan is stopped, slow, noisy or turning the wrong way.

   2. Check the supply and the control signal. Measure the voltage at the motor terminals or the EC fan connector with the fan called for. For an EC fan also measure the control signal (0–10 V or PWM) and read any status lamp or fault code on the fan electronics. Check fan delay settings and thermostats in the controller: a fan that is held off on purpose is not faulty.

      ✓ Check: Supply voltage is within the rated range on the nameplate, and the control signal asks for the fan to run; otherwise the fault is upstream (contactor, fuse, controller output, wiring).

   3. Isolate, lock off and make sure the fan cannot start. Switch off and lock the isolator, prove the circuit dead with a tested two-pole tester, and wait for the fan to stop completely. Remember that an EC fan can restart on its own when the signal returns, and that a fan wind-milling in a draught can generate voltage.

      ✓ Check: 0 V on the motor terminals and the blades are blocked or stationary.

   4. Turn the fan by hand and check it mechanically. Turn the impeller by hand: it should spin freely and evenly, without scraping, axial play or a rough bearing. Check that the blades are tight on the shaft and not cracked, and that the guard and the coil are free of ice, dirt and loose objects.

      ✓ Check: The fan turns freely and quietly and the blades are undamaged; a stiff or rough motor must be replaced.

   5. Measure the motor (AC motors). With the motor disconnected, measure the winding resistance between the terminals and compare the windings with each other and with the data sheet; test the run capacitor (see the capacitor guide). Measure the insulation resistance to earth with an insulation tester – never with the fan electronics or an EC motor connected. Check that the motor's internal thermal protection is closed when the motor is cold.

      ✓ Check: Windings are equal where they should be and match the data sheet, insulation resistance is well above the limit your electrical rules require, and the thermal protection is closed when cold.

   6. Restore, run and measure the current. Reconnect from your photo, fit guards and covers, remove the lock and start. Measure the running current on each phase and compare it with the nameplate, and check the direction and the air flow.

      ✓ Check: The fan runs smoothly in the right direction, the current is at or below the nameplate value and the pressures settle (head pressure falls on a condenser, suction pressure rises and ice clears on an evaporator).

   Test the start/run capacitor and the start relay

   Faulty capacitors and start relays are common reasons why a compressor or fan motor hums, overheats or will not start.

   1. Isolate and lock off. Switch off and lock the isolator (or unplug the unit) and remove the cover of the compressor terminal box. Set the multimeter to AC volts and measure between phase and neutral, and between each phase and earth.

      ✓ Check: The meter shows 0 V AC on the incoming terminals.

   2. Photograph, discharge and disconnect the capacitor. Photograph the wiring with your phone so that you can put it back the same way. Discharge the capacitor through a resistor of about 20 kΩ, 5 W with insulated leads, held across the terminals for a few seconds. Never short it with a screwdriver: the spark damages the terminals and the capacitor. Then pull off the cables.

      ✓ Check: The meter shows 0 V DC directly across the capacitor terminals.

   3. Measure the capacitance. Set the multimeter to capacitance (µF) and measure across the terminals. Compare with the rating printed on the capacitor. Tecumseh gives the tolerances: a start capacitor −0/+20 % of its rating, a run capacitor ±10 %. A swollen or leaking capacitor, or one that reads 0 µF, is faulty.

      ✓ Check: The measured value is within the tolerance of the printed rating.

   4. Check the start relay. Pull the relay off the compressor pins and look for burn marks or a smell of burnt plastic. A current relay is tested upright: L–S open, L–M closed (Tecumseh). A PTC starter must cool for about 3 minutes before a resistance reading means anything; compare it with the value in the compressor or starter data sheet (usually a few to some tens of ohms at room temperature).

      ✓ Check: The relay contacts are as described, or the PTC reads its data-sheet value; OL (open) means the part is faulty.

   5. Replace the faulty part and test run. Fit a new part with exactly the same data: capacitance in µF, voltage rating and relay type. Mount the relay the right way up. Reconnect from your photo, refit the cover, switch on and start the unit. If capacitors or relays fail again, look for the cause: low supply voltage, short cycling or a relay of the wrong type.

      ✓ Check: The compressor starts smoothly, without humming and without the motor protection tripping.

6. 6
   Flowchart D

   Low evaporating temperature because of too little refrigerant or a restriction.

   Remedy: Follow the low-pressure troubleshooting.

   Continue with: System trips on low pressure →