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Itho Daalderop Heat Pump Error Code A1 21

Call a professionalReviewed November 2026Checked against manufacturer documentation
What this code means
A1 21: Compressor protection tripped: the compressor starting current is too high or the temperature protection is not working properly

Code 21 groups two compressor protections: an excessive starting current and a temperature protection that does not work properly. The compressor is the heart of the refrigerant circuit; testing and replacement is installer work, and work on the circuit itself is for an F-gas certified installer.

Applies to: Itho Daalderop WPU 5G (including WPU-I, WPU-C, WPU-E, WPU-GE, WPU-KC and WPU-COE 5G)

Fix level
Call a professional
The checks on this page are preliminary. The likely fix involves a part, wiring or the control board.
Time to run the checks
About 15–30 minutes
For the 3 checks on this page — not a full repair.
You may need
No special tools
The checks below use basic household items only.
Parts that may be involved
Compressor temperature protection, Contactor (compressor relay) (+1 more below)
Parts are model-specific; see the note below before ordering.

Common causes

  1. The compressor starting current is too high, for example due to a stiff-running or faulty compressor
  2. The compressor temperature protection is not working properly or is faulty
  3. The contactor (compressor relay) is worn or faulty

What to check, in order

Safety first. Switch the heat pump off at the breaker before checking anything. Never open the refrigerant circuit or the electrical boxes — refrigerant work requires F-gas certification. The checks on this page stay on the outside: power, settings, filters and pressure where your manual allows it.
  1. Write down the code and leave the heat pump switched off for at least half an hour, so the compressor can cool down.
  2. Then try one fresh start. Do not restart the unit repeatedly in a row.
  3. If the code returns, call your installer and quote the code; the temperature protection, the compressor relay and the compressor itself must be tested.

Clear the code and test

  1. Fix what the checks above found first — a filter cleaned, a setting corrected, power restored. Clearing the code without fixing the cause only brings it back.
  2. Make sure the heat pump has power and the controller is set to heating or hot water, as you need it.
  3. Restart the unit as your manual describes and give it time: a heat pump can take ten to twenty minutes to start a full cycle.
  4. If the unit runs a full cycle with no code, the fault is cleared. If the code returns, write down the code and what the unit was doing (heating, hot water or defrosting) before calling a technician — it speeds up the diagnosis.

When to call a professional

This code usually points to a faulty part, wiring or the control board on a Itho Daalderop heat pump. Run the checks above to rule out the simple causes, then book a technician — and book one straight away if any of these apply:

  • The code comes back after you have run the checks and a test cycle.
  • You smell burning, see sparks or scorch marks, or the breaker trips.
  • Water is leaking and you cannot stop it.
  • The fix needs panels opened beyond what your manual describes, or a part tested with a meter, and you are not set up for that.

When you call, give the brand, the code A1 21, your model number (on a label on the type plate, usually on the side of the unit) and the checks you already ran. It shortens the visit.

Repair or replace?

This fault is in professional territory: the fix usually means a major part or board-level work, and the labour adds up. If your machine is already well into its service life, get the repair quote first, then compare it with the price of a current equivalent model before you decide. Paying a large share of a new machine's price to repair an old one is rarely the better deal; replacing a machine over one cheap fault is wasteful too.

Parts that may be involved

Compressor temperature protectionContactor (compressor relay)Compressor

Parts are model-specific. Find your model number first — usually on a label on the type plate, usually on the side of the unit — and match the part to that number before ordering.

Questions people ask about A1 21

What does A1 21 mean on a Itho Daalderop heat pump?

A1 21 means: Compressor protection tripped: the compressor starting current is too high or the temperature protection is not working properly. Code 21 groups two compressor protections: an excessive starting current and a temperature protection that does not work properly. The compressor is the heart of the refrigerant circuit; testing and replacement is installer work, and work on the circuit itself is for an F-gas certified installer.

Can I fix Itho Daalderop A1 21 myself?

Only the preliminary checks. Code A1 21 usually means a part, wiring or the control board has failed, which is a technician job. Run the checks to rule out simple causes before you book.

How do I clear Itho Daalderop A1 21?

Fix what the checks above found first — a filter cleaned, a setting corrected, power restored. Clearing the code without fixing the cause only brings it back. Make sure the heat pump has power and the controller is set to heating or hot water, as you need it. Restart the unit as your manual describes and give it time: a heat pump can take ten to twenty minutes to start a full cycle.

Sources checked

The meaning and checks for this code were checked against the sources above in November 2026. Codes vary by model and region; if your manual lists a different meaning for your exact model, follow the manual.

Other Itho Daalderop heat pump codes

Itho Daalderop A1 1Temperature sensor T1 (outdoor) is faulty or reads outside the normal operating rangeItho Daalderop A1 10Temperature sensor T11 (heating return) is faulty or reads outside the normal operating rangeItho Daalderop A1 11Temperature sensor T10 (heating flow) is faulty or reads outside the normal operating rangeItho Daalderop A1 12The pressure sensor of the heating circuit passes on no usable readingItho Daalderop A1 13Current measuring coil 1 is not working properly or is faultyItho Daalderop A1 14Current measuring coil 2 is not working properly or is faultyItho Daalderop A1 15The flow sensor of the source circuit is not working properly or is faultyItho Daalderop A1 16An automatic function or setting has been switched to manual operationItho Daalderop A1 17The high-pressure switch (pressostat) in the refrigerant circuit has trippedItho Daalderop A1 18The high-pressure switch (pressostat) is not working properly or is faultyItho Daalderop A1 19The water pressure of the heating system is too low; the heat pump does not work until the pressure is restoredItho Daalderop A1 2Temperature sensor T3 (storage vessel, low) is faulty or reads outside the normal operating rangeItho Daalderop A1 20The water pressure of the heating system is low; the heat pump does keep workingItho Daalderop A1 22The phase sequence in the wall socket is not clockwise (right-rotating)Itho Daalderop A1 23There is no clockwise rotating field on the incoming 3-phase supply of the heat pumpItho Daalderop A1 24The current draw of the compressor is too lowItho Daalderop A1 25The current draw of the electric heating element is too lowItho Daalderop A1 26The current draw of the compressor is too highItho Daalderop A1 27The current draw of the electric heating element is too highItho Daalderop A1 28The source return temperature (sensor T8) is too lowItho Daalderop A1 29The source supply temperature (sensor T9) is too lowItho Daalderop A1 3Temperature sensor T2 (storage vessel, high) is faulty or reads outside the normal operating rangeItho Daalderop A1 30The liquid temperature (sensor T7) is too highItho Daalderop A1 31The discharge-gas temperature (sensor T6) is too highItho Daalderop A1 32The injection temperature (sensor T4) is too lowItho Daalderop A1 33The discharge-gas temperature is too high relative to the liquid temperature and the expansion valve is fully openItho Daalderop A1 34The heating flow temperature (sensor T10) is too highItho Daalderop A1 36There is a communication fault between the heat pump and the room thermostatItho Daalderop A1 37The compressor draws current without being drivenItho Daalderop A1 38The relay of the electric heating element sticksItho Daalderop A1 4Temperature sensor T4 (injection) is faulty or reads outside the normal operating rangeItho Daalderop A1 42Temperature sensors T8 (source return) and T9 (source supply) are not working properly or are faultyItho Daalderop A1 43An unsuitable thermostat has been connected to the heat pumpItho Daalderop A1 5Temperature sensor T5 (suction gas) is faulty or reads outside the normal operating rangeItho Daalderop A1 6Temperature sensor T6 (discharge gas) is faulty or reads outside the normal operating rangeItho Daalderop A1 7Temperature sensor T7 (liquid) is faulty or reads outside the normal operating rangeItho Daalderop A1 8Temperature sensor T8 (source return) is faulty or reads outside the normal operating rangeItho Daalderop A1 9Temperature sensor T9 (source supply) is faulty or reads outside the normal operating range

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