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Itho Daalderop Heat Pump Error Code A1 20

DIY-friendlyReviewed November 2026Checked against manufacturer documentation
What this code means
A1 20: The water pressure of the heating system is low; the heat pump does keep working

Code 20 is a warning, not a blockage: the pressure is on the low side, but the heat pump keeps working. Top up to between 1.5 and 2 bar to stop the pressure falling further and the blocking code 19 from following.

Also shown as: A01 20

Applies to: Itho Daalderop WPU 5G and older WPU types with an MFT thermostat (shown there as A01)

Fix level
DIY-friendly
Most people can run the checks on this page with basic household items.
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
Expansion vessel, Inlet combination
Parts are model-specific; see the note below before ordering.

Common causes

  1. Water is escaping from the heating system, for example through a small leak at a fitting, radiator or pipe
  2. After bleeding or maintenance the system was not topped up enough
  3. The expansion vessel has lost its pre-charge pressure, so the pressure drops again at every heat-up

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. Read the water pressure on the thermostat or the heat pump's display.
  2. Top up the heating system with the filling hose until the pressure is between 1.5 and 2 bar, and check visible pipes and radiators for leaks.
  3. Keep an eye on the pressure over the next few days. If it drops again or the message turns into code 19, call your installer.

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

Most cases are solved by the checks above. Book a technician 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 20, 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.

Parts that may be involved

Expansion vesselInlet combination

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 20

What does A1 20 mean on a Itho Daalderop heat pump?

A1 20 means: The water pressure of the heating system is low; the heat pump does keep working. Code 20 is a warning, not a blockage: the pressure is on the low side, but the heat pump keeps working. Top up to between 1.5 and 2 bar to stop the pressure falling further and the blocking code 19 from following.

Can I fix Itho Daalderop A1 20 myself?

Usually, yes. Code A1 20 is rated DIY-friendly: the checks on this page cover the common causes and use basic household items only. If the code returns after the checks and a test cycle, book a technician.

How do I clear Itho Daalderop A1 20?

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 21Compressor protection tripped: the compressor starting current is too high or the temperature protection is not working properlyItho 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

See all Itho Daalderop heat pump error codes