HC-01 on a Daikin Altherma: Second tank temperature sensor problem

What it means

The manual says: Second tank temperature sensor problem.

Part involved: thermistor / pressure sensor.

When it shows: Domestic hot water tank thermistor input is out of range.

Likely causes:

• Faulty domestic hot water tank thermistor.; Faulty hydro PCB.

Is it safe?

The heat pump keeps running.

Resetting itself: yes.

Manual reset: Automatic reset when resistance is within range.

What you can check

• Read the full error code and sub-code shown on the user interface (Malfunctioning screen; press the help button for Daikin's long description) and note it, with the date/time, for your installer.
• Daikin documents this code as resetting automatically once the condition clears; if it keeps coming back, contact your installer.
• Do NOT bleed radiators while an error/malfunction symbol is displayed: Daikin operation manuals warn refrigerant may have leaked into the water circuit - ventilate and call your installer.
• If heating/hot water has stopped, the operation manual lets the user confirm on the Malfunctioning screen whether the backup/booster heater may take over as emergency heating (much higher running cost) until the installer attends.

What the engineer will check

• Perform a check of the domestic hot water tank thermistor. ; Perform a check of the hydro PCB.

Who to ask for: Heat pump engineer (Daikin-trained).

Certification: Electrical competence to BS 7671 for sensor/PCB wiring; F-gas if a refrigerant-side sensor must be replaced.

What you might notice

• No heating
• Intermittent fault

What owners and engineers report

9 owner and engineer reports mention this.

• No heating on the Altherma: 7 reports
• Intermittent fault on the Altherma: 2 reports

Sources

• Service manual Daikin Altherma 3 H HT ECH2O EPRA14-18D + ETSH/ETSX(B)16P30/50E (ESIE22-11A) ↗ · ESIE22-11A · page 63 · Reviewed 23 September 2026

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026