AC on a Daikin Altherma LT: Booster heater thermal protector is open

What it means

The manual says: Booster heater thermal protector is open.

Part involved: booster heater / thermal protector.

When it shows: The error is generated in case the temperature of the booster heater rises above specific temperature.

Likely causes:

• Malfunction of the booster heater relay [K3M]; Incorrect parameter of the solar pump station (maximum tank temperature is too high, etc.).

Is it safe?

The manual does not say whether the heat pump stops.

Resetting itself: unknown.

Manual reset: Reset via the remote controller (Inspection/Test button) after the cause is fixed; some codes need a manual reset of the remote-controller memory (service manual section 1.4).

What you can check

• Read the full error code shown on the user interface and note it, with the date/time, for your installer.
• Check the heating-system water pressure reading on the gauge/display: Daikin operation manuals ask users to check it is above 1 bar. If it is below that, report it to your installer (do not open the unit).

What the engineer will check

• Follow the service-manual troubleshooting flow chart for this code: check malfunction of the booster heater relay [K3M]; check incorrect parameter of the solar pump station (maximum tank temperature is too high, etc.).

Who to ask for: Heat pump installer with electrical competence.

Certification: Electrical competence to BS 7671 for heater/thermal cut-out work.

What you might notice

• No heating
• No hot water
• Unexpected backup / auxiliary heater use

What owners and engineers report

1 owner and engineer report mentions this.

• No heating on the Altherma LT (low temperature split): 1 report

Sources

• Service Manual ALTHERMA ERHQ006~008BAV3 / ERLQ006~008BAV3(9) / EKHBH(X)008BA (ESIE10-02) ↗ · ESIE10-02 · page 148 · Reviewed 23 September 2026

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

Reviewed: 23 September 2026