F.1120 on a Vaillant heat pump: Immersion heater: Phase failure
Short answer
On a Vaillant heat pump, F.1120 means: Immersion heater: Phase failure. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 2 sources
What it means
On the aroTHERM plus (VWL ../6 A)
The manual says: Immersion heater: Phase failure.
Part involved: Phase failure.
When it shows: Internal circuit breaker trips on short circuit or failure of one (230 V) or several (400 V) current-carrying phases of the electric back-up heater.
Likely causes:
- Internal circuit breaker has switched off because of a short circuit or failure of one (230 V) or several (400 V) current-carrying phases of the electric back-up heater; anti-legionella function not guaranteed if the heater is defective.
On the flexoTHERM exclusive (VWF ../4)
The manual says: Immersion heater: Phase failure.
Part involved: Phase failure.
Likely causes:
- The circuit breaker in the electronics box has been triggered; Electric back-up heater defective; Poorly tightened electrical connections; Mains voltage too low; Blocked by energy supply company for over three hours.
Is it safe?
On the aroTHERM plus (VWL ../6 A)
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Safety lockout |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no - the fault remains displayed until the cause is eliminated.
Manual reset: yes - by a competent person, who eliminates the fault and resets the internal circuit breaker (operating instructions).
On the flexoTHERM exclusive (VWF ../4)
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Safety lockout |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no - the fault remains displayed until the cause is eliminated.
Manual reset: yes - by a competent person who resets the internal circuit breaker (operating instructions).
What you can check
- Note the full code and text shown on the display (and the date/time it appeared). Check the heat pump supply breaker/isolator (and back-up heater breaker, if separate) at the consumer unit is on; if a breaker has tripped more than once, leave it off and call an engineer - do not open any casing. Tell your installer: the back-up heater's safety device must be investigated and reset by a competent person.
What the engineer will check
On the aroTHERM plus (VWL ../6 A)
- Check the electric back-up heater and its supply, reset the internal circuit breaker, check electrical connections and measure supply voltage (fault list per flexoTHERM table for the same code).
Who to ask for: Heat pump engineer with electrical competence (qualified electrician for supply/phase faults).
Certification: Competent electrician (BS 7671 / Part P) for supply-side work.
On the flexoTHERM exclusive (VWF ../4)
- Check the electric back-up heater and its power supply, and reset the circuit breaker; Check the electrical connections; Measure the voltage at the electrical connection for the electric back-up heater.
Who to ask for: Heat pump engineer with electrical competence (qualified electrician for supply/phase faults).
Certification: Competent electrician (BS 7671 / Part P) for supply-side work.
What you might notice
What owners and engineers report
11 owner and engineer reports mention this.
- No hot water on the aroTHERM plus (VWL ../6 A): 5 reports
- Hot water not hot enough on the aroTHERM plus (VWL ../6 A): 3 reports
- Insufficient heating (house not warm enough) on the aroTHERM plus (VWL ../6 A): 3 reports
Sources
- uniTOWER (VIH QW 190/6 E) operating, installation and maintenance instructions ↗ · 0020291525_05 · page uniTOWER VIH QW 190/6 E 0020291525_05 p.11; VWZ MEH 97/6 hydraulic station 0020291544_03 p.11; aroTHERM split hydraulic station VWL ../5 IS 0020257319_05 p.11 · Reviewed 23 September 2026
- flexoTHERM exclusive VWF 57/4 - 197/4 400V installation and maintenance instructions ↗ · 0020213394_05 · page flexoTHERM 400 V 0020213394_05 p.56; flexoTHERM 230 V 0020213395_04 p.50 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026