F.1100 on a Vaillant heat pump: Immersion heater: Safety cut-out open
Short answer
On a Vaillant heat pump, F.1100 means: Immersion heater: Safety cut-out open. The heat pump keeps running with some functions stopped. 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: Safety cut-out open.
Part involved: Safety cut-out open.
Likely causes:
- The safety cut-out on the electric back-up heater is open due to:; Insufficient volume flow or air in the building circuit; The immersion heater is operated when the building circuit is not filled; Immersion heater operation at flow temperatures above 89 °C trips the safety fuse of the safety cut-out and requires that the fuse be replaced; Feed-in of external heat into the building circuit.
On the flexoTHERM exclusive (VWF ../4)
The manual says: Immersion heater: Safety cut-out open.
Part involved: Safety cut-out open.
Likely causes:
- The safety cut-out on the electric back-up heater is open due to:; Insufficient volume flow or air in the building circuit; The immersion heater is operated when the building circuit is not filled; Immersion heater operation at flow temperatures above 110 °C trips the safety fuse of the safety cut-out and requires that the fuse be replaced; Feed-in of external heat into the building circuit.
Is it safe?
On the aroTHERM plus (VWL ../6 A)
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Safety lockout |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: no - the fault remains displayed until the cause is eliminated.
Manual reset: partly automatic: the safety cut-out resets when the heater cools (below approx. 55 °C per flexoTHERM docs) and reset is pressed; if the safety fuse has blown (overtemperature) the cut-out must be replaced by an engineer.
On the flexoTHERM exclusive (VWF ../4)
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Safety lockout |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: no - the fault remains displayed until the cause is eliminated.
Manual reset: yes - after the cause is eliminated, press the reset button (indoor unit) or reset via the system control; if it recurs after several resets contact customer service.
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 circulation in the building circuit pump; If required, open the stopcocks; Replace the safety cut-out; Reduce or stop the external heat that is entering; Check that the dirt filter that is in place is permeable.
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 circulation in the building circuit pump; If required, open the stopcocks. The safety cut-out is automatically reset as soon as the temperature at the fuse falls below 55 °C again. Also press reset . If the safety cut-out is still open at an electric back-up heater temperature below 55 °C, temperatures above 110 °C were reached and the safety fuse tripped; Replace the safety cut-out; Reduce or stop the external heat that is entering.
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.63; aroTHERM split hydraulic station VWL ../5 IS 0020257319_05 p.61 · 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.55; 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