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Heat pumps · Error codes · Vaillant
F.734Vaillant

F.734 on a Vaillant heat pump: Condensation temperature too low

Short answer

On a Vaillant heat pump, F.734 means: Condensation temperature too low. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.

Reviewed 23 September 2026 · 3 sources

What it means

On the aroTHERM plus (VWL ../6 A)

The manual says: Condensation temperature too low.

Part involved: Condensation temperature too low.

Likely causes:

  • The temperature in the heating circuit is too low, outside of the operating characteristics; Refrigerant volume too low.

On the flexoTHERM exclusive (VWF ../4)

The manual says: Condensation temperature too low.

Part involved: Condensation temperature too low.

Likely causes:

  • Temperature in the building circuit (heating mode) or environment circuit (cooling mode) too low for compressor operation; Refrigerant volume too low; Leak-tightness test.

On the geoTHERM mini (VWS 30/4, VWS 36/4.1)

The manual says: Condensation temperature too low.

Part involved: Condensation temperature too low.

Likely causes:

  • Temperature in the building circuit (heating mode) or environment circuit (cooling mode) too low for compressor operation; Refrigerant volume too low; Leak-tightness test Appendix.

Is it safe?

Does the heat pump stop?Yes
SeverityFault — needs a manual reset once the cause is fixed
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 - 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

On the aroTHERM plus (VWL ../6 A)

  • Note the full code and text shown on the display (and the date/time it appeared). Check the outdoor unit's air inlet and outlet are not blocked by leaves, debris, snow or ice build-up and clear any loose debris from around it (do not open the casing, poke the fins or use tools). The operating instructions allow the reset button on the indoor unit (or a reset via the system control) to be pressed once; if the code returns, do not keep resetting - contact your installer or Vaillant. If you smell gas/solvent near the outdoor unit or hear hissing, switch the heat pump off at the isolator, keep ignition sources away and call an engineer.

On the flexoTHERM exclusive (VWF ../4)

  • Note the full code and text shown on the display (and the date/time it appeared). Check the outdoor unit's air inlet and outlet are not blocked by leaves, debris, snow or ice build-up and clear any loose debris from around it (do not open the casing, poke the fins or use tools). Do not keep resetting the unit; report the code to your installer (the operating instructions for this product were not checked for a homeowner reset procedure). If you smell gas/solvent near the outdoor unit or hear hissing, switch the heat pump off at the isolator, keep ignition sources away and call an engineer.

On the geoTHERM mini (VWS 30/4, VWS 36/4.1)

  • Note the full code and text shown on the display (and the date/time it appeared). Check the outdoor unit's air inlet and outlet are not blocked by leaves, debris, snow or ice build-up and clear any loose debris from around it (do not open the casing, poke the fins or use tools). Do not keep resetting the unit; report the code to your installer (the operating instructions for this product were not checked for a homeowner reset procedure). If you smell gas/solvent near the outdoor unit or hear hissing, switch the heat pump off at the isolator, keep ignition sources away and call an engineer.

What the engineer will check

On the aroTHERM plus (VWL ../6 A)

  • Check the EEV (does the EEV move to the limit stop? Use the sensor/actuator test); Check the compressor inlet sensor; Check the refrigerant filling volume (see technical data); Check whether the 4-port diverter valve is in an intermediate position and is not correctly switched; Checking the high-pressure sensor; Check the pressure sensor in the heating circuit.

Who to ask for: F-gas certified refrigeration engineer with heat pump experience (R290 flammable-refrigerant competence required for aroTHERM plus / GeniaAir).

Certification: F-gas Category I (Regulation (EU) 517/2014 as retained in GB) certificate; flammable (R290) refrigerant training for R290 units.

On the flexoTHERM exclusive (VWF ../4)

  • Check the EEV (does the EEV move to the limit stop? Use the sensor/actuator test); Check the compressor inlet sensor, high-pressure sensor and low-pressure sensor; Check the refrigerant volume (see Technical data).

Who to ask for: F-gas certified refrigeration engineer with heat pump experience (R290 flammable-refrigerant competence required for aroTHERM plus / GeniaAir).

Certification: F-gas Category I (Regulation (EU) 517/2014 as retained in GB) certificate; flammable (R290) refrigerant training for R290 units.

On the geoTHERM mini (VWS 30/4, VWS 36/4.1)

  • Check the EEV (does the EEV move to the limit stop? Use the sensor/actuator test); Check the compressor inlet sensor, high-pressure sensor and low-pressure sensor; Check the refrigerant volume (700 g).

Who to ask for: F-gas certified refrigeration engineer with heat pump experience (R290 flammable-refrigerant competence required for aroTHERM plus / GeniaAir).

Certification: F-gas Category I (Regulation (EU) 517/2014 as retained in GB) certificate; flammable (R290) refrigerant training for R290 units.

What you might notice

What owners and engineers report

14 owner and engineer reports mention this.

  • No heating on the aroTHERM plus (VWL ../6 A): 8 reports
  • Keeps stopping / cutting out on the aroTHERM plus (VWL ../6 A): 6 reports

Sources

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

Reviewed: 23 September 2026