F.729 on a Vaillant aroTHERM plus: Compressor outlet temperature too low
Short answer
On a Vaillant heat pump, F.729 means: Compressor outlet 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 · 1 source
What it means
The manual says: Compressor outlet temperature too low.
Part involved: Compressor outlet temperature too low.
Likely causes:
- The compressor outlet temperature is lower than 0 °C for more than 10 minutes or the compressor outlet temperature is lower than -10 °C even though the heat pump is in the operating characteristics.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — 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
- 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.
What the engineer will check
- Checking the high-pressure sensor; Checking that the EEV is working correctly; Check the condenser outlet temperature sensor (subcooling); Check whether the 4-port diverter valve is in the intermediate position; Checking the refrigerant volume for overfilling.
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
- Heat pump control interface module (VWZ AI) operating and installation instructions ↗ · 0020291573_01 · page VWZ AI appliance interface 0020291573_01 p.34; uniTOWER VIH QW 190/6 E 0020291525_05 p.60; aroTHERM split hydraulic station VWL ../5 IS 0020257319_05 p.57 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026