S.312 on a Vaillant heat pump: Building circuit: Return temperature too low Return temperature in the building circuit too low for compressor to start. Heating: Return temperature < 5 °C. Cooling: Return temperature < 10 °C. Cooling: Check that the 4-port diverter valve works correctly
Short answer
On a Vaillant heat pump, S.312 means: Building circuit: Return temperature too low Return temperature in the building circuit too low for compressor to start. Heating: Return temperature < 5 °C. Cooling: Return temperature < 10 °C. Cooling: Check that the 4-port diverter valve works correctly. The heat pump keeps running with some functions stopped. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 3 sources
What it means
On the aroTHERM plus (VWL ../6 A)
The manual says: Building circuit: Return temperature too low Return temperature in the building circuit too low for compressor to start. Heating: Return temperature < 5 °C. Cooling: Return temperature < 10 °C. Cooling: Check that the 4-port diverter valve works correctly.
Part involved: Heat pump control (status/Live Monitor).
When it shows: Building circuit: Return temperature too low Return temperature in the building circuit too low for compressor to start. Heating: Return temperature < 5 °C. Cooling: Return temperature < 10 °C. Cooling: Check that the 4-port diverter valve works correctly.
Likely causes:
- Operating status reported by the heat pump control (not a fault in itself).
On the flexoTHERM exclusive (VWF ../4)
The manual says: Building circuit: Return temperature too low.
Part involved: Heat pump control (status/Live Monitor).
When it shows: See official description.
Likely causes:
- Operating status reported by the heat pump control (not a fault in itself).
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
The manual says: Building circuit inlet Return temperature too low.
Part involved: Heat pump control (status/Live Monitor).
When it shows: See official description.
Likely causes:
- Operating status reported by the heat pump control (not a fault in itself).
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Information only |
| Engineer needed? | No |
The heat pump keeps running with some functions stopped.
Resetting itself: yes - status changes automatically with operating state.
Manual reset: not applicable.
What you can check
- Operating status only - no action needed. If heating or hot water is not being delivered for a long period, note the status code and tell your installer.
What the engineer will check
- None required unless heating/hot water is not delivered; then review status history, settings and application limits.
Who to ask for: Vaillant-trained heat pump installer/engineer (MCS-certified installer).
Certification: MCS installer certification; Vaillant product training recommended.
What you might notice
What owners and engineers report
1 owner and engineer report mentions this.
- Slow heat-up on the aroTHERM plus (VWL ../6 A): 1 report
Sources
- Heat pump control interface module (VWZ AI) operating and installation instructions ↗ · 0020291573_01 · page VWZ AI appliance interface 0020291573_01 p.33; uniTOWER VIH QW 190/6 E 0020291525_05 p.58; VWZ MEH 97/6 hydraulic station 0020291544_03 p.50; aroTHERM split hydraulic station VWL ../5 IS 0020257319_05 p.55 · 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.49; flexoTHERM 230 V 0020213395_04 p.44 · Reviewed 23 September 2026
- geoTHERM mini VWS 36/4.1 230 V installation manual ↗ · 0020250181_00 · page geoTHERM mini 0020250181_00 p.36 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026