21 on a Stiebel Eltron heat pump: Outside temperature sensor: The heating system runs constantly with the flow temperature for manual mode
Short answer
On a Stiebel Eltron heat pump, 21 means: Outside temperature sensor: The heating system runs constantly with the flow temperature for manual mode. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the LWZ 5/8 CS Premium, LWZ 5 S Plus, LWZ 5/8 S Trend, LWZ 5 S Smart (integral ventilation units)
The manual says: Outside temperature sensor: The heating system runs constantly with the flow temperature for manual mode.
Part involved: temperature/pressure sensor.
When it shows: The heating system runs constantly with the flow temperature for manual mode.
Likely causes:
- The measured sensor values have been constantly outside the permissible range for a specified period.
On the LWZ 304 SOL / LWZ 404 SOL (integral ventilation units)
The manual says: Outside temperature sensor: Outside sensor values outside permissible range; heating runs at manual-mode flow temperature.
Part involved: temperature/pressure sensor.
When it shows: Outside sensor values outside permissible range; heating runs at manual-mode flow temperature.
Is it safe?
On the LWZ 5/8 CS Premium, LWZ 5 S Plus, LWZ 5/8 S Trend, LWZ 5 S Smart (integral ventilation units)
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: no.
Manual reset: electronics reset button / STB reset per manual.
On the LWZ 304 SOL / LWZ 404 SOL (integral ventilation units)
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
Manual reset: unknown.
What you can check
- Note the fault number shown on the programming unit display.
- Check the appliance breakers/MCBs at the consumer unit are on.
What the engineer will check
On the LWZ 5/8 CS Premium, LWZ 5 S Plus, LWZ 5/8 S Trend, LWZ 5 S Smart (integral ventilation units)
- Press the electronics reset button for one second. If the fault recurs, inform your qualified contractor.
Who to ask for: Heat pump service engineer (electrical/controls).
Certification: Qualified electrician competence for live-work on appliance electrics.
On the LWZ 304 SOL / LWZ 404 SOL (integral ventilation units)
- Follow remedy in LWZ 304/404 SOL installation manual section 9.4; causes largely parallel LWZ 5-8 list.
Who to ask for: Heat pump service engineer (electrical/controls).
Certification: Qualified electrician competence for live-work on appliance electrics.
What you might notice
Sources
- COMMISSIONING AND NOTIFICATION LIST LWZ 8 CS Premium / LWZ 5 CS Premium / LWZ 5 S Plus / LWZ 8 CS Trend / LWZ 8 S Trend / LWZ 5 S Trend / LWZ 5 S Smart (doc-00081766) ↗ · A 338916-45679-9862 · page 18 · Reviewed 23 September 2026
- INSTALLATION LWZ 304 SOL / LWZ 404 SOL (English, via ManualsLib manual 1279865) ↗ · page 43 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026