40 on a Stiebel Eltron heat pump: Float switch, bottom panel: The heat pump and the ventilation are switched off
Short answer
On a Stiebel Eltron heat pump, 40 means: Float switch, bottom panel: The heat pump and the ventilation are switched off. The heat pump stops until the fault clears or is reset. 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: Float switch, bottom panel: The heat pump and the ventilation are switched off.
Part involved: ventilation module / fans / air sensors.
When it shows: The heat pump and the ventilation are switched off.
Likely causes:
- Defrost pan overflowing or leak in the heating or DHW circuit of the appliance; may reset automatically.
On the LWZ 304 SOL / LWZ 404 SOL (integral ventilation units)
The manual says: Float switch, bottom panel: Heat pump and ventilation switched off (condensate/defrost pan overflow or leak).
Part involved: ventilation module / fans / air sensors.
When it shows: Heat pump and ventilation switched off (condensate/defrost pan overflow or leak).
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)
Switch the heat pump off at its isolator and call an engineer.
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: yes.
Manual reset: unknown.
On the LWZ 304 SOL / LWZ 404 SOL (integral ventilation units)
Switch the heat pump off at its isolator and call an engineer.
| Does the heat pump stop? | Yes |
|---|---|
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: unknown.
Manual reset: unknown.
What you can check
On the LWZ 5/8 CS Premium, LWZ 5 S Plus, LWZ 5/8 S Trend, LWZ 5 S Smart (integral ventilation units)
- Note the fault number shown on the programming unit display.
- Check the appliance breakers/MCBs at the consumer unit are on.
- Look for visible water leaking from the appliance; if present switch off at the isolator and call an engineer.
On the LWZ 304 SOL / LWZ 404 SOL (integral ventilation units)
- 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)
- Check for leaks. Check the float switch is working correctly.
Who to ask for: MVHR/ventilation-competent heat pump engineer.
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: MVHR/ventilation-competent heat pump engineer.
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 44 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026