15 on a Stiebel Eltron heat pump: DHW temperature: DHW heating blocked until 22:00 h if set DHW temperature not reached within 'MAX DHW HTG DUR.'; retried, then blocked until 22:00 next day
Short answer
On a Stiebel Eltron heat pump, 15 means: DHW temperature: DHW heating blocked until 22:00 h if set DHW temperature not reached within 'MAX DHW HTG DUR.'; retried, then blocked until 22:00 next day. 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: DHW temperature: DHW heating blocked until 22:00 h if set DHW temperature not reached within 'MAX DHW HTG DUR.'; retried, then blocked until 22:00 next day.
Part involved: water flow / pump / booster heater.
When it shows: DHW heating blocked until 22:00 h if set DHW temperature not reached within 'MAX DHW HTG DUR.'; retried, then blocked until 22:00 next day.
Likely causes:
- Booster heater stages not enabled or blocked by high limit safety cut-out; DHW diverter valve not working; 'MAX FLOW TEMP DHW' set too low.
On the LWZ 304 SOL / LWZ 404 SOL (integral ventilation units)
The manual says: DHW temperature: Set DHW temperature not reached in the defined time.
Part involved: unspecified.
When it shows: Set DHW temperature not reached in the defined time.
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? | Yes |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
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
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.
- Check the DHW set temperature and schedule on the programming unit.
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/reset high limit safety cut-out (see 02); try 'DHW ECO'; test diverter valve by setting DHW to 10 °C and 50 °C; if the fault recurs, inform your qualified contractor.
Who to ask for: Heating / 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: Heating / 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 17 · 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