E042 on a Viessmann Vitocal 100-A: Insufficient thermal exchange alarm (blocks: Machine/DHW)
Short answer
On a Viessmann heat pump, E042 means: Insufficient thermal exchange alarm (blocks: Machine/DHW). 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: Insufficient thermal exchange alarm (blocks: Machine/DHW).
Part involved: Insufficient thermal exchange alarm.
When it shows: The alarm is triggered when the delivery probe of the heat pu mp detects a temperature of 60°C. The compressor is stopped, and if present, auxiliary heating systems work with the relative offset until the sanitary setpoint detected by the sanitary probe is fulfilled or ….
Likely causes:
- The alarm is triggered when the delivery probe of the heat pu mp detects a temperature of 60°C. The compressor is stopped, and if present ….
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Not stated.
Manual reset: Placing the controller in OFF mode resets alarms and hour counters (6167577 s.13, printed p.34); if alarms persist when switched on, contact technical assistance.
What you can check
- Note the E-code (e.g. E006) shown on the Vitocal 100-A controller display and give it to your installer.
- Per 6167577 s.13, switching the controller to OFF resets alarms; if the alarm is still present when switched back on, stop and contact your installer/Viessmann technical assistance (do not keep cycling it).
- Check the heat pump breaker/isolator in the consumer unit is ON and the outdoor unit is not blocked by leaves, debris or snow.
What the engineer will check
- Read alarm log (Hist) on controller; contact Viessmann technical assistance if alarm persists.
Who to ask for: Viessmann-trained heat pump engineer with F-gas refrigeration certification.
Certification: F-gas certificate (UK F-gas Regulations; Viessmann: refrigerant work by authorised refrigeration engineers only).
What you might notice
Sources
- Installation/service instructions Vitocal 100-A outdoor unit controller (6167577, 9/2020) ↗ · 6167577 VSA00001 · page 36 (PDF p.38) table 13.2; detail s.13.13 printed p.34-35 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026