78 on a Viessmann Vitotronic 200: Room temp. sensor HC1: No frost protection mode via room temperature sensor; No room temperature hook-up; No room temperature-dependent control
Short answer
On a Viessmann heat pump, 78 means: Room temp. sensor HC1: No frost protection mode via room temperature sensor; No room temperature hook-up; No room temperature-dependent control. The manual does not say whether the heat pump stops. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Room temp. sensor HC1: No frost protection mode via room temperature sensor; No room temperature hook-up; No room temperature-dependent control.
Part involved: Room temp. sensor HC1.
When it shows: No frost protection mode via room temperature sensor; No room temperature hook-up; No room temperature-dependent control.
Likely causes:
- Lead break, room temperature sensor, heating circuit without mixer A1/HC1.
Is it safe?
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: Not stated per code; Vitotronic messages remain until the cause is removed and are acknowledged with OK (6150133 p.82). Unremedied acknowledged faults are redisplayed the following day.
Manual reset: Acknowledge with OK on the programming unit (user level permitted per 5817913 p.56-57) after the cause is remedied.
What you can check
- Note the 2-character message code and message text shown on the Vitotronic 200 display (press OK for details) and give them to your heating contractor (operating instructions 5817913 p.56).
- You may acknowledge the message once with OK as described in the Vitotronic 200 operating instructions; if it reappears, do not keep acknowledging it - call your contractor.
- Check the room thermostat/remote control and time-program settings are as expected.
What the engineer will check
- Check remote control. Replace remote control if required: See "Vitotrol" installation and service instructions.
Who to ask for: Viessmann-trained heat pump installer/service engineer.
What you might notice
Sources
- Service instructions Vitotronic 200 type WO1C heat pump control unit (air/water split & monobloc, brine/water) (6150133, 2023-07) ↗ · 6150133 VSA00004 · page 95 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026