failure-on-sensor-e2x-t8-heat-transfer-fluid-out on an IVT Greenline HE: Activated when the sensor's value indicates a temperature lower than 0 °C. To be able to make hot water, T8 is set at a temperature calculated according to the following formula: T8 = T9 + Compressor x 7K + 0.07K x current output in operation. The active compressor provides Compressor = 1 and current output in operation means additional heat in %. Compressor operation and 50% additional heat then give T8 = T9 + 10.5K. Compressor off (Compressor = 0) and no additional heat (0%) give T8 = T9
Short answer
On an IVT heat pump, failure-on-sensor-e2x-t8-heat-transfer-fluid-out means: Activated when the sensor's value indicates a temperature lower than 0 °C. To be able to make hot water, T8 is set at a temperature calculated according to the following formula: T8 = T9 + Compressor x 7K + 0.07K x current output in operation. The active compressor provides Compressor = 1 and current output in operation means additional heat in %. Compressor operation and 50% additional heat then give T8 = T9 + 10.5K. Compressor off (Compressor = 0) and no additional heat (0%) give T8 = T9. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Activated when the sensor's value indicates a temperature lower than 0 °C. To be able to make hot water, T8 is set at a temperature calculated according to the following formula: T8 = T9 + Compressor x 7K + 0.07K x current output in operation. The active compressor provides Compressor = 1 and current output in operation means additional heat in %. Compressor operation and 50% additional heat then give T8 = T9 + 10.5K. Compressor off (Compressor = 0) and no additional heat (0%) give T8 = T9.
Part involved: E2x.T8.
When it shows: Activated when the sensor's value indicates a temperature lower than 0 °C. To be able to make hot water, T8 is set at a temperature calculated according to the following formula: T8 = T9 + Compressor x 7K + 0.07K x current output in operation. The active compressor provides Compressor = 1 and current output in operation means additional heat in %. Compressor operation and 50% additional heat then give T8 = T9 + 10.5K. Compressor off (Compressor = 0) and no additional heat (0%) give T8 = T9.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: Yes - The value of the sensor indicates >0 °C.
What you can check
- Note the full alarm text including the component designation (e.g. E21.T6) - IVT asks for it when you contact the dealer. Contact the dealer if the alarm remains active for more than three hours or recurs often. Check the heating-system pressure gauge is in the range given in the user guide - do not top up yourself unless you have been shown how; check radiator/underfloor valves and room thermostats are open.
What the engineer will check
- Measure sensor and cable resistance against the IVT sensor table; check connections on the I/O board; replace sensor or cable.
Who to ask for: heat pump engineer.
What you might notice
Sources
- Installer Guide Greenline HE C6-C11 E6-E11 1x230V (6 720 802 419 (2012/06) en) ↗ · 6 720 802 419 (2012/06) · page 6 720 802 419 p.69; 6 720 643 409 p.86; 6 720 643 415 p.31 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026