Heat pumps · Error codes · IVT
GT8-GT9IVT · Greenline HT Plus
GT8-GT9 on an IVT Greenline HT Plus: High heat transfer fluid delta T8/T9: temperature difference between sensors T8 (GT8) and T9 (GT9) too high
Short answer
On an IVT heat pump, GT8-GT9 means: High heat transfer fluid delta T8/T9: temperature difference between sensors T8 (GT8) and T9 (GT9) too high. 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: High heat transfer fluid delta T8/T9: temperature difference between sensors T8 (GT8) and T9 (GT9) too high.
Part involved: flow/return sensors GT8/GT9.
Likely causes:
- Not enough flow over the heat pump (heat carrier pump jammed, valves closed); particle filter on hot side clogged.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Manual reset: Ackn. button on Rego 637 control panel.
What you can check
- Check the heating-system pressure gauge is in the range given in the user guide (Thermia/Danfoss: 1-1.5 bar when cold) - do not top up yourself unless you have been shown how; check radiator/underfloor valves and room thermostats are open. Press the Ackn. button on the control panel once (the manual says there is no risk in resetting an alarm; the heat pump restarts within 15 minutes if heat is needed). If the alarm returns, contact your installer.
What the engineer will check
- Check heat carrier pump; open valves; clean hot-side particle filter.
Who to ask for: heat pump engineer.
What you might notice
Sources
- Greenline HT Plus C and E Guide to installation, commissioning and maintenance (Art. no 290410-29, v1.1) ↗ · 290410-29 v1.1 · page p.37 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026