Heat pumps · Error codes · Joule
81Joule
81 on a Joule heat pump: No temperature difference across the evaporator
Short answer
On a Joule heat pump, 81 means: No temperature difference across the evaporator. The heat pump keeps running with some functions stopped. 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 Modul-AIR exhaust air heat pump
The manual says: No temperature difference across the evaporator.
On the Victorum exhaust air heat pump
The manual says: No Temp diff across Evaporator.
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: Automatic restart after 1 hour.
What you can check
- Read and note the code shown on the controller display, then contact your installer or service engineer.
- Check the outdoor unit/air grilles are not blocked by debris, leaves or snow.
What the engineer will check
On the Modul-AIR exhaust air heat pump
- Manual 'possible solution': Check the fan operation, possible blockage in the duct system.
- Check fan motor, wiring and PWM signal.
- Check fan blade for obstruction or ice.
Who to ask for: heat pump engineer.
Certification: Electrical competence.
On the Victorum exhaust air heat pump
- Manual 'possible solution': Check fan operation or a possible blockage in the duct system.
- Check fan motor, wiring and PWM signal.
- Check fan blade for obstruction or ice.
Who to ask for: heat pump engineer.
Certification: Electrical competence.
What you might notice
Sources
- Joule Modul-AIR All-E, Modul-AIR All-E with Green Comfort Module & Modul-AIR Aqua - Installation & Maintenance Manual ENG-0026-07 (UK/IE) ↗ · ENG-0026-07 · page 104 · Reviewed 23 September 2026
- Victorum Pre-Plumb & Victorum Compact Exhaust Air Heat Pump Installation & Maintenance Manual ENG-0014-8 ↗ · ENG-0014-8 · page 96 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026