low-pressure-error on a Thermia heat pump: Evaporation pressure has fallen too low
Short answer
On a Thermia heat pump, low-pressure-error means: Evaporation pressure has fallen too low. The heat pump stops until the fault clears or is reset. 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 Atec
The manual says: Evaporation pressure has fallen too low.
Part involved: low pressure transmitter.
Likely causes:
- Cable break/loose cable to LP transmitter; transmitter fault; lack of refrigerant; drier blockage; evaporator blocked on refrigerant side. (causes from the equivalent Danfoss DHP-AQ service instructions VMGFJ202).
On the Diplomat
The manual says: Tripped low pressure switch. Compressor stopped.
Part involved: low pressure switch.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Manual reset: Set operating mode OFF then back to the desired mode.
What you can check
On the Atec
- Note the exact alarm text. Atec user manual: contact the service technician. Check the outdoor unit is not blocked by snow, ice, leaves or debris and that airflow around it is clear. Acknowledge once (operating mode OFF then back); if it remains, contact a service technician (user manual 086L0130 p.25).
On the Diplomat
- Note the exact alarm text. Look at the brine expansion tank: the fluid level should be visible (user guide check). Do not refill or open the ground-loop circuit yourself - call the installer. Acknowledge once by setting operating mode OFF then back; 'Other alarm message: reset the alarm; if the alarm remains, contact a service technician' (VUGFD302 p.16).
What the engineer will check
On the Atec
- Check transmitter cabling; verify break pressure with gauges; check superheat/charge (superheat >9°C with EXV steady at 100% indicates significant refrigerant loss); leak test; drier dT; nitrogen purge of evaporator.
Who to ask for: heat pump engineer.
Certification: F-Gas category I certificate (refrigerant circuit work).
On the Diplomat
- Brine strainer, air, taps, brine pump, antifreeze concentration, collector sizing, expansion valve, charge, drier (VMGFC302 Table 13). Causes per the equivalent Danfoss service instructions VMGFC302 (same Danfoss-era controller) - not stated in the Thermia user manual.
Who to ask for: heat pump engineer.
Certification: F-Gas category I certificate (refrigerant circuit work).
What you might notice
Sources
- Service Instructions Atec (VIIFM102) - silo.tips copy ↗ · VIIFM102 · page p.10 · Reviewed 23 September 2026
- User manual Atria, Atria Duo, Atria Duo Optimum, Atria Optimum, Comfort, Diplomat, Diplomat Duo, Diplomat Duo Optimum (G2), Diplomat Optimum (G2) - 086U6297 Rev. 6 EN ↗ · 086U6297 Rev. 6 · page printed p.41 (PDF p.43) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026