high-pressure-error on a Thermia heat pump: High pressure switch has deployed
Short answer
On a Thermia heat pump, high-pressure-error means: High pressure switch has deployed. 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: High pressure switch has deployed.
Part involved: high pressure switch.
Likely causes:
- Air in heating system; blocked strainer; closed thermostats/valves; defective/jammed circulation pump; closed main tap; cable break to HP switch; wrong/faulty HP switch; timer shunt closing circuit; non-return valve wrong way or too stiff; large pressure drop; blocked condenser (water or refrigerant side); overfilled refrigerant circuit. (causes from the equivalent Danfoss DHP-AQ service instructions VMGFJ202).
On the Diplomat
The manual says: Tripped high pressure switch. Compressor stopped.
Part involved: high 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: the heating circuit is the high pressure circuit - check and if necessary rectify the circuit level (1-1.5 bar cold; top up if below 0.8 bar per user manual p.25) then reset. 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. 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. 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. 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
- Bleed heating system; clean strainer; check pump voltage and 0-10 V signal; check HP switch and cabling; check non-return valve; condenser dT test (<3 K or >13 K suggests blockage); check charge.
Who to ask for: heat pump engineer.
Certification: F-Gas category I certificate (refrigerant circuit work).
On the Diplomat
- See Danfoss service instructions VMGFC302 Table 14 (same design): bleed heating system, clean strainer, check pump, valves, HP switch, non-return valve, condenser, charge. 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