low-pressure-error on a Danfoss heat pump: Evaporation pressure has fallen too low
Short answer
On a Danfoss 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 DHP-AQ
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.
On the DHP-H
The manual says: Tripped low pressure switch. Compressor stopped.
Part involved: low pressure switch.
Likely causes:
- Blocked brine strainer; air in brine circuit; closed brine taps; defective/jammed brine pump; broken/loose cable to LP switch; LP switch opening too early; wrong anti-freeze type or concentration; ground collector too short or too long; expansion valve defective/mis-set; low refrigerant charge; blocked drier; evaporator blocked (water or refrigerant side).
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 DHP-AQ
- Note the exact alarm text. Check the outdoor unit is not blocked by snow, ice, leaves or debris and that airflow around it is clear. The manual allows acknowledging by setting operating mode OFF then back once; if the alarm returns, call an engineer.
On the DHP-H
- Note the exact alarm text. The manual allows acknowledging by setting the operating mode to OFF and then back to the previous mode - do this once; if the alarm returns, leave it and call an engineer. 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.
What the engineer will check
On the DHP-AQ
- 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 DHP-H
- Clean brine strainer; bleed brine circuit; confirm valves open and brine pump running; check antifreeze with refractometer; compare collector length to design; check superheat and expansion valve; leak test; check drier dT (>1 K = blocked); evaporator dT test (2-6 K normal).
Who to ask for: heat pump engineer.
Certification: F-Gas category I certificate (refrigerant circuit work).
What you might notice
Sources
- Service Instructions DHP-AQ (VMGFJ202) ↗ · VMGFJ202 · page p.10; pp.15-16 · Reviewed 23 September 2026
- Service instructions DHP-A, DHP-A Opti, DHP-AL, DHP-AL Opti, DHP-C, DHP-H, DHP-H Opti, DHP-H Opti Pro, DHP-L, DHP-L Opti, DHP-L Opti Pro (VMGFC302) ↗ · VMGFC302 (© 2010; PDF regenerated 2024-07-10 on assets.danfoss.com) · page printed p.35, pp.37-39 (Table 13); PDF pp.37, 39-41 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026