Heat pumps · Error codes · Navien
F18Navien · PEM750
F18 on a Navien PEM750: IPM current sampling error
Short answer
On a Navien heat pump, F18 means: IPM current sampling error. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 24 September 2026 · 1 source
What it means
The manual says: IPM current sampling error.
Part involved: compressor inverter drive board.
Likely causes:
- Check ambient/water temperatures, pressures, discharge/suction temperatures, EEV step, compressor frequency and current.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: unknown.
What you can check
- Note the code shown on the controller and the model number on the unit's rating label before contacting your installer.
- Check that the heat pump's isolator and consumer-unit breaker are on; do not open any casing.
What the engineer will check
- Check supply voltage and phase at the unit terminals.
- Check compressor U/V/W connections and compressor winding resistance.
- Check drive board heatsink cooling and fan.
- Replace the inverter/driver board if faults persist.
Who to ask for: Heat pump engineer.
Certification: Heat pump installer competence; F-gas and flammable-refrigerant (R32/R290) handling competence for any refrigerant-circuit work.
What you might notice
Sources
- Navien Air to Water Heat Pump UK Service Manual (EN, 250408) ↗ · 250408 · page printed p.71 (PDF p.72) · Reviewed 24 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 24 September 2026