F1 on a Baxi heat pump: DC generatrix (DC bus) voltage too low
Short answer
On a Baxi heat pump, F1 means: DC generatrix (DC bus) voltage 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 · 3 sources
What it means
On the Assure Monobloc ASHP (Assure Mono 5/7/9/12/16)
The manual says: DC generatrix (DC bus) voltage too low.
Part involved: inverter module, compressor, PFC.
Likely causes:
- Check power supply; check LED and PN voltage; check IGBT/diodes and inverter board; check rectifier bridge; at compressor start usually main board, at fan start possibly inverter board.
On the HP40 / Baxi Assure HP50 Monobloc ASHP
The manual says: Low DC generatrix (DC bus) voltage protection.
Part involved: inverter module, compressor, PFC.
Likely causes:
- Check power supply; check PN voltage / inverter board LED; check IGBT and diodes; check rectifier bridge. At compressor start usually main board; at fan start possibly inverter board.
On the HP60 / Baxi Assure HP55 High Temperature Monobloc ASHP (R290)
The manual says: Protection against DC bus undervoltage.
Part involved: inverter module, compressor, PFC.
Is it safe?
On the Assure Monobloc ASHP (Assure Mono 5/7/9/12/16)
| Does the heat pump stop? | Yes |
|---|---|
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: unknown.
Manual reset: Power OFF/ON after correcting cause (ISM 13.4).
On the HP40 / Baxi Assure HP50 Monobloc ASHP
| Does the heat pump stop? | Yes |
|---|---|
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: unknown.
Manual reset: Installer: correct cause then power OFF/ON (ISM 13.4).
On the HP60 / Baxi Assure HP55 High Temperature Monobloc ASHP (R290)
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
Manual reset: unknown.
What you can check
- Note the exact code shown on the controller and when it appeared.
- Check the outdoor unit is not blocked by leaves, debris, snow or covers and has clear airflow.
- Check the heat pump's consumer-unit breaker / isolator is on (do not open any panels).
- If the code persists or returns, contact a heat pump engineer.
What the engineer will check
On the Assure Monobloc ASHP (Assure Mono 5/7/9/12/16)
- Check power supply; check LED and PN voltage; check IGBT/diodes and inverter board; check rectifier bridge; at compressor start usually main board, at fan start possibly inverter board.
Who to ask for: heat pump engineer with electrical competence.
Certification: Qualified electrician / manufacturer-trained heat pump engineer; F-gas if refrigerant circuit is opened.
On the HP40 / Baxi Assure HP50 Monobloc ASHP
- Check power supply; check PN voltage / inverter board LED; check IGBT and diodes; check rectifier bridge. At compressor start usually main board; at fan start possibly inverter board.
Who to ask for: heat pump engineer with electrical competence.
Certification: Qualified electrician / manufacturer-trained heat pump engineer; F-gas if refrigerant circuit is opened.
On the HP60 / Baxi Assure HP55 High Temperature Monobloc ASHP (R290)
- Diagnose per manufacturer service procedure (manual gives description only).
Who to ask for: heat pump engineer with electrical competence.
Certification: Qualified electrician / manufacturer-trained heat pump engineer; F-gas if refrigerant circuit is opened.
What you might notice
Sources
- Installation and owner's manual - Heat Pump air/water 'inverter' Assure Mono 5, 7, 9, 12, 16 ↗ · Back cover shows '7762758 - v05 - 19082020' (same number as the Assure Split manual - probably shared back-cover artwork) · page 76 · Reviewed 23 September 2026
- Installation and Service Manual - Heat Pump air/water 'inverter' HP40/50-4-1PHMB to HP40/50-13-3PHMB (United Kingdom) ↗ · 784570-01 (v1) · page 76 · Reviewed 23 September 2026
- Installation, user and service manual - Reversible monobloc air-to-water heat pump, outdoor unit HP60-4-1-PHMB to HP60-13-3-PHMB ↗ · 7888668 - v03 - 17072025 · page 58 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026