F06 on a Warmflow heat pump: DC Bus Underload

What it means

The manual says: DC Bus Underload.

Part involved: compressor driver / inverter board.

Likely causes:

• The DC bus voltage is lower than the underload protection limit.

Is it safe?

The heat pump stops until the fault clears or is reset.

What you can check

• Note the exact code shown on the controller (a photo helps) and report it to your installer or the manufacturer's service line.
• Check the heat pump's supply breaker/isolator at the consumer unit is on (do not open any casing).

What the engineer will check

• Check the input voltage measurement.

Who to ask for: heat pump electrical / inverter.

Certification: Electrical competence; manufacturer-trained heat pump engineer.

What you might notice

• Heat pump will not start
• Keeps stopping / cutting out

What owners and engineers report

1 owner and engineer report mentions this.

• Keeps stopping / cutting out on the Zeno A-Series air source heat pump (R32) AS01-R32/AS02-R32/AS03-R32: 1 report

Sources

• Warmflow High Efficiency Variable Speed Air Source Heat Pumps A-Series AS01-R290 / AS02-R290 / AS03-R290 - User, Installation & Service Instructions, Guarantee (9009049 Rev D) ↗ · 9009049 Rev D · page 59 · Reviewed 23 September 2026
• Warmflow High Efficiency Variable Speed Air Source Heat Pumps A-Series AS01-R32 / AS02-R32 / AS03-R32 - User, Installation & Service Instructions (9007723 Issue C) ↗ · 9007723 Issue C · page 58 · Reviewed 23 September 2026
• Warmflow High Efficiency Variable Speed Air Source Heat Pumps A-Series AS01 / AS02 / AS03 (R410A Zeno) - User, Installation & Service Instructions (Issue 7) ↗ · Issue 7 · page 50 · Reviewed 23 September 2026

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026