Heat pumps · Error codes · WaterFurnace
B0WaterFurnace · Envision NSKW / TruClimate 100 hydronic 50 Hz (06-17 kW)
B0 on a WaterFurnace Envision NSKW / TruClimate 100 hydronic 50 Hz: Brown-Out (B0): Control voltage falls below 18VAC for 10-15 continuous seconds; all operation disabled
Short answer
On a WaterFurnace heat pump, B0 means: Brown-Out (B0): Control voltage falls below 18VAC for 10-15 continuous seconds; all operation disabled. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Brown-Out (B0): Control voltage falls below 18VAC for 10-15 continuous seconds; all operation disabled.
When it shows: no-retry fault.
Likely causes:
- Control voltage falls below 18VAC for 10-15 continuous seconds; all operation disabled.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: yes.
What you can check
- Note the code and any text shown on the display/controller.
- Check the heat pump isolator and consumer-unit breaker are on.
What the engineer will check
- Diagnose per NSKW installation manual troubleshooting.
- Check 24VAC control transformer supply.
Who to ask for: heat pump engineer.
Sources
- NSKW Installation Manual - Geothermal Hydronic Heat Pump 50 Hz, NSKW 06 to 17 kW (IM1066WN 03/16) ↗ · IM1066WN 03/16 · page 27 (printed 28) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026