A1 on a Kensa heat pump: Freeze protection activated - low or no water flow on ground (cold) side
Short answer
On a Kensa heat pump, A1 means: Freeze protection activated - low or no water flow on ground (cold) side. 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 · 11 sources
What it means
On the Compact (single compressor)
The manual says: Freeze protection activated - low or no water flow on ground (cold) side.
Part involved: Ground circuit / freeze protection.
When it shows: Legacy panel controller: 'Display flashes fault code A1'.
Likely causes:
- Ground-side flow low/none, pump not operating, antifreeze missing; can occur after months of very cold weather.
On the Twin Compressor Compact
The manual says: Freeze protection activated - low or no water flow on ground (cold) side.
Part involved: Ground circuit / freeze protection.
When it shows: Legacy panel controller: 'Display flashes fault code A1'.
Likely causes:
- Ground-side flow low/none, pump not operating, antifreeze missing; can occur after months of very cold weather.
On the Evo
The manual says: Ground return temperature T5 below the Heating Mode anti-freeze limit (single or left-hand compressor).
Part involved: T5 ground return sensor / ground circuit.
When it shows: Error level per manual: Fault or Warning.
Likely causes:
- Insufficient ground-side flow, ground pump failure, low antifreeze concentration, incorrect ground temperature settings.
On the High Temperature Compact
The manual says: Freeze protection activated - low or no water flow on ground (cold) side.
Part involved: Ground circuit / freeze protection.
When it shows: Legacy panel controller: 'Display flashes fault code A1'.
Likely causes:
- Ground-side flow low/none, pump not operating, antifreeze missing; can occur after months of very cold weather.
On the High Temperature Plantroom
The manual says: Freeze protection activated - low or no water flow on ground (cold) side.
Part involved: Ground circuit / freeze protection.
When it shows: Legacy panel controller: 'Display flashes fault code A1'.
Likely causes:
- Ground-side flow low/none, pump not operating, antifreeze missing; can occur after months of very cold weather.
On the Hybrid Twin Compressor
The manual says: Freeze protection activated - low or no water flow on ground (cold) side.
Part involved: Ground circuit / freeze protection.
When it shows: Legacy panel controller: 'Display flashes fault code A1'.
Likely causes:
- Ground-side flow low/none, pump not operating, antifreeze missing; can occur after months of very cold weather.
On the Plantroom
The manual says: Freeze protection activated - low or no water flow on ground (cold) side.
Part involved: Ground circuit / freeze protection.
When it shows: Legacy panel controller: 'Display flashes fault code A1'.
Likely causes:
- Ground-side flow low/none, pump not operating, antifreeze missing; can occur after months of very cold weather.
On the Q80 Series (Kensa Q)
The manual says: Ground return temperature T5 below the Heating Mode anti-freeze limit (single or left-hand compressor).
Part involved: T5 ground return sensor / ground circuit.
When it shows: Error level per manual: Fault or Warning.
Likely causes:
- Insufficient ground-side flow, ground pump failure, low antifreeze concentration, incorrect ground temperature settings.
On the Shoebox
The manuals for this range word it in different ways:
- Freeze protection activated - low or no water flow on ground (cold) side.
- Ground return temperature T5 below the Heating Mode anti-freeze limit (single or left-hand compressor).
Part involved: Ground circuit / freeze protection; T5 ground return sensor / ground circuit.
When it shows: Legacy panel controller: 'Display flashes fault code A1'. Error level per manual: Fault or Warning.
Likely causes:
- Ground-side flow low/none, pump not operating, antifreeze missing; can occur after months of very cold weather.
- Insufficient ground-side flow, ground pump failure, low antifreeze concentration, incorrect ground temperature settings.
On the Shoebox NX
The manual says: Ground return temperature T5 below the Heating Mode anti-freeze limit (single or left-hand compressor).
Part involved: T5 ground return sensor / ground circuit.
When it shows: Error level per manual: Fault or Warning.
Likely causes:
- Insufficient ground-side flow, ground pump failure, low antifreeze concentration, incorrect ground temperature settings.
Is it safe?
On the Compact (single compressor)
| 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 - approx. 4 h with machine off (per manual).
Manual reset: unknown.
On the Twin Compressor Compact
| 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 - approx. 4 h with machine off (per manual).
Manual reset: unknown.
On the Evo
| 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 - clears when condition clears (per manual action text).
Manual reset: unknown.
On the High Temperature Compact
| 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 - approx. 4 h with machine off (per manual).
Manual reset: unknown.
On the High Temperature Plantroom
| 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 - approx. 4 h with machine off (per manual).
Manual reset: unknown.
On the Hybrid Twin Compressor
| 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 - approx. 4 h with machine off (per manual).
Manual reset: unknown.
On the Plantroom
| 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 - approx. 4 h with machine off (per manual).
Manual reset: unknown.
On the Q80 Series (Kensa Q)
| 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 - clears when condition clears (per manual action text).
Manual reset: unknown.
On the Shoebox
| 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 - approx. 4 h with machine off (per manual).
Manual reset: unknown.
On the Shoebox NX
| 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 - clears when condition clears (per manual action text).
Manual reset: unknown.
What you can check
On the Compact (single compressor)
- Note the code shown on the controller/LED; check the heat pump isolator and consumer-unit breaker are on; check thermostat/timer settings. Kensa manuals say the A1 lockout auto-resets after about 4 hours with the machine off once the cause is removed; report the code to your installer.
On the Twin Compressor Compact
- Note the code shown on the controller/LED; check the heat pump isolator and consumer-unit breaker are on; check thermostat/timer settings. Kensa manuals say the A1 lockout auto-resets after about 4 hours with the machine off once the cause is removed; report the code to your installer.
On the Evo
- Note the code shown on the controller/LED; check the heat pump isolator and consumer-unit breaker are on; check thermostat/timer settings.
On the High Temperature Compact
- Note the code shown on the controller/LED; check the heat pump isolator and consumer-unit breaker are on; check thermostat/timer settings. Kensa manuals say the A1 lockout auto-resets after about 4 hours with the machine off once the cause is removed; report the code to your installer.
On the High Temperature Plantroom
- Note the code shown on the controller/LED; check the heat pump isolator and consumer-unit breaker are on; check thermostat/timer settings. Kensa manuals say the A1 lockout auto-resets after about 4 hours with the machine off once the cause is removed; report the code to your installer.
On the Hybrid Twin Compressor
- Note the code shown on the controller/LED; check the heat pump isolator and consumer-unit breaker are on; check thermostat/timer settings. Kensa manuals say the A1 lockout auto-resets after about 4 hours with the machine off once the cause is removed; report the code to your installer.
On the Plantroom
- Note the code shown on the controller/LED; check the heat pump isolator and consumer-unit breaker are on; check thermostat/timer settings. Kensa manuals say the A1 lockout auto-resets after about 4 hours with the machine off once the cause is removed; report the code to your installer.
On the Q80 Series (Kensa Q)
- Note the code shown on the controller/LED; check the heat pump isolator and consumer-unit breaker are on; check thermostat/timer settings.
On the Shoebox
- Note the code shown on the controller/LED; check the heat pump isolator and consumer-unit breaker are on; check thermostat/timer settings. Kensa manuals say the A1 lockout auto-resets after about 4 hours with the machine off once the cause is removed; report the code to your installer.
- Note the code shown on the controller/LED; check the heat pump isolator and consumer-unit breaker are on; check thermostat/timer settings.
On the Shoebox NX
- Note the code shown on the controller/LED; check the heat pump isolator and consumer-unit breaker are on; check thermostat/timer settings.
What the engineer will check
On the Compact (single compressor)
- Check antifreeze added and commissioning, pump operation, flow restrictions; wait ~4 h (machine off) for automatic reset.
Who to ask for: GSHP / ground-loop installer.
Certification: MCS-certified GSHP installer recommended.
On the Twin Compressor Compact
- Check antifreeze added and commissioning, pump operation, flow restrictions; wait ~4 h (machine off) for automatic reset.
Who to ask for: GSHP / ground-loop installer.
Certification: MCS-certified GSHP installer recommended.
On the Evo
- Follow Kensa fault table actions: Insufficient ground-side flow, ground pump failure, low antifreeze concentration, incorrect ground temperature settings. Check ground pump operation and flow, purge air, check antifreeze concentration (refractometer, protected to -10 C or lower), check ground temperature settings.
Who to ask for: GSHP / ground-loop installer.
Certification: MCS-certified GSHP installer recommended; Kensa-trained.
On the High Temperature Compact
- Check antifreeze added and commissioning, pump operation, flow restrictions; wait ~4 h (machine off) for automatic reset.
Who to ask for: GSHP / ground-loop installer.
Certification: MCS-certified GSHP installer recommended.
On the High Temperature Plantroom
- Check antifreeze added and commissioning, pump operation, flow restrictions; wait ~4 h (machine off) for automatic reset.
Who to ask for: GSHP / ground-loop installer.
Certification: MCS-certified GSHP installer recommended.
On the Hybrid Twin Compressor
- Check antifreeze added and commissioning, pump operation, flow restrictions; wait ~4 h (machine off) for automatic reset.
Who to ask for: GSHP / ground-loop installer.
Certification: MCS-certified GSHP installer recommended.
On the Plantroom
- Check antifreeze added and commissioning, pump operation, flow restrictions; wait ~4 h (machine off) for automatic reset.
Who to ask for: GSHP / ground-loop installer.
Certification: MCS-certified GSHP installer recommended.
On the Q80 Series (Kensa Q)
- Follow Kensa fault table actions: Insufficient ground-side flow, ground pump failure, low antifreeze concentration, incorrect ground temperature settings. Check ground pump operation and flow, purge air, check antifreeze concentration (refractometer, protected to -10 C or lower), check ground temperature settings.
Who to ask for: GSHP / ground-loop installer.
Certification: MCS-certified GSHP installer recommended; Kensa-trained.
On the Shoebox
- Check antifreeze added and commissioning, pump operation, flow restrictions; wait ~4 h (machine off) for automatic reset.
- Follow Kensa fault table actions: Insufficient ground-side flow, ground pump failure, low antifreeze concentration, incorrect ground temperature settings. Check ground pump operation and flow, purge air, check antifreeze concentration (refractometer, protected to -10 C or lower), check ground temperature settings.
Who to ask for: GSHP / ground-loop installer.
Certification: MCS-certified GSHP installer recommended; MCS-certified GSHP installer recommended; Kensa-trained.
On the Shoebox NX
- Follow Kensa fault table actions: Insufficient ground-side flow, ground pump failure, low antifreeze concentration, incorrect ground temperature settings. Check ground pump operation and flow, purge air, check antifreeze concentration (refractometer, protected to -10 C or lower), check ground temperature settings.
Who to ask for: GSHP / ground-loop installer.
Certification: MCS-certified GSHP installer recommended; Kensa-trained.
What you might notice
Sources
- Single Compressor Compact Heat Pump Manual ↗ · Version 11 · page 41 · Reviewed 23 September 2026
- Twin Compressor Compact Heat Pump Manual ↗ · Version 14 · page 38 · Reviewed 23 September 2026
- Kensa Evo Heat Pump Installation manual ↗ · V32 · page 50 · Reviewed 23 September 2026
- High Temperature Single Compressor Compact Heat Pump Manual ↗ · Version 5 · page 41 · Reviewed 23 September 2026
- High Temperature Plant Room Heat Pump Manual ↗ · v4.1 · page 30 · Reviewed 23 September 2026
- Hybrid Twin Compressor Compact Heat Pump Manual ↗ · Version 4 · page 38 · Reviewed 23 September 2026
- Plant Room Heat Pump Manual ↗ · Version 11.1 · page 30 · Reviewed 23 September 2026
- Q80 Series Heat Pump Operational Manual ↗ · Version 2 · page 23 · Reviewed 23 September 2026
- Shoebox Heat Pump Installation and Commissioning Manual (ground array) ↗ · Version 4 · page 33 · Reviewed 23 September 2026
- Shoebox Heat Pump Operational Manual (User Manual) ↗ · Version 8 · page 26 · Reviewed 23 September 2026
- Kensa Shoebox NX heat pump Installation manual ↗ · V12 · page 40 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026