H21 on a Panasonic heat pump: Abnormal water pressure
Short answer
On a Panasonic heat pump, H21 means: Abnormal water pressure. 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: Abnormal water pressure.
Part involved: Water pressure sensor.
When it shows: During operation of cooling and heating, the water pressure detected by the indoor water pressure sensor is used to determine water pressure error. Service-manual table judgement: Continue for 300 sec. (L).
Likely causes:
- 1 Faulty water pump. 2 Water leak in system. 3 Faulty connector connection. 4 Faulty water pressure sensor. 5 Faulty indoor unit PCB (main).
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Not stated in the Panasonic documents opened; protection counters lock out after the stated number of occurrences.
Manual reset: User: 'Error Reset' in the remote-controller Quick Menu (H/J/K/L controllers) or OFF/ON; engineer must clear the underlying cause (service manuals: 'When error happens, the power has to be reset').
What you can check
- Read and note the code shown blinking on the remote controller (also stored under System Check > Error history). Per the Panasonic operating instructions the user may switch the system OFF/ON at the remote controller and use 'Error Reset' in the Quick Menu; if the code returns, leave the system and contact the installer (UK user guide lists Panasonic technical support as the fallback). Check the water pressure gauge reading where it is visible without removing any cover: the UK user guide gives 0.05-0.3 MPa (0.5-3 bar) as the normal range; if outside it, contact the installer.
What the engineer will check
- Check system water pressure and repressurise/find leak; check expansion vessel charge; check water pressure sensor connector and output; check water pump and indoor PCB.
Who to ask for: Heat pump / heating engineer (hydraulics).
Certification: None statutory for water-side work; G3 unvented-cylinder competence if cylinder safety devices are involved.
What you might notice
Sources
- Service Manual Hydromodule WH-SDC0509L3E5/L6E5 with WH-WDG05LE5/07LE5/09LE5 (Order No PAPAMY2307068CE) ↗ · PAPAMY2307068CE · page 189 (error codes table); 194 (self-diagnosis) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026