H99 on a Panasonic heat pump: Indoor heat exchanger freeze prevention
Short answer
On a Panasonic heat pump, H99 means: Indoor heat exchanger freeze prevention. 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: Indoor heat exchanger freeze prevention.
Part involved: Indoor (water) heat exchanger anti-freeze protection.
When it shows: During anti-freezing control in cooling operation, when the indoor refrigerant liquid temperature < 0°C.
Likely causes:
- 1 Faulty water pump. 2 Insufficient water flow rate in system. 3 Water leak in system. 4 2 way valve partially closed. 5 Clogged expansion valve or strainer. 6 Refrigerant shortage (refrigerant leakage). 7 Faulty indoor refrigerant liquid temperature sensor. 8 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 water flow (pump, strainer, air, 2-way valve); check for water leaks; check refrigerant charge/leaks; check expansion valve/strainer; check refrigerant liquid temperature sensor and indoor PCB.
Who to ask for: F-gas certified refrigeration / heat pump engineer (Panasonic-trained).
Certification: UK F-gas certification (Category I) for any refrigerant-circuit work; flammable-refrigerant competence for R32 (J/K) and R290 (L/M) units.
What you might notice
Sources
- Service Manual Hydromodule WH-SDC0509L3E5/L6E5 with WH-WDG05LE5/07LE5/09LE5 (Order No PAPAMY2307068CE) ↗ · PAPAMY2307068CE · page 190 (error codes table); 219 (self-diagnosis) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026