Panasonic Aquarea code H99 means the unit's freeze prevention has kicked in because the indoor heat exchanger is getting too cold. In a cold snap it often turns up alongside H42, a low pressure fault that can follow an iced outdoor coil.
The checks below are safe to try, but if H99 comes back after one reset, book an Aquarea repair rather than resetting again. For every other code, see our full guide to Panasonic Aquarea error codes.
Please read first: this guide is here to help you understand what your Panasonic Aquarea heat pump is telling you. Every effort has been made to keep the information accurate, but manufacturers update their manuals and codes can differ between models and software versions, so always check the official Panasonic Aquarea manual for changes before acting on anything here. Do not attempt anything that could be dangerous, and avoid any work that could void your warranty. If in doubt, call a qualified heat pump engineer.
What does H99 mean on a Panasonic Aquarea heat pump?
H99 is Panasonic's indoor heat exchanger freeze prevention code. The unit has detected that the heat exchanger inside is getting close to freezing and has stepped in to protect it, and Panasonic lists a faulty indoor heat exchanger or a refrigerant shortage as the causes.
H42 is a related cold-weather code for low compression pressure in heating. It triggers when the pipe sensor reads outside its range five minutes after the compressor starts, and Panasonic's list of causes includes a dirty outdoor coil and restricted airflow.
Both codes protect expensive parts, so treat them as a reason to investigate rather than something to keep clearing.
Is ice on a heat pump normal?
Yes, some frost on the outdoor coil is normal in cold, damp weather, and the Aquarea removes it with regular defrost cycles. While it defrosts, the fan may stop, you may see steam and the unit can be quieter than usual for a few minutes. Our guide to the heat pump defrost cycle explains what normal looks like.
It is a fault when the ice stays. A coil that remains solid white between defrosts, ice creeping over the casing or fan guard, or a growing block of ice underneath all point to a problem. Our page on a frozen outside unit shows what to look for.
Why does H99 or H42 appear in cold weather?
These codes show up in cold weather because the refrigerant runs colder and the outdoor coil collects frost, so any weakness in flow or airflow is exposed. Panasonic lists these causes:
- H99: a faulty indoor heat exchanger or a refrigerant shortage
- H42: a dirty outdoor coil, restricted airflow, a part-closed service valve, a faulty fan motor, low refrigerant or a leak, a blocked expansion valve or filter, a faulty sensor or the outdoor PCB
General engineering knowledge, not from the Panasonic manuals, adds these underlying causes:
- Very low water flow, which lets the plate heat exchanger get cold enough to trigger H99
- Sludge or magnetite in the system, which a heat pump powerflush can clear
- A blocked strainer or filter, or an air lock in the pipework
- Low system pressure, covered in our guide to a heat pump losing pressure
- A coil left iced after failed defrosting, which restricts airflow and can bring on H42
- A frozen or blocked condensate drain, or snow and leaves piled round the unit
- A unit sitting too low, so snow and ice build up beneath it
Our article on heat pumps in cold weather explains why performance dips in a cold spell.
What can you safely check yourself?
A few outside checks are safe, as long as you stay away from the inside of the unit.
- Keep the power on. Frost protection needs electricity, so do not switch off at the outdoor isolator unless an engineer tells you to.
- Clear snow, leaves and debris from around, above and under the outdoor unit, working from outside the casing.
- Check the condensate drain path is clear and not frozen. If it has iced up, thaw it with lukewarm water, never boiling.
- Read the pressure gauge, if one is fitted, and check that the isolation valves under the unit and the radiator valves are open.
- If you are going away, use the holiday or frost settings rather than turning the system off. Our guide to heat pump holiday settings explains how.
- Do one Error Reset from the controller's Quick Menu, following the steps in your user guide. If the code comes straight back, the fault is still there.
What not to do:
- Never chip, scrape or knock ice off the coil or fan.
- Never use boiling water, a heat gun or a hairdryer on the coil.
- Never remove panels or covers.
- Never touch the refrigerant pipework; only an F-Gas certified engineer can work on it.
When should you call an engineer?
Call an engineer if H99 or H42 returns after one reset, or if the outdoor unit stays iced after you have cleared around it. Other clear signs:
- The house is getting cold or the hot water has stopped
- The pressure gauge keeps dropping
- Ice is forming on the pipework or casing, not just the coil
- H62 or H20 also appears, pointing to a flow or pump problem
- The fan grinds or will not turn
Both Panasonic cause lists include a refrigerant shortage, and refrigerant work is legally restricted to F-Gas certified engineers. If the heat pump has stopped altogether, our guide to common air source heat pump problems covers other things to rule out.
How can you stop it happening again?
An annual service is the most reliable way to prevent H99 and H42, because it keeps the coil clean, the airflow clear and the water flowing freely. Our annual service checklist shows what a thorough visit covers.
General engineering knowledge, not from the Panasonic manuals:
- Have the magnetic filter and strainer cleaned at every service.
- Make sure the system has the right inhibitor, plus antifreeze or freeze-protection valves where your installer designed it that way.
- Keep outdoor pipework well insulated.
- Stand the unit on raised feet with a clear path for condensate to drain away.
- Keep the clearances in your manual and site the unit away from drifting snow, falling leaves and roof run-off.
Our heat pump winter tips have more ideas for the colder months.
Other Panasonic codes you might see in a cold snap
These Aquarea codes often appear alongside H99 or H42 in freezing weather. Each links to our full Panasonic code guide.
| Code | Meaning | What to do |
|---|---|---|
| H65 | Defrost water circulation fault (two-fan units) | Engineer |
| F43 | Outdoor defrost sensor fault | Engineer |
| H62 | Water flow fault | Check valves are open, reset once, then engineer |
| H75 | Low water temperature control | Engineer |
| H21 | Water pressure fault | Check the gauge and for leaks, then engineer |
| F15 | Outdoor fan motor locked | Clear ice or debris from outside, then engineer |
Need help with a frozen Panasonic heat pump?
If your Aquarea is iced up or showing H99 or H42, our emergency heat pump repair service can get an engineer to you fast, with same-day visits where possible. Our F-Gas certified engineers test flow, airflow, sensors and the refrigerant circuit, then fix the cause rather than clearing the code.
We carry out Panasonic Aquarea repairs across the UK, we are MCS accredited, and every repair has a 12-month guarantee. To keep freeze faults away, book annual heat pump servicing or choose one of our heat pump cover plans. Our Panasonic heat pump page has more on the brand.
Call 0333 041 5130 or contact us online.
Frequently asked questions
Is H99 the same as the heat pump being frozen outside?
Not exactly. H99 is about the indoor heat exchanger getting too cold, while an iced outdoor coil is more likely to show as H42. The two can happen together in a cold snap, which is why we cover them in one guide.
Does H99 mean my heat pump has a refrigerant leak?
Not always. Panasonic lists a refrigerant shortage as one cause, but a faulty heat exchanger or very low water flow can produce the same code. Only an F-Gas engineer can test the refrigerant charge and find a leak if there is one.
Is it safe to leave the heat pump on overnight with H99 showing?
Usually it is better to leave the power on so frost protection can work, as long as nothing looks damaged. Clear what you can from outside and book an engineer. If the fan grinds or you see ice inside the fan guard, call us straight away.
Can I use the heating while the code is showing?
The heat pump may stop or deliver little heat until the fault is fixed. Depending on your installation, a back-up heater may keep some warmth or hot water going, but it costs more to run. Treat it as a short-term stopgap.
Will H99 come back next winter if I just reset it?
It is likely if the cause is low flow, sludge or low refrigerant, because none of those fix themselves. A reset only clears the code. Having the cause found and an annual service booked is what stops it returning.
