A frozen Hitachi Yutaki outdoor unit usually shows up as low pressure code 104 or as overload codes 35 or 38, because a coil caked in ice cannot pull heat from the air. A light frost that clears on its own is normal, but solid ice plus a code is not.
If a code comes back after one restart, book an engineer rather than resetting again. We handle Hitachi heat pump repairs across the UK. For every other alarm, see our full guide to Hitachi heat pump error codes.
Please read first: this guide is here to help you understand what your Hitachi 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 Hitachi 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 do 104, 35 and 38 mean on a Hitachi heat pump?
These three codes are refrigerant-side protection alarms that a heavily iced outdoor coil can trigger. Hitachi does not have a single "frozen unit" code, so the code you see depends on how the ice affects the system. All three are listed in the Yutaki S80 documents, so check your manual if you have a different Yutaki model.
| Code | Hitachi meaning | Main factor Hitachi lists |
|---|---|---|
| 104 | Low pressure control activated | Suction pressure below 0.15 MPa for 1.5 minutes, or below 0.1 MPa for 3 seconds |
| 35 | Pump down overload | Heat exchanger blocked or short-circuited, or a broken outdoor fan |
| 38 | Heating overload | Heat exchanger blocked, or expansion valve closed |
One third-party source gives code 35 a different meaning (an indoor setting error), so treat the manual for your exact unit as the final word. Either way, the unit stops to protect the compressor.
Is ice on a heat pump normal?
A thin layer of frost on the outdoor coil is normal in cold, damp weather, and the unit clears it with regular defrosts. During a defrost you may see steam rising from the unit and water dripping underneath, then heating carries on as before.
It becomes a fault when ice keeps building between defrosts, covers the fins in a solid sheet or encases the fan. That is when codes such as 104, 35 or 38 tend to appear. Our guide to the heat pump defrost cycle explains what a healthy defrost looks like, and heat pump frozen outside unit covers the warning signs across all makes.
Why do these codes appear in cold weather?
They appear when ice or a blockage stops the outdoor coil working, and cold, wet weather is when that happens most. Hitachi's own documents name a blocked heat exchanger, a broken outdoor fan or a closed expansion valve for codes 35 and 38, and low suction pressure for 104.
The causes below are general engineering knowledge, not from the Hitachi manual:
- Failed or weak defrost. A faulty defrost sensor or valve lets ice build up between cycles until the coil blocks.
- Poor water flow. Sludge, a blocked strainer or an air lock means less warm water is available to melt the coil during defrost.
- Low system pressure. A slow leak lowers flow and can trigger pressure alarms alongside the refrigerant codes. See heat pump losing pressure.
- Frozen or blocked condensate drain. Meltwater that cannot drain away refreezes under and around the coil.
- Snow, leaves or debris. Anything blocking airflow through the fins speeds up icing and can strain the fan.
- Unit sitting too low. A unit on the ground with no raised feet can sit in its own meltwater or in drifting snow.
- Low refrigerant. A leak drops suction pressure, which makes the coil run colder and ice faster, and is a common reason for code 104.
If the unit stops altogether rather than just icing, our guide to common air source heat pump problems covers the wider checks.
What can you safely check yourself?
You can make a few outside checks without tools, as long as you never open the unit. Work through these in order:
- Keep the power on. Frost protection and defrost need power, so do not leave the heat pump switched off at the isolator in freezing weather.
- Clear around the unit. From outside the casing, brush away snow, leaves and debris from the front, back, top and underneath. Use your hands or a soft brush only.
- Check the condensate drain. Make sure meltwater can run away from the base. If the drain path is iced, thaw it with lukewarm water only, never boiling.
- Check the pressure gauge and valves. Look at the system pressure and confirm the isolation valves near the indoor unit are open.
- Check your settings. If you are going away, use the holiday or frost setting instead of turning the system off. Our guide to heat pump holiday settings explains why.
- Restart once. Hitachi does not publish a homeowner reset sequence, so the safe option is a single power restart: switch off at the isolator, wait a few minutes and switch straight back on.
What not to do
- Never chip, scrape or prise ice off the coil or fan.
- Never use boiling water, a heat gun or a hairdryer on the coil.
- Never remove panels or touch anything inside the unit.
- Never touch the refrigerant circuit. That work is restricted by law to F-Gas certified engineers.
When should you call an engineer?
Call an engineer as soon as a code returns after one restart, or straight away if the coil is a solid block of ice. Other clear triggers are:
- Ice still covering the coil several hours after you cleared the area around the unit.
- The fan not turning, or turning with a scraping or knocking sound.
- Code 104 or 105, which point at refrigerant pressure and need an F-Gas certified engineer.
- System pressure that keeps falling, or water appearing where it should not.
- No heating or hot water in freezing weather, especially with elderly or vulnerable people at home.
Only an F-Gas certified engineer can legally find, repair and recharge a refrigerant leak.
How can you stop it happening again?
Most repeat freezing comes down to maintenance and where the unit sits, so a few changes go a long way. These points are general engineering advice rather than Hitachi instructions:
- Book an annual service. An engineer checks the defrost sensors, coil, fan, pressure and flow before winter. Our annual service checklist shows what is covered.
- Keep the water side clean. A clean magnetic filter and strainer keep flow up. Badly sludged systems may need a heat pump powerflush.
- Protect the pipework. Make sure external pipes are well insulated, and that antifreeze, inhibitor or freeze-protection valves are in place where your installer fitted them.
- Lift and drain. The unit should sit on raised feet or a stand with a clear path for meltwater to drain away.
- Mind the location. Keep the unit clear of drifting snow, roof run-off and plants, with the clearances in your manual.
For day-to-day habits through the cold months, read our heat pump winter tips and our guide to running a heat pump in cold weather.
Other Hitachi codes you might see in a cold snap
A freezing spell can trigger other Hitachi alarms alongside, or instead of, 104, 35 and 38. Codes marked * are listed for the Yutaki S80 only. Each code links to its entry in our Hitachi error code guide.
| Code | Hitachi meaning | Why it can show in cold weather |
|---|---|---|
| 105* | Pressure difference too low | Iced coil or refrigerant loss |
| 51* | Inverter fin temperature fault | Blocked heat exchanger or fan fault |
| 57* | DC fan motor protection | Fan blocked by ice or debris |
| 08 | Compressor-top temperature too high | Locked fan or refrigerant shortage |
| 70 / P70 | No water flow, or pump fault | Air lock, sludge or blocked strainer |
| 83 / P83 | Water pressure not detected | Low pressure from a leak |
Need help with a frozen Hitachi heat pump?
We look after Hitachi heat pumps across the UK, and our F-Gas certified engineers deal with iced coils, defrost faults and refrigerant leaks. If your home is cold, our emergency heat pump repair team offers same-day visits where possible.
We find the cause before fixing anything, then carry out the heat pump repair it needs, backed by a 12-month guarantee. To stop it happening next winter, book annual heat pump servicing or spread the cost with one of our cover plans.
Call 0333 041 5130 or contact us online.
Frequently asked questions
Can I cover my Hitachi outdoor unit in winter?
No, do not wrap or cover the unit, because it needs free airflow through the coil to work and to defrost. If snow or roof run-off is a problem, ask an engineer about a canopy that keeps the clearances in your manual.
Will a frozen outdoor unit damage my heat pump?
A light frost does no harm. Running for long periods with a solid block of ice strains the fan and compressor, which is exactly why alarms like 104, 35 and 38 stop the unit. Acting early usually keeps the repair small.
Why does my unit ice up more just above freezing than in a hard frost?
As a general rule, air a few degrees above freezing holds more moisture, so frost builds on the coil faster. Very cold air tends to be drier, so the unit may defrost less often even though it is colder.
Will the backup heater keep my home warm while the unit is iced?
Many Yutaki systems include an electric backup heater, but whether it steps in depends on how your installer set the system up. It is a stopgap, not a fix.
Does freeze damage count under my warranty?
That depends on your warranty terms and whether the unit has been serviced as required. Keep your service records and installation paperwork handy, and check the terms before anyone works on the unit.
