F.741 and F.283 on a Vaillant heat pump both mean the outdoor unit has struggled to de-ice itself. They usually show up in a cold, damp spell, often with a thick layer of ice on the back of the unit.
A single code after a hard frost can clear once conditions ease. If it keeps coming back, book a heat pump repair so an engineer can find the cause before the system loses heat altogether.
This guide covers the two defrost codes in detail. For every other F code, see our full Vaillant heat pump error codes guide.
Please read first: this guide is here to help you understand what your Vaillant 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 Vaillant 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 F.741 or F.283 mean on a Vaillant heat pump?
F.741 means the water returning to the heat pump dropped below 13°C while the outdoor unit was de-icing. Published Vaillant code tables link it to too little water volume in the heating system, and the listed fix is to make sure the system has its minimum volume and that the backup heater is enabled.
F.283 means de-icing was unsuccessful. This code comes from the Glow-worm GeniaAir list, which shares Vaillant Group hardware, so check your own manual to confirm it applies to your model.
The causes listed for F.283 are a backup heater that cannot supply enough heat, too little heat in the system and ice left on the evaporator, which is the finned coil on the back of the outdoor unit. Both codes point to the same problem: the unit cannot borrow enough heat from your heating water to melt its own ice.
Is ice on a heat pump normal?
Yes, a light layer of frost on the outdoor coil is normal in cold, damp weather, and the unit clears it with a defrost cycle. One UK repair guide puts a normal defrost at roughly 2 to 10 minutes, during which you may see steam, hear a whoosh and notice the fan stop.
It becomes a fault when the ice builds into a thick, solid block, the defrost never seems to finish or the unit shows F.741 or F.283. Our guide to the heat pump defrost cycle explains what a healthy defrost looks like, and our page on a heat pump frozen outside unit covers heavier icing.
Why does F.741 or F.283 appear in cold weather?
F.741 and F.283 appear in cold weather because that is when the unit defrosts most often and needs the most heat from the system to do it. Published tables give the main reasons, and our engineers see several more underlying causes.
Causes from published Vaillant and Glow-worm tables:
- Too little water volume in the heating system (F.741)
- Backup heater not enabled or not supplying enough heat (F.741, F.283)
- Too little heat in the system to complete de-icing (F.283)
- Ice remaining on the evaporator (F.283)
Other causes (general engineering knowledge, not from the manual):
- Low flow from sludge, a blocked strainer or filter, or air locks
- Low system pressure, which also reduces flow
- A frozen or blocked condensate drain, so meltwater refreezes under the unit
- Snow, leaves or debris blocking airflow through the coil
- The unit sitting too low, so snow and meltwater build up around its base
- A failing defrost sensor, reversing valve or low refrigerant charge
A UK repair guide also lists restricted airflow, a dirty coil and refrigerant or reversing valve faults for ice that will not clear. If your pressure keeps dropping too, read our guide to a heat pump losing pressure.
What can you safely check yourself?
You can safely make a few outside checks without opening the unit. Work through them in order.
- Keep the power on. Frost protection only works with power, so do not leave the heat pump switched off at the isolator in freezing weather.
- Clear around the unit. From outside, brush snow, leaves and debris away from the sides, top and base of the outdoor unit so air can flow and meltwater can drain.
- Check the condensate drain. Make sure the drain path under the unit is clear and not frozen. If it is iced, thaw it only with lukewarm water, never boiling.
- Check pressure and valves. Read the pressure gauge and make sure any heating valves you closed are open again, as low pressure and closed valves both cut flow.
- Check settings, not the switch. If you are going away, use holiday or frost settings rather than turning the system off. Our guide to heat pump holiday settings explains how.
- Reset once. Note the code, switch off at the isolator, wait about five minutes and switch back on. Then leave the power on.
What not to do:
- Never chip, scrape or prise ice off the coil, as the fins and pipes damage easily.
- Never pour boiling water or use a heat gun on the coil.
- Never remove the unit's panels or covers.
- Never touch the refrigerant circuit. That work is restricted to F-Gas certified engineers.
When should you call an engineer?
Call an engineer if F.741 or F.283 returns after one reset, or if ice keeps building even though the unit has power and clear airflow. Book a visit straight away if:
- The house is losing heat or hot water has stopped
- A thick block of ice stays on the coil for hours
- Water is leaking, or the pressure gauge keeps falling
- The unit makes grinding or unusual noises
- You suspect a refrigerant or reversing valve problem
Checking system volume and the backup heater is an engineer's job. Anything involving the refrigerant circuit, including the reversing valve or the charge, needs an F-Gas certified engineer by law.
How can you stop it happening again?
The best way to stop F.741 and F.283 coming back is to keep water flowing freely and give the outdoor unit room to breathe and drain. These steps are general engineering knowledge rather than Vaillant instructions.
- Annual service. An engineer checks system volume, pressure, strainers, the backup heater and the coil. Our heat pump annual service checklist shows what is covered.
- Clean filters. A magnetic filter or strainer that is cleaned every year keeps sludge from cutting flow. Badly sludged systems may need a heat pump powerflush.
- Freeze protection. Where fitted, antifreeze or freeze protection valves should be checked, and the inhibitor level kept correct.
- Insulate pipes. Insulate the outdoor flow and return pipes fully.
- Raise and drain. Keep the unit on raised feet with a clear path for meltwater to drain away.
- Location. Keep the unit clear of plants, fences and drifting snow, and away from dripping gutters.
Our heat pump winter tips cover more ways to prepare before the first frost.
Other Vaillant codes you might see in a cold snap
Cold weather can trigger other Vaillant codes alongside or instead of a defrost fault. Each one is explained in our Vaillant error code guide.
| Code | What it means | Cold weather link |
|---|---|---|
| F.733 | Evaporation temperature too low | Weak airflow through an iced or blocked coil |
| F.718, F.785 | Fan 1 or fan 2 blocked | Snow, ice or leaves stopping the fan |
| F.022 | Heating circuit pressure low | Low pressure cuts flow and heat for defrosting |
| F.788, F.820 | Heating pump fault or not signalling back | Poor flow through the heating circuit |
Need help with a frozen Vaillant heat pump?
We fix frozen and iced-up Vaillant heat pumps across the UK. Our F-Gas certified engineers can check system volume, flow, the backup heater and the refrigerant side in one visit, and our repairs carry a 12-month guarantee.
If you have lost heating or hot water in a cold snap, our emergency heat pump repair team aims for same-day visits where possible. For a code that keeps returning, book a standard heat pump repair.
Regular heat pump servicing catches low pressure and blocked strainers before winter, and our cover plans spread the cost. Call 0333 041 5130 or contact us online.
Frequently asked questions
Will my Vaillant heat pump still heat the house with F.741 showing?
It may run in a limited way or stop until the fault clears, depending on the cause. If the backup heater is enabled it can help keep some heat going, but it costs more to run. Do not rely on it for long; get the cause fixed.
Can F.741 damage my heat pump?
The code itself is a protective stop, so it is there to prevent damage. Repeated failed defrosts and heavy ice can strain the fan and coil over time, which is why a returning code needs checking.
Does F.283 apply to all Vaillant heat pumps?
F.283 comes from the Glow-worm GeniaAir code list, which shares Vaillant Group hardware. It may not appear on every aroTHERM model, so check the manual for your unit.
Why does my heat pump struggle more on damp days than dry, frosty ones?
Damp air near freezing carries more moisture, so frost builds on the coil faster and the unit has to defrost more often. Our guide to running a heat pump in cold weather explains how performance changes as temperatures drop.
What if the heat pump shows no code but still is not heating?
Not every problem raises a code. Our guide to an air source heat pump not working covers other common causes and checks.
