Viessmann code 21 means low evaporation pressure in the refrigerant circuit, and in cold weather it often goes hand in hand with an iced-up outdoor unit. The meaning comes from an independent repair guide that covers several Vitocal models, so check your own manual too.
The checks below are safe to try, but if code 21 returns or the unit stays frozen, book a Viessmann heat pump repair rather than restarting it again. For other codes, see our full guide to Viessmann heat pump error codes.
Please read first: this guide is here to help you understand what your Viessmann 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 Viessmann 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 code 21 mean on a Viessmann heat pump?
Code 21 means the evaporation pressure in the refrigerant circuit is too low. The evaporator is the outdoor coil that picks up heat from the air, so a low reading there tells you the unit is struggling to absorb heat.
The published remedy is to review the heat pump's control settings, adjust the heating circuit setpoints or check the refrigerant charge. A closely related code, 34, means the minimum evaporation temperature was not reached, possibly because of too little refrigerant.
Viessmann keeps its full fault descriptions in an installer document, and these two-character codes come from a repair guide covering several Vitocal models. Newer units such as the Vitocal 250-A use F-numbers instead, so if your display shows something different, note it with the subscriber number and check your manual.
Is ice on a heat pump normal?
Yes, a light frost on the outdoor coil is normal in cold, damp weather, and a Vitocal clears it with regular defrost cycles. During a defrost the fan may stop, steam may rise from the unit and it can go quiet for a few minutes. Our guide to the heat pump defrost cycle explains what normal looks like.
It is a fault when ice builds up and stays. A coil that remains solid white, ice spreading over the casing or fan guard, or a block of ice growing underneath all suggest defrosting is not keeping up. Our page on a frozen outside unit shows what to look for.
Why does code 21 appear in cold weather?
Code 21 is more likely in cold weather because the outdoor coil is working in cold air and collecting frost, which pulls evaporation pressure down. The published sources say less about causes than we would like:
- Code 21: no specific cause is given; the remedy is to review the control settings or check the refrigerant charge
- Code 34: possibly too little refrigerant
- Code D6: reported alongside an interrupted defrost
General engineering knowledge, not from a Viessmann manual, fills in the likely underlying causes:
- An iced or blocked coil after failed defrosting, which starves the evaporator of air
- Snow, leaves or stored items blocking airflow round the unit
- A frozen or blocked condensate drain, so meltwater refreezes under the unit
- A unit sitting too low, so snow and ice build up beneath it
- Low water flow from sludge, a blocked strainer or filter, or an air lock, which a heat pump powerflush can help with
- Low system pressure, covered in our guide to a heat pump losing pressure
- A faulty defrost sensor or valve, or low refrigerant, both engineer jobs
Our article on heat pumps in cold weather explains why output drops 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.
- Look at the pressure gauge, if one is fitted, and check that radiator and isolation 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.
- Write down the code and the subscriber number shown beside it. Viessmann's manual describes faults being cleared by a technician in the Service menu, with no homeowner reset step, so do not keep power cycling the unit to clear it.
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 code 21 stays on the display or returns after you have cleared the unit, or if the outdoor coil stays iced. 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
- D7 appears as well, as running on after a flow fault can damage the compressor
- F.1007 appears, which locks the unit out until a certified professional diagnoses it
The published remedy for code 21 includes checking the refrigerant charge, and refrigerant work is legally restricted to F-Gas certified engineers. If the heat pump has stopped altogether, our guide to an air source heat pump not working covers other things to rule out.
How can you stop it happening again?
An annual service is the best protection against code 21 returning, because it keeps the coil clean, airflow clear and the refrigerant and water sides checked. Our annual service checklist shows what a proper visit covers.
General engineering knowledge, not from a Viessmann manual:
- 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 Viessmann codes you might see in a cold snap
These Vitocal codes can appear alongside code 21 in freezing weather. Meanings come from published sources covering several Vitocal models, so check your manual, and each links to our full Viessmann code guide.
| Code | Meaning | What to do |
|---|---|---|
| 34 | Minimum evaporation temperature not reached | Clear airflow, then F-Gas engineer |
| 35 | Minimum suction gas temperature not reached | F-Gas engineer |
| 33 | Minimum return temperature not reached | Engineer |
| D6 | Flow controller fault with interrupted defrost | Clear ice from outside, then engineer |
| D7 | Flow controller fault, likely low water flow | Check valves, do not keep restarting, call an engineer |
Need help with a frozen Viessmann heat pump?
If your Vitocal is iced up or showing code 21, our emergency heat pump repair service can get an engineer to you quickly, with same-day visits where possible. Our F-Gas certified engineers read the stored fault list and subscriber numbers, then test airflow, flow and the refrigerant circuit to find the real cause.
We carry out Viessmann heat pump repairs across the UK, we are MCS accredited, and every repair has a 12-month guarantee. To keep cold-weather faults at bay, book annual heat pump servicing or pick one of our heat pump cover plans. Our Viessmann heat pump page has more on the brand.
Call 0333 041 5130 or contact us online.
Frequently asked questions
Why does my Vitocal show an F-number instead of code 21?
Code formats vary between Vitocal generations. Older and related units use two-character codes such as 21 or D7, while newer models such as the Vitocal 250-A use F-numbers. If yours shows an F-code, note it with the subscriber number and check your manual or our main Viessmann guide.
Does code 21 mean my heat pump is low on refrigerant?
Not necessarily. The published remedy includes checking the refrigerant charge, but an iced coil or blocked airflow can pull evaporation pressure down in the same way. Only an F-Gas engineer can measure the charge and confirm a leak.
Is it safe to leave the heat pump on overnight with code 21 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 ice has built up inside the fan guard, call us straight away.
Will an engineer be able to see when the fault happened?
Yes, on the Vitocal 250-A the controller stores the 10 most recent faults, including ones that have cleared. Engineers view them in the Service menu, which helps show whether code 21 only appears on the coldest nights.
Can I still use the heating while code 21 is showing?
The heat pump may stop or give out little heat until the cause is fixed. Depending on your installation, a back-up heater may keep some warmth going, but it usually costs more to run. Treat it as a short-term stopgap.
