What it means
Vaillant's VWZ AI fault table calls this 'Building circuit: Return temp. too low'. The message stays until the return temperature is above 20 °C. Vaillant's status code list gives 16 minutes as the longest a defrost should take.
What to check, in order
- Leave it switched on
Vaillant says the message clears once the return water is back above 20 °C. Keep the heat pump powered, as it needs power for frost protection.
- Tell your installer if it keeps happening
Vaillant's fix is to make sure the system holds its minimum water volume, if needed by fitting a series return cylinder, and to switch on the electric back-up heater to help raise the return temperature.
When to call your installer
If it happens often, ask your installer to check the system's water volume and to enable the electric back-up heater in the heat pump's control panel and in the system control. These are the remedies in Vaillant's fault table.
A repair visit will usually check the water flow and pressure, the temperature sensors and the refrigerant circuit.
- Note the model and serial number. They're on a label on the rating plate on the outdoor unit, and on the indoor unit or cylinder.
- Write down the exact code, and what the appliance was doing when it appeared.
- Check whether it's still under warranty. If it is, the repair may cost you nothing.
Good to know
- Vaillant's outdoor unit instructions say vapour rising from the unit while it defrosts in damp weather is normal.
Quick answers
What does the Vaillant aroTHERM F741 error code mean?
While the outdoor unit was defrosting, the water coming back from your heating system dropped below 13 °C. The compressor stays blocked until the return water warms up again. Vaillant's VWZ AI fault table calls this 'Building circuit: Return temp. too low'. The message stays until the return temperature is above 20 °C. Vaillant's status code list gives 16 minutes as the longest a defrost should take.
Can I fix Vaillant aroTHERM F741 myself?
Engineer only (installer visit if it repeats). Start with: leave it switched on; tell your installer if it keeps happening. If it happens often, ask your installer to check the system's water volume and to enable the electric back-up heater in the heat pump's control panel and in the system control. These are the remedies in Vaillant's fault table.
Is Vaillant aroTHERM F741 worth repairing?
Worth fixing. A one-off can clear by itself. If it keeps happening, the system needs a settings change or more water volume, which is an installer job.
Checked against
- Vaillant heat pump appliance interface VWZ AI (for aroTHERM plus VWL ..5/6 A): operating and installation instructions (PDF, 0020291573_01)
- Vaillant aroTHERM plus VWL 35/6 to 125/6 A: operating, installation and maintenance instructions, Nordic edition (PDF, 0020297933_03)
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.