Vaillant fault code F.84
Fault code F.84 on a Vaillant boiler (including ecoCOMPACT and ecoFIT pro) means: Flow and return sensors swapped. Reset the appliance once. If the code comes back, a part is usually faulty, often a temperature sensor. You can check the water pressure and settings yourself; leave work on gas-carrying parts to a certified gas engineer.
After a safety alert (gas, flue gas, combustion), reset no more than once and call in a certified gas engineer. If you smell gas, call your national gas emergency number or 112 immediately.
Cause
Fault code F.84 on a Vaillant boiler indicates: Flow and return sensors swapped.
Applies to
Vaillant ecoCOMPACT, ecoFIT pro, ecoTEC classic, ecoTEC exclusive, ecoTEC plus, ecoTEC pro classic, ecoTEC pure, VC 1006/5-5 R3, VC 1206/5-5 R3, VC 806/5-5 R3, VHR 18-24 44625 EXP R4, VHR 18-24/5-3 EXP R6, VHR 18-24/5-3 R4, VHR 18-24/5-3 R6, VHR 18-24/7-2.
Matching parts
Vaillant parts are available from Instalpunt; shipping to EU countries from €17.95 (€24.95 for more distant countries), usually 3–5 working days.
Parts by Vaillant series: ecoTEC plus · ecoTEC pro.
Based on the cause, these original Vaillant parts fit (always check the type number of your appliance):
- Vaillant pressure sensor VHR18-22/3C24-28/3C VHR CW3 4/3 for ecoTEC plus and ecoTEC pro etc. € 24.18
- Vaillant NTC sensor flow/return VHR18-22/3C24-28/3C for ecoTEC plus and ecoTEC pro etc. € 16.99
- Vaillant flow sensor complete VHR18-22/3C24-28/3C VHR CW3 4/3 for ecoTEC plus and ecoTEC pro etc. € 62.73
Further reading on this cause
Other Vaillant codes with the same cause
- F.00 — Open circuit of flow temperature sensor
- F.01 — Open circuit of return temperature sensor (NTC)
- F.02 — Open circuit of DHW outlet sensor
- F.03 — Open circuit of cylinder temperature sensor/domestic hot water temperature sensor
- F.05 — Open circuit in outdoor temperature sensor
- F.06 — Open circuit of inside flue gas sensor
Fix it yourself or call an engineer?
Always consult the manual for your appliance and work safely. If in doubt or if the fault comes back, call in a qualified professional. A question about a part? Write to us at info@instalpunt.nl.
Frequently asked questions
What does Vaillant boiler fault code F.84 mean?
Flow and return sensors swapped.
How do I fix fault code F.84 on a Vaillant boiler?
First check the cause described above. Then restart the appliance using the reset button. If the message comes back, a part usually needs replacing or the fault needs to be repaired by a qualified professional.
Which parts do I need for Vaillant fault code F.84?
That depends on the exact cause. In the Vaillant parts overview at Instalpunt you will find the current price and stock for each part.
Can I fix fault code F.84 myself?
You can reset the appliance and check things like the water pressure or settings yourself. Work on gas-carrying parts must be left to a certified gas engineer.
All Vaillant boiler faults and codes at a glance →