Viessmann F.70 fault codewhat it means and how to fix it
An internal fault in the HBMU heat management unit of a 2020-on Vitodens. Nearly twenty numbers say the same: reset once, and if it returns the unit is replaced. Engineer only.
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What F.70 means
The HBMU heat management unit is the brain of the numeric-table Vitodens — the board that runs the burner, reads the sensors and talks to the programming unit — and the table gives it a long run of self-diagnosis rows. F.70 is "HBMU heat management unit internal fault", measure "Replacing the HBMU heat management unit". F.73 is "Internal communication error" and F.77 "HBMU heat management unit data memory"; F.161 a data memory access fault, F.162 "Processor low voltage", F.163 a checksum error; F.364 "Internal fault"; F.386, F.388, F.396, F.400, F.402, F.404, F.406, F.408, F.410, F.417 and F.418 simply "HBMU heat management unit faulty"; F.453 a sequence synchronisation error, F.474 "Error in scheduled program run monitoring". Every one is "Burner in a fault state". Where the measures differ it is only in whether Viessmann allow a reset first: most read "Reset the appliance. If the fault re-occurs, replace the HBMU heat management unit"; F.70, F.364 and the "faulty" rows go straight to replacement.
A control board that fails its own checks is not a thing anyone repairs on the wall, and Viessmann's chapter on replacing the unit includes loading the correct parameter set for the boiler type — F.454, F.527 and F.528 are the rows for a unit flashed with the wrong one, which is why a replacement is a contractor job with the right tooling and not a part swap. The householder's options are narrow: one reset with the arrow keys, because a checksum or memory error can be a one-off after a power cut, and if the code comes back, a call. It is worth saying what these codes are not. They are not F.78 or F.103, the programming unit losing contact with the HBMU (the boiler keeps running); not F.59, low mains voltage; and not the electrode earth-fault rows (F.385 to F.405), which name the HBMU but are usually the electrode.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Transient after a power interruption | A brown-out or a supply cut mid-cycle leaves a memory or checksum error that clears on one reset. The reason Viessmann allow the reset on most rows. | Common |
| Heat management unit failed | A component on the board, or its stored data, gone for good. The code returns after every reset. Replaced and re-parameterised. | Common |
| Wrong parameter set loaded | After a previous board change. F.454, F.527 and F.528 name it; the fix is to flash the correct set. | Rare |
| Water or heat damage to the board | A leak from above, or a heat cell opened without protecting the PCB as Viessmann warn. Found on inspection. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£0–£550
Time on site
2 hours
A one-off cleared by a reset costs nothing. A heat management unit fitted and parameterised is the top of the range.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
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Viessmann burner control unit (control unit PCB / HBMU heat management unit)
£250–£500Authorised Viessmann parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Viessmann fault codes
- F0Get it checkedInternal fault in the burner control unit. Viessmann's only measure on the two-character units is "Replace control unit"; the Vitotronic B2HB allows one reset first. Not a householder repair.Related: electronics and power
- F.59Get it checkedMains undervoltage on a 2020-on Vitodens: the burner locks out and the pump stops until the supply recovers. If the voltage is good and it recurs, Viessmann replace the fan unit.Related: electronics and power
- FdGet it checkedBurner control unit fault, which Viessmann first blame on the ignition electrodes, their cables or electrical interference, and only then on the unit. FF is the same row. Engineer job.Related: electronics and power
- FFGet it checkedOn the two-character units FF is the same burner control unit fault as Fd. On a Vitotronic 200-W it means an internal fault or a disabled reset key, and Viessmann say restart the appliance.Related: electronics and power
- 0CGet it checkedMains voltage too low. The burner is blocked while the supply is out of range and restarts on its own when it recovers. Viessmann's measure on the WB1A is to contact the electricity supplier.Related: electronics and power
- b7Get it checkedOn the B1HC-generation 100-W and 111-W, b7 is emergency mode after a control unit fault and Viessmann say to switch the boiler off and on. On a Vitotronic unit it is a coding card fault.Related: electronics and power
Own a Viessmann boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.