Viessmann [F.299] fault codewhat it means and how to fix it

F.299 is the least alarming row in the numeric table: "Date/time wrong — Real-time clock fault — Check the date and time; amend if necessary." The 050-W B0HA/B0KA and B1GA tables add the second half: "Reset the date and time. If this does not rectify the fault, replace the HBMU heat management unit." The boiler keeps heating; what is affected is anything that depends on the clock — time programmes, the daily 07:00 re-display of acknowledged faults, the timestamps in the fault history, and the summertime changeover Viessmann say is automatic. A wrong clock after a long power cut is the innocent version; a clock that drifts or resets by itself with the power on is the clock circuit on the heat management unit failing, and that is the only case that costs anything.

Setting the time is in the operating instructions and is one of the few things on these boilers the householder is told to adjust: hold the menu button for about four seconds to enter the main menu, select the clock, confirm with OK, then use the arrows to set the hour and the minutes, confirming each. The introductory pages say that "the date and time were set by your heating contractor" and that "all settings are retained if there is a power failure", which is why a boiler that comes back from a power cut with the wrong time has, strictly, reported a fault rather than a nuisance. Note the date when you correct it. If F.299 returns within weeks with no interruption to the supply, the contractor will want to know that when deciding whether the heat management unit needs to be changed.

1. 1

   Hold the menu button for about four seconds to open the main menu, select the clock with the arrow keys and confirm with OK, then set the hour (arrows, OK) and the minutes (arrows, OK), and press the menu button repeatedly to exit — Viessmann's "Setting the time" procedure.

2. 2

   Make a note of the date you corrected it. If the time is wrong again without a power cut, tell your heating contractor; the B0HA and B1GA tables say the heat management unit is replaced if resetting the clock does not cure F.299.