Viessmann [F.89] fault codewhat it means and how to fix it

Three rows in the numeric table are the pump speaking for itself. F.89 — "No central heating and no DHW heating — Internal circulation pump blocked. — Check circulation pump. Replace if necessary." F.799 — "No DHW heating, no central heating — Central heating circuit pump reports an electrical fault. No flow rate. — Perform a power reset. If this occurs repeatedly, replace the heating circuit pump." F.982 — "No central heating, no DHW heating — Circulation pump heating circuit 1 running dry. — Check the DEV; check the circulation pump." The DEV is the diaphragm expansion vessel, because a pump runs dry when there is no water at it to move, and the usual reason is a system whose pressure has collapsed. None of the three is "Burner in a fault state" — the burner has simply not been asked to light, since the control unit will not fire a heat exchanger with no water moving through it.

The pump on these boilers is an electronically controlled unit driven by a PWM signal from the HBMU heat management unit, and Viessmann's repair chapter describes its two status LEDs: constant green for a pump running without external control, flashing green for one under the boiler's control, and constant red for "pump failure". A blocked pump is one whose rotor has stuck — sludge and magnetite from an old system are the classic — and F.89 is the unit reporting it cannot turn. F.799 is the pump's electronics tripping, for which Viessmann's first measure is simply to switch the power off and on. F.982 sends the contractor to the expansion vessel and the system pressure before the pump. A householder can do one useful thing for F.799, which is the power reset Viessmann name, and one for F.982, which is to check the pressure reading; anything involving the pump itself is behind the casing. F.75 is the related row where the unit infers no flow from the temperature sensors rather than hearing it from the pump.