Viessmann C5 error (primary circulation fault) — diagnosis guide

The C5 error on a Viessmann boiler indicates a primary circulation fault — either the pump isn’t running, the flow rate is too low, or a mechanical blockage prevents water flow through the heat exchanger. The boiler locks out to avoid local overheating.

Symptoms

• Boiler stopped, C5 displayed
• Cold radiators despite heating demand
• Sometimes abnormal pump noise just before lockout
• Pressure occasionally fluctuating (indicator of circulation issues)
• Often accompanied by banging/clicking in the pipework

Affected Viessmann models

• Vitodens 100-W, 200-W (compact wall-hung boilers)
• Vitodens 222-W, 242-S (with integrated DHW tank)
• Vitopend 100 / 200
• Vitocal 200-S, 222-S (heat pumps with similar primary circuit)

Common causes

• Seized pump at startup — rotor stuck after summer dormancy (30%)
• Failed pump — capacitor or motor defect
• Air in the circuit — prevents pump priming
• Sludge in the primary heat exchanger — reduced flow
• All radiator valves closed — pump runs against closed circuit
• Clogged sludge filter on recent installs (post-2015)

Checks before calling

1. Listen for pump start-up (humming sound) on heating demand
2. Check pressure (1.2–1.8 bar cold)
3. Verify at least one radiator is open (one thermostat unblocked)
4. Bleed each radiator (air is a common cause)
5. If the boiler hasn’t run for 3+ months: the pump may just be seized — a manual un-sticking via the central bolt can work

Intervention costs

• Diagnosis + pump un-sticking: on request
• Circulation pump replacement: on request
• Full circuit bleed: on request
• Sludge filter clean: on request
• Full power-flush (désembouage): on request

Frequently asked questions

My boiler shows C5 and the pump is turning — now what?

If the pump runs but the fault persists, it’s probably insufficient flow (sludge or air). Bleed first, then check the filter.

Manual pump un-sticking — what is that?

On most pumps, a central bolt (slotted screw) gives access to the motor shaft. With a flat screwdriver, turn gently 2–3 times to un-stick the pump. Only with the boiler cold and power cut. In Luxembourg, this is a professional-only intervention.

How long before the fault returns?

If only un-sticking was done: the pump may last 1–2 more seasons. Beyond that, replacement. If sludge was the cause: the fault recurs until a proper power-flush.

Need an English-speaking Viessmann specialist in Luxembourg?

Weber & Fils is certified on all Viessmann boilers and heat pumps installed in Luxembourg. Their technicians speak English fluently and stock the most common parts.

For quotes and emergency call-outs: weberetfils.lu/en or call +352 20 60 22 22 — 24/7.

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