Viessmann EC Fault Code: Circulation Lockout Explained

What does the Viessmann EC fault code mean?

The EC fault code appears on older Viessmann boiler models and signals a circulation-related burner lockout. In plain terms, the boiler has detected that water is not moving around the system at the required rate. To protect itself from overheating, it shuts the burner down and displays the EC code. The root cause is usually somewhere in the circulation circuit — the pump, the pipework, or the system condition — rather than the burner itself.

Common causes

• Pump airlock or blockage Common

  A seized or air-locked circulating pump is the most frequent trigger. Sludge built up in the system can coat the pump shaft and impeller, stopping it from spinning freely. Without a working pump, water cannot circulate, so the boiler locks out almost immediately after firing.

• Minimum flow rate not met Common

  Even with a functioning pump, restrictions elsewhere — partially closed isolation valves, a blocked strainer, or a heavily sludged system — can reduce flow below the boiler's minimum threshold, triggering the same lockout.

• System pressure too low Sometimes

  Viessmann recommends keeping system pressure between 1.0 and 1.5 bar when cold. A pressure drop below this range can impair circulation enough to cause the EC fault, and is something a homeowner can check and correct themselves.

• Blocked or fouled heat exchanger Sometimes

  A build-up of scale or sludge inside the heat exchanger restricts water flow through the boiler itself, creating a localised circulation problem that the boiler interprets as a system-wide fault.

• Faulty flow or temperature sensor Rare

  If the sensor that monitors water movement or temperature has drifted out of calibration or failed outright, it may report inadequate circulation even when the pump is running normally, causing a spurious lockout.

• PCB fault Rare

  The printed circuit board manages communication between the pump, sensors, and burner. A PCB fault can prevent the boiler from correctly reading or responding to circulation data, leading to an EC lockout.

How to fix it

1. Check the system pressure gauge DIY safe

   Look at the pressure gauge on the boiler — it should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1.0 bar, top it up using the filling loop (the pair of flexible braided hoses with small taps, usually beneath the boiler). Open both taps slowly until the needle reaches around 1.2 bar, then close both taps fully. Do not overfill beyond 1.5 bar.

2. Bleed trapped air from the radiators DIY safe

   Air pockets in radiators reduce circulation efficiency and can contribute to localised overheating. Using a radiator bleed key, open the bleed valve on each radiator in turn (starting upstairs) until water — not air — trickles out, then close it. Check the pressure gauge again afterwards and top up if it has dropped.

3. Reset the boiler — but limit attempts to two DIY safe

   Consult your boiler's manual for the correct reset procedure, as it varies by Viessmann model (typically a button press or dial sequence). Viessmann permits up to three resets within 24 hours before entering a safety lockout, but repeatedly resetting without addressing the underlying cause causes unnecessary wear. If the EC code returns after two resets, stop and move to the next step.

4. Check that all isolation valves are fully open DIY safe

   Walk around your heating system and confirm that any lockshield or isolation valves on radiators and pipework are in the fully open position. A partially closed valve can restrict flow enough to trigger an EC lockout. This is a visual check only — do not attempt to work on any gas pipework or internal boiler components.

5. Call a Gas Safe registered engineer to inspect the pump and circulation circuit Gas Safe engineer

   If the fault persists after the above checks, a Gas Safe engineer needs to assess the circulating pump for airlocks, blockages, or mechanical failure, and measure actual flow rates against the boiler's minimum specification. They may need to power-flush the system, clean or replace the pump, or adjust flow settings. This work must not be attempted by an unqualified person.

6. Engineer to inspect the heat exchanger, sensors, and PCB if circulation components are sound Gas Safe engineer

   Should the pump and flow checks pass, the engineer will need to test the flow and temperature sensors for accuracy and inspect the PCB. A faulty sensor or PCB will require replacement by a qualified professional — these are not user-serviceable components.

Parts you may need

• Circulating pump (compatible replacement) · from £120
• Flow or temperature sensor · from £35
• Magnetic system filter (e.g. Magnaclean or Fernox TF1) · from £75
• PCB (printed circuit board) · from £220

The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.

Typical repair cost

Expect to pay roughly £120–£370, depending on the underlying cause.

Frequently asked questions