Low
Filling on Viessmann Vitodens 100-W WB1A

Your boiler is currently in a special setting used by heating engineers to add water to the system, so it is temporarily paused from normal operation.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

What the manual says

Manufacturer manual

System filling mode (weather-compensated control unit)

From this model's manual
Verified against the Viessmann Vitodens 100 W Wb1a installation & service manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

DIY fix possible

Locate the external filling loop on the pipework beneath your boilerEnsure both valves on the filling loop are fully closed.Restart the boiler using the reset button or power cycle to exit the software-driven filling mode.

Find a Gas Safe engineer
Search the official register →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like Filling: no call-out fees, no surprise bills.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

Affiliate links. We may earn a commission at no extra cost to you.

Related fault codes

F2View →F4View →

Related guides you might find useful

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →

New boiler cost 2026

Realistic UK prices for a swap.

Read →