What the F2 code means
On the Vitodens 050-W and 100-W, Viessmann's service instructions describe F2 as "temperature limiter has responded". The temperature limiter, sometimes called the high limit thermostat, is a safety device that cuts the burner if the water in the boiler gets far hotter than it should. It works separately from the normal temperature control, so when it trips, something has stopped heat being carried away from the boiler.
The boiler puts the burner into a fault state and shows F2 until it is reset. On the Vitodens 100-W the fault symbol usually flashes with the letter R to show a lockout. Older Vitodens generations use F2 for the same kind of fault, but always check your own manual.
Because the limiter is a last line of defence, we treat F2 as an engineer fault. There are a few simple checks you can make first, but a boiler that overheats more than once needs proper investigation.
Common causes of F2
Viessmann's service instructions list the heating system fill level, the circulation pump, venting the system and the limiter and its connecting leads. These are the usual causes behind those checks.
Low water pressure
If there is not enough water in the system, circulation suffers and the heat exchanger overheats. This is the first thing to look at.
Air in the system
Air trapped in radiators, pipework or the pump stops water moving freely, so heat builds up inside the boiler.
Pump problems
A pump that is stuck, slow or failed cannot move enough water. Pumps often stick after a quiet summer.
Restricted flow
Sludge in older systems, a blocked filter or most radiator valves being closed can all reduce flow through the boiler.
Faulty limiter or wiring
Less often, the limiter itself or its connecting leads are at fault, so it trips when the boiler is not actually overheating.
A quick way to tell them apart
If F2 appeared soon after you bled radiators, drained the system or noticed the pressure dropping, low water or air is the likely culprit. If the pressure is steady and the radiators are hot all over, but the boiler still trips, the pump, a restriction or the limiter itself becomes more likely, and that needs an engineer.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Low pressure | Gauge below 1 bar when cold | You, via the filling loop |
| Air in radiators | Radiators cold at the top, gurgling | You, by bleeding radiators |
| Air in the pump or boiler | Noisy boiler after draining or refilling | Gas Safe engineer |
| Pump fault | Humming, radiators stay cool | Gas Safe engineer |
| Sludge or blocked filter | Cold patches at the bottom of radiators | Gas Safe engineer |
| Faulty limiter or leads | F2 with good pressure and circulation | Gas Safe engineer |
Safe checks you can do yourself
- Let the boiler cool down before you do anything else.
- Check the pressure gauge or display with the system cold. If it is below 1 bar, top up with the filling loop until it reaches around 1 to 1.5 bar, or the figure in your manual, then close the valves fully. Our guide on how to repressurise a boiler explains each step.
- If any radiators are cold at the top, bleed them with the heating off, then check the pressure again.
- Check your thermostat, timer and radiator thermostatic valves so that several radiators are open.
- Reset the boiler as your manual describes. On the Vitodens 100-W, press and hold R for about two seconds. Stop after two or three attempts.
Do not remove the casing to vent the pump or test the limiter. These parts sit next to the burner and gas valve and must only be handled by a Gas Safe registered engineer. If you hear loud banging or smell burning, switch the boiler off at the fused spur and get it checked.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if F2 comes back after the pressure has been corrected and the radiators bled, or if the boiler has overheated more than once. You should also call if the pressure keeps dropping, as that points to a leak or a problem with the expansion vessel.
The engineer will check the pump, vent the boiler properly, test the temperature limiter and its leads, and look for restrictions such as sludge or a blocked filter. Tell them the pressure reading, whether you have topped up or bled radiators, and any noises you have heard.
Typical repair costs in the UK
If topping up and bleeding clear the fault for good, there is no cost. Otherwise, these ranges are typical. Your final bill depends on the model, parts and area.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Temperature limiter replacement | Typically £100 to £200 |
| Pump replacement | Typically £250 to £450 fitted |
| Magnetic filter fitted | Typically £150 to £250 |
| System clean or powerflush | Typically £400 to £700 |
How to stop F2 coming back
Check the pressure monthly and keep it in the normal range when cold. Run the heating briefly now and then over the summer so the pump does not stick. If you keep losing pressure, see our guide on a boiler leaking water, and if it climbs too high when hot, our guide on boiler pressure that is too high explains why. A yearly service lets an engineer check the pump and system water quality, and our boiler service cost guide covers what is included.