Boiler fault codes
Viessmann F2 Fault Code: The Temperature Limiter Tripped
Published
Viessmann display
F2
What it means
The safety temperature limiter has responded: water inside the boiler got hotter than it should, so the burner locked out. Viessmann’s Vitodens 100-W service manual lists F2 as “the temperature limiter has responded”.
Safe to check yourself
- Read the pressure gauge with the boiler cold; Viessmann asks for at least 1.0 bar, so top up with the filling loop if it is lower
- Make sure the radiator valves and any valves on the pipes under the boiler are open
- Bleed radiators that are cold at the top once the system has cooled
- Let the boiler cool for at least half an hour before a reset, because the limiter will not reset while the water is still hot
- Reset once using your model’s reset method, and note the code before you do
Call an engineer if
- F2 comes back after one reset with the pressure correct
- The pressure gauge keeps falling, or water is dripping from the boiler or the pressure relief pipe outside
- The pipes or radiators near the boiler are banging, hissing or boiling
- You smell gas: open windows, turn the meter off if you can reach it safely, avoid switches, and call 0800 111 999 from outside
F2 on a Viessmann boiler means the safety temperature limiter has tripped because the water inside the boiler overheated. The boiler shuts the burner and locks out. Usual causes are low pressure, air in the system, a failing pump or a blocked heat exchanger. Check the pressure, let it cool, reset once, then call if F2 returns.
What does F2 mean on a Viessmann boiler?
Every Vitodens boiler has a temperature limiter that shuts the burner down if the water reaches a fixed maximum. On older models it is a separate switch on the heat exchanger; on newer ones it is electronic and works through the flow temperature sensor. When the normal controls fail to keep the temperature down and the limiter opens, the burner locks out and the display shows F2.
The fault tables agree on this. The UK installation and service instructions for the Vitodens 100-W give the cause as the temperature limiter having responded, and Viessmann’s North American fault list for the Vitodens 200-W describes F2 as the fixed high limit switch being open. Both point the engineer to the same checks: fill level, circulation pump, air in the system, then the limiter itself.
Do not confuse it with F1, which on the same boilers means the maximum flue gas temperature was exceeded. F1 is about the flue, F2 is about the water. Newer Vitodens models with a black-and-white button display show numbered fault messages rather than two-character codes, so if your display looks different, check the model label and its own manual.
Why has my Viessmann overheated?
The boiler overheats when heat cannot leave it fast enough. The usual causes, starting with the ones you can check yourself:
- Low system pressure or air in the water. With too little water moving through the heat exchanger, a few seconds of firing is enough to trip the limiter. A gauge well below 1 bar when cold is the giveaway.
- The circulation pump has stopped or is weak. You may notice the boiler firing but the radiators staying cool, or hear the pump humming without moving water.
- Closed valves. Radiators all turned off, a zone valve stuck shut, or isolation valves left closed after other work mean the water has nowhere to go.
- Sludge or scale in the heat exchanger. Magnetite from older radiators narrows the waterways, and the boiler overheats even though the pump runs.
- A failed limiter or a loose connection to it. Less common, but it produces F2 even when the water is not hot.
How to reset a Viessmann F2 safely
Viessmann’s service manual says that if the boiler will not reset after a fault shutdown while the water is below about 95°C, the engineer should test the limiter. In practice that means a reset straight after the fault often does nothing: the water is still too hot. Switch the heating demand off, wait for the boiler to cool, and fix the pressure first if it is low. Our guide to a filling loop that will not work helps if topping up is the problem.
On many Vitodens 100-W models the reset is on the rotary selector: turn it briefly to the reset position and back. On newer models with a button display, follow the reset step in the operating instructions, which usually means holding the marked buttons for a few seconds. Viessmann is clear that fault messages should not be acknowledged several times in quick succession. One reset, then watch the boiler through a full heating cycle.
If the boiler is making loud banging or boiling noises, or water is coming out of the pressure relief pipe outside, switch the boiler off at the controls and leave it off. Banging and boiling point to a real overheat, and relief-pipe discharge means the pressure has hit about 3 bar, so check the filling loop taps are shut.
What an engineer does about F2
The engineer checks the cold fill pressure and the expansion vessel charge first, since a flat vessel makes the pressure swing and can leave the system short of water. Then they confirm the pump is actually moving water by feeling the flow and return pipes and, where needed, freeing or replacing the pump. They bleed air from the boiler and the system, and test the limiter’s continuity with a multimeter. A heat exchanger choked with sludge needs a chemical clean or power flush and a magnetic filter to stop it coming back.
Gas work is carried out by Gas Safe registered engineers. You pay the £60 call-out, and the repair price is agreed on site before any work starts. If the boiler is showing a code about the flame rather than the temperature, see the Viessmann F4 fault page, and for a wider breakdown our boiler repairs page explains how a visit works.
Questions people ask
Can I reset F2 myself?
Yes, once. Let the boiler cool, check the pressure is at least 1.0 bar, make sure the radiator valves are open, then reset. If F2 comes back, leave it off and call an engineer, because repeated resets do not fix an overheat.
Is a Viessmann F2 fault dangerous?
The lockout is the boiler protecting itself, so the code on its own is not dangerous. It becomes urgent if the pipes are banging or boiling or water is discharging outside: switch the boiler off and call an engineer. If you smell gas, ventilate, turn off at the meter if safe, use no switches, get out and call 0800 111 999.
Why does F2 happen when the heating first comes on?
At start-up the burner fires hard. If the pump is sticking, the pressure is low or most radiators are shut, the small amount of water in the heat exchanger heats up in seconds and trips the limiter.