What Viessmann publishes
On the Vitodens 100-W, the installation and service instructions give F2 as "Burner in a fault state", cause "Temperature limiter has responded", with the measures listed in this order: check heating system fill level, check circulation pump, vent the system, check the temperature limiter and connecting cables, press Reset. Viessmann's detailed Vitodens fault code document describes the same code as the fixed high limit switch being open, with the same first three remedies.
On the current Vitodens 100-W generation, which uses numeric codes rather than F codes, the equivalent is 345: burner locked out with automatic enabling after the appliance cools down, cause "Temperature limiter has responded". Viessmann's measures there are more detailed and worth reading in full, because they name the whole family of causes: ensure adequate heat transfer, check the heating system fill level, check the pre-charge pressure in the diaphragm expansion vessel, check whether the flow rate is sufficient, check the three-way diverter valve, and vent the system.
So F2 is a heat-transfer fault. The burner produced heat faster than the water could carry it away, a safety device opened, and the boiler shut down. That is what the code says, and it is the whole of what the code says.
Where a leak fits
The first item on Viessmann's own remedy list is the system fill level, and that is not an accident. Water is what moves heat out of the heat exchanger. If the system has lost enough water, or has drawn in enough air through the space the lost water left, the flow past the heat exchanger is not sufficient and the limiter trips.
That makes a leak an indirect cause of F2 rather than the thing F2 reports. The test is the pressure gauge:
| What you find | What it means |
|---|
| Gauge low, tops up, stays up | Air or a one-off loss. Vent the radiators, run the heating, and watch the gauge for a fortnight. |
| Gauge low, tops up, falls again within days | The system is losing water. This is the case where F2 is downstream of a leak, and the leak is worth finding. |
| Gauge normal and steady, F2 still returns | Not a water question. Circulation pump, blocked or sludged heat exchanger, seized diverter valve, failed limiter or its wiring. A heating engineer's job. |
Most F2 faults fall into that third row, which is why this page is not a sales pitch. If the gauge is steady, we are not the people to call.
Do not just keep pressing Reset
Viessmann's procedure ends with pressing Reset, and on the current generation the boiler re-enables itself automatically once it has cooled. Neither of those is a repair. A limiter that responds is doing the job it exists for, which is preventing an overheating heat exchanger from being damaged, and an appliance that is repeatedly driven into that state does not last.
If the code returns more than once, stop resetting and get the cause diagnosed. Note what the boiler was doing when it locked out, because it narrows the list: on hot water only points at the plate heat exchanger or the diverter valve, on heating only points at circulation, and on both points at the pump, the fill level or the limiter itself.
What to check before calling anyone
- The pressure gauge, cold. Write down the reading and the date. It is the single most useful thing you can hand an engineer.
- Radiators for air. Cold at the top and warm at the bottom means air, and air reduces the flow that carries heat away.
- Whether one radiator or circuit is cold. A dead zone points at circulation or a stuck valve rather than at water loss.
- The discharge pipe outside. If it drips when the boiler fires, the system is venting water and the pressure will keep falling whatever you do.
If after topping up the gauge holds for a fortnight, the F2 had nothing to do with water loss and you need a heating engineer. If it falls again, read on.
Where the water goes when it is a leak
Check what you can see first: radiator valve spindles and tail nuts, the underside of radiators for a rust bloom, towel rails, and the floor and wall around the boiler. If that is all dry, the loss is under a floor, inside a wall, or in the appliance. A pinhole in a 15mm heating pipe under screed can drop the gauge repeatedly for months without producing a visible damp patch, because the screed absorbs the water and the heating dries the surface.
Before anyone lifts a floor, the expansion vessel needs ruling out. A vessel that has lost its air charge sends the pressure high when the heating runs and below normal when it cools, venting through the relief valve each cycle. Viessmann's own remedy list for the limiter fault includes checking the vessel's pre-charge pressure, so it is on the same short list as the fill level, and it is far cheaper to fix than to search for.
If it is a leak, what we do
The circuit is isolated and pressure tested first, which proves the loss is real and sizes it, then traced with thermal imaging and, on a drained circuit, tracer gas, so the floor is opened at the point of the leak rather than searched. We work to a fixed hourly rate, settled before we attend rather than after, and if we do not find the leak you do not pay for the time we spent looking. We then carry out the repair and the making good.
If the pressure is steady, skip us and call a heating engineer. This list shows which fault codes across the brands actually mean water loss, and this one covers topping up properly and how often is too often.
