Viessmann 28 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 28 fault code mean?
Fault code 28 on Viessmann boilers signals that the boiler has shut itself down because its fixed high-limit thermostat has been triggered, or because the internal fuse F2 has blown. In both cases, the boiler detected dangerously high temperatures and cut out to protect itself and your home. The code can also appear when the ionisation current reads too high during a calibration cycle — which itself is often a symptom of a circulation problem, trapped air, or a struggling pump preventing heat from being carried away from the heat exchanger efficiently. This is a safety lockout and should not be repeatedly reset without addressing the underlying cause.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Poor circulation or pump failure Common
When the circulating pump is worn, seized, or running at reduced speed, heated water cannot move away from the heat exchanger quickly enough. The temperature climbs until the high-limit thermostat trips and shuts the boiler down. A failing pump is one of the most frequent triggers for code 28 in older Viessmann installations.
- Low system pressure Common
Viessmann boilers need the system pressure to sit between 1.0 and 1.5 bar when cold. If the pressure has dropped below 1.0 bar there is simply not enough water in the circuit for the pump to move effectively, which starves the heat exchanger of cooling flow and can quickly lead to an overheat trip.
- Sludge or debris restricting flow Common
Years of use without a magnetic filter or regular powerflushing can allow black iron oxide sludge and debris to accumulate in the pipework and heat exchanger. This narrows flow passages, reduces circulation, and creates hot spots that repeatedly trigger the high-limit safety. Radiators that are cold at the bottom while the boiler is running are a telltale sign.
- Blown internal fuse F2 Sometimes
Viessmann boilers contain an internal fuse (F2) on the PCB or control board wiring that protects certain components. If this fuse blows — often due to an electrical spike or a failing component drawing excess current — the boiler logs code 28 and locks out. Identifying and replacing this fuse safely requires access to the boiler's internals and must be carried out by a qualified engineer.
- Ionisation current too high during calibration Sometimes
On some Viessmann models, the ionisation sensor monitors the burner flame quality during a calibration phase at start-up. If the reading exceeds the expected threshold — which can happen when the sensor is contaminated, the burner is dirty, or circulation issues are causing abnormal combustion conditions — the boiler logs code 28 and shuts down.
- Blocked or fouled heat exchanger Rare
If sludge, scale, or debris has built up inside the heat exchanger itself rather than just the wider pipework, thermal transfer is badly impaired. The heat exchanger runs hot, the high-limit trips, and the fault becomes persistent. Repeated overheat lockouts without addressing this cause can permanently damage the heat exchanger.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If the needle is below 1.0 bar, use the filling loop (the flexible braided hoses with small valves, usually found beneath the boiler) to top up. Open both valves slowly and watch the gauge, stopping when it reaches approximately 1.2 bar. Close both valves fully. If pressure is already correct, move to the next step.
- Bleed your radiators DIY safe
Trapped air in the system can create hot spots that cause the high-limit to trip. Starting from the lowest radiator in the house and working your way up, use a radiator bleed key to open the bleed valve at the top corner of each radiator just enough to let any trapped air hiss out. Close the valve as soon as water appears. After bleeding all radiators, recheck the boiler pressure and top up again if needed.
- Reset the boiler — but no more than twice DIY safe
Once pressure is correct and radiators have been bled, try resetting the boiler using the reset button or procedure described in your user manual. Viessmann boilers allow up to three resets within a 24-hour window before entering a deeper safety lockout. If the fault clears and the boiler runs normally, monitor it over the next day or two. If code 28 returns, do not keep resetting — repeated resets without resolving the root cause add unnecessary wear to the igniter and control board.
- Inspect for any obvious blockages around the boiler DIY safe
Check that all radiator valves and any isolation valves near the boiler are fully open. A partially closed valve can starve the system of flow just as effectively as a pump failure. Also confirm there are no kinks or obvious damage to any visible pipework.
- Call a Gas Safe registered engineer to inspect the pump and internal components Gas Safe engineer
If the fault code persists after the basic checks above, a Gas Safe registered engineer needs to assess the circulating pump, check for internal sludge, inspect the heat exchanger for blockage, and test or replace the internal fuse F2 if required. These tasks involve accessing the boiler's sealed components and gas-side parts and must not be attempted by a homeowner. Ideally, choose an engineer with Viessmann-specific experience, as Viessmann recommend that diagnosis of this code is carried out by a Viessmann-trained installer.
- Arrange a powerflush if sludge is found Gas Safe engineer
If the engineer finds evidence of sludge throughout the system — cold spots at the bottom of radiators, discoloured water when bleeding — a full system powerflush may be recommended. This is carried out by a qualified heating engineer using specialist equipment and typically takes a full day. A magnetic system filter (such as a Magnaclean or Fernox TF1) should then be fitted to prevent sludge from returning.
Parts you may need
- Circulating pump (compatible replacement) · from £120
- High-limit thermostat / overheat sensor · from £45
- Internal fuse F2 (PCB protection fuse) · from £8
- Ionisation electrode / sensor · from £35
- Magnetic system filter (e.g. Magnaclean Pro2) · from £85
- Radiator bleed key · from £3
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–£400, depending on the underlying cause.
Frequently asked questions
Is it safe to keep resetting my Viessmann boiler when code 28 appears?
No. Viessmann boilers are designed to tolerate up to three resets within a 24-hour period, but resetting repeatedly without fixing the cause does nothing other than delay the inevitable and can accelerate wear on the igniter and PCB. If the boiler clears after one or two resets but code 28 keeps coming back, stop resetting and call a Gas Safe registered engineer to find the root cause.
How much does it cost to fix a Viessmann fault code 28 in the UK?
For most people, the repair falls between £120 and £400. A diagnostic visit and simple fix such as a fuse replacement or sensor swap typically costs £80 to £200. Replacing a circulating pump usually runs to £200–£400 all in. A powerflush, if the system is heavily sludged, adds around £350–£500. Heat exchanger replacement is a less common outcome but can cost £500–£900 or more depending on the model — if an engineer recommends this and the boiler is more than ten years old, it is worth comparing that cost against a new boiler installation.
Why does my Viessmann boiler show code 28 but the pressure looks fine?
Correct pressure is necessary but not always sufficient. If the pump is worn or partially seized, it may be moving water too slowly even with adequate pressure, causing the heat exchanger to overheat. Sludge blockages can have the same effect. An ionisation calibration fault is another possibility that has nothing to do with pressure. A Gas Safe engineer can carry out flow-rate checks, pump tests, and combustion analysis to identify the exact cause.
Will annual servicing prevent fault code 28 from recurring?
In most cases, yes. A thorough annual service includes checking pump operation, measuring system pressure and flow rate, cleaning or inspecting the heat exchanger, and checking sensors — all things that catch the early signs of problems that lead to code 28. Viessmann's warranty terms also typically require annual servicing by a qualified Gas Safe engineer, so keeping up with services protects both your boiler and any remaining warranty cover.