Andrews E28 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews E28 fault code mean?
The E28 fault code on Andrews boilers (which share their platform and fault-code conventions with Baxi Group products, including Baxi and Potterton) indicates that the boiler has detected a problem related to the flue thermistor (NTC sensor) or has registered an issue consistent with a blocked or obstructed flue, or a frozen condensate pipe. In either scenario the boiler locks out as a safety precaution and will not restart until the root cause is resolved and the fault is cleared. The flue NTC is a small temperature-sensing probe that monitors exhaust gas temperatures in the flue; if it reads outside expected parameters — whether through sensor failure, wiring loss, or actual abnormal flue conditions — the boiler registers E28 and shuts down. Note: Andrews Water Heaters is part of the Baxi Group; always cross-reference with your specific Andrews model manual or contact Andrews Technical Support to confirm applicability to your exact unit.
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
- Frozen condensate pipe Common
During cold spells the external section of the condensate pipe — which carries acidic waste water away from the condensing boiler — can freeze solid. This blocks drainage, causes the boiler to detect abnormal back-pressure or temperature conditions, and triggers E28. It is one of the most common winter causes of this code across all Baxi-platform boilers.
- Flue NTC thermistor failure Common
The flue-gas temperature sensor (NTC thermistor) can degrade or fail outright over time, causing it to send readings that fall outside the boiler's expected range. The control board interprets this as a potentially unsafe flue condition and locks the boiler out with E28. Sensor failure is more common on older units or those without recent servicing.
- Flue obstruction Sometimes
A partial or full blockage at the flue terminal — caused by debris, a bird nest, compacted snow, or a damaged flue cowl — restricts exhaust gas flow. This can elevate flue temperatures or produce abnormal NTC readings, triggering E28. External inspection of the flue terminal is a straightforward first check.
- Loose or damaged NTC wiring Sometimes
The wiring harness connecting the flue NTC sensor to the PCB can develop loose connections, chafed insulation, or corrosion — particularly in units exposed to moisture ingress. An intermittent or open-circuit connection produces erratic or absent sensor signals that the boiler reads as a fault.
- NTC sensor incorrectly seated Sometimes
If the flue NTC probe has been disturbed — for example, after recent servicing — it may not be fully inserted into its pocket or may have slipped out of position. A displaced sensor cannot measure flue gas temperature accurately and will generate spurious E28 faults.
- Gas valve fault Rare
A failing gas valve can cause erratic burner behaviour or prevent ignition altogether. The resulting abnormal combustion conditions can produce unexpected flue temperatures that cause the NTC to flag E28, even though the sensor itself is healthy. Gas valve faults are less common but should be considered if other causes are ruled out.
- PCB (control board) fault Rare
A faulty printed circuit board may misinterpret a perfectly good NTC signal, or may fail to supply the correct reference voltage to the sensor circuit. PCB issues typically present alongside other unexplained faults and are usually identified by a Gas Safe engineer after other causes are eliminated.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on your Andrews boiler (usually marked with a flame or reset symbol) and hold it for the period specified in your user guide — typically around 3 seconds. If the boiler fires up and runs normally, monitor it closely over the next few hours. If E28 returns, do not keep resetting; move to the next steps instead. Repeated resets without fixing the root cause can cause further damage.
- Check whether the condensate pipe is frozen DIY safe
If the fault appeared during or after a cold snap (typically below 0 °C), inspect the condensate pipe — usually a white or grey plastic pipe (often 21.5 mm or 32 mm diameter) exiting through an external wall. If it feels solid or you can see ice, thaw it carefully by applying warm (not boiling) water using a watering can, or wrap a warm damp cloth around the frozen section and allow it to melt gradually. Work from the boiler end outward. Once thawed, reset the boiler. Consider insulating the pipe to prevent recurrence.
- Inspect the external flue terminal for blockages DIY safe
Go outside and visually check the flue outlet (the pipe or grille on the external wall or roof from which exhaust gases exit). Look for obvious obstructions: compacted snow, a bird nest, leaves, or a damaged or dislodged flue cowl. If there is loose debris you can safely remove without climbing to a dangerous height, clear it. If the terminal appears damaged or the obstruction is inaccessible, this is a job for an engineer.
- Confirm your gas supply is working DIY safe
Check that other gas appliances in your property (hob, gas fire) are working normally. If they are not, the problem may be with your gas supply rather than the boiler itself — contact your gas supplier or call the National Gas Emergency line (0800 111 999) immediately. Also confirm that the gas isolation valve on the pipe leading to the boiler is fully open (handle parallel to the pipe).
- Check system pressure DIY safe
Look at the pressure gauge on your Andrews boiler. For most condensing boilers the cold pressure should sit between 1.0 and 1.5 bar. If it reads below 1.0 bar, top up via the filling loop (two small valves, usually found beneath the boiler) following the instructions in your user manual until pressure reaches approximately 1.2–1.5 bar, then close both valves firmly. Low pressure alone is unlikely to cause E28 but resolving it rules out a compounding issue.
- If E28 persists, call a Gas Safe registered engineer Gas Safe engineer
If none of the above steps clear the fault, or if the code returns shortly after resetting, a qualified engineer needs to attend. They will test the flue NTC resistance with a multimeter to confirm whether the sensor is within specification, check and reseat wiring connections between the NTC and PCB, inspect the full flue run for damage or partial blockages, assess the gas valve and burner performance, and test the PCB if all other components check out. Do not attempt to access or replace the NTC sensor, gas valve, or PCB yourself — this work must be carried out by a Gas Safe registered engineer.
Parts you may need
- Flue NTC thermistor (temperature sensor) · from £25
- NTC sensor wiring harness · from £20
- Flue terminal cowl/guard · from £15
- Condensate pipe insulation lagging · from £10
- Gas valve (Baxi-platform compatible) · from £120
- PCB (printed circuit board) · from £200
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 £100–£320, depending on the underlying cause.
Frequently asked questions
Can I fix the Andrews E28 fault myself without calling an engineer?
Some E28 faults can be resolved without an engineer. Thawing a frozen condensate pipe, clearing visible flue obstructions, and resetting the boiler are all safe homeowner actions. However, if the fault is caused by a failed NTC sensor, damaged wiring, a faulty gas valve, or a PCB problem, you must call a Gas Safe registered engineer. Working on gas components or internal boiler electrics without Gas Safe registration is both dangerous and illegal in the UK.
How much does it cost to fix an Andrews E28 fault in the UK?
Most E28 repairs fall in the range of £100–£320 including parts and labour. A flue NTC sensor replacement typically costs £100–£200 all-in, wiring repairs are usually covered within a standard call-out fee of around £80–£150, and a condensate pipe repair or thaw is often low-cost or DIY. More involved repairs — such as a gas valve replacement (around £200–£350 including labour) or a PCB (£250–£450 including labour) — are less common causes of E28 but can push the total higher. Always get a written quote before work begins.
Why does my Andrews boiler keep coming back with E28 after I reset it?
If E28 keeps returning after resetting, the underlying fault has not been resolved. Common repeat-fault scenarios include a condensate pipe that refreezes because it has not been insulated, a flue NTC sensor that is genuinely failing (intermittent faults can temporarily clear on reset), or a loose wiring connection that briefly makes contact then loses it again. More than two or three resets without a successful run is a clear sign that you need a Gas Safe engineer to diagnose the root cause properly.
Is the Andrews E28 code the same as on Baxi or Potterton boilers?
Andrews Water Heaters is part of the Baxi Group and many Andrews condensing products share the same underlying control platform and fault-code library as Baxi and Potterton boilers. E28 carries a consistent meaning — flue NTC or flue/condensate fault — across these related brands. That said, Andrews also produces commercial and semi-commercial water heaters with their own control systems, so you should always check your specific model's documentation or contact Andrews Technical Support on the Baxi Group helpline to confirm the exact meaning for your unit.