What E28 means
E28 means your Baxi has lost a trustworthy signal from its flue temperature sensor and has locked out. Most of the time the sensor or its wiring needs attention from a Gas Safe registered engineer, although a blocked flue or condensate drain can also be behind it on some models.
A condensing boiler sends its exhaust gases out through a flue. The flue NTC is a thermistor that tells the control board how hot those gases are, which lets the boiler protect the flue and the heat exchanger from overheating. When the sensor reads open, shorted or wildly out of range, the board cannot do that job and shuts the burner down.
The Megaflo System HE IE manual (UK Comp No 5129838) groups E28 with E20 and says both "indicate faulty components". The Duo-tec Combi HE A manual (UK Comp No 5122323) uses the same "Flue NTC Fault" description.
Baxi and Potterton boilers that use E28
| Boiler | Document | Wording |
|---|---|---|
| Baxi Duo-tec Combi HE A (24, 28, 33, 40) | Installation and Servicing Instructions, p.52 | Flue NTC Fault |
| Baxi Duo-tec 2 Combi GA | Installation and Servicing Instructions 7206334-08, p.45 | Flue NTC Fault |
| Baxi Assure Combi 25, 30, 36 (and LPG) | Assure Combi User Guide, Table 9 | Code 28, "Flue NTC fault" |
| Baxi Megaflo System HE IE | Installation and Servicing Instructions, section 18.2 | Flue NTC Fault, a faulty component |
| Potterton Gold Combi 24, 28, 33 | User Guide, p.8 | Flue NTC Fault, Air Pressure Switch Fault, Blocked Flue, Blocked Condensate or Wiring Fault |
The Gold Combi is built on the same Baxi platform, and its longer description is a useful clue to what else an engineer will look at on a Baxi showing E28.
Possible causes and how they differ
A failed flue sensor
This is the cause every Baxi document names. If the sensor itself has failed, E28 tends to appear the moment the boiler tries to fire and comes back straight after a reset.
Wiring or connection fault
A loose plug or damaged lead between the sensor and the board gives the same symptom. It can be intermittent, showing up some days and not others.
Blocked flue or blocked condensate
The Gold Combi wording lists both. A condensing boiler drains acidic water through its condensate pipe, and if that pipe freezes or blocks, the water backs up. A flue terminal that is covered or damaged stops the gases leaving properly. These causes are more likely when the code appears in freezing weather or after building work near the flue.
Air pressure switch
Also named in the Gold Combi table. This is an internal part and an engineer will test it if the sensor checks out.
E28 is not the same as E130
Baxi tables also list E130, "Flue NTC Operated". The Duo-tec manual explains that E130 means overheating in the flue system: the sensor is working and has measured a temperature that is too high. E28 means the sensor reading itself cannot be trusted.
Checks you can safely make
- Look at the flue from outside. From the ground or a window, check that the terminal is not covered by anything such as a bag, sheeting or plant growth. Do not touch, move or try to clear the flue. Our boiler flue guide explains what a healthy terminal looks like.
- Check the condensate pipe. In freezing weather, follow the plastic pipe outside and thaw any frozen section with warm (not boiling) water or a hot water bottle. Our frozen condensate pipe guide covers the steps.
- Check the power supply. Make sure the fused spur is on and the display is lit.
- Try one reset. If you have cleared a frozen condensate pipe, reset the boiler as the user guide describes. Stop after 2 or 3 attempts.
If a carbon monoxide alarm sounds, or you notice fumes, sooty marks or a smell near the boiler, switch the boiler off, open windows, get everyone into fresh air and call the National Gas Emergency Service on 0800 111 999. Never seal up or cover a flue.
What the engineer will check
The Duo-tec Combi HE A fault-finding chart gives the flue sensor's cold resistance as about 20kΩ at 25°C, double the figure for the heating sensor. The engineer will measure the sensor against that reference, inspect its plug and the lead back to the board, and test the air pressure switch where fitted. They should also inspect the flue route and terminal and the condensate trap and pipe, since a blockage in either can bring the code back after a new sensor has been fitted.
Typical repair costs in the UK
| Job | Typical cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Flue temperature sensor (NTC thermistor) replaced | Typically £100 to £200 |
| Sensor wiring or connector repair | Typically £100 to £180 |
| Flue repair or adjustment | Typically £150 to £400 |
| Condensate pipe reroute or insulation | Typically £80 to £200 |
When to call a Gas Safe registered engineer
Book an engineer if E28 returns after one or two resets, if the flue looks damaged, or if the code keeps coming back in cold spells even after you thaw the condensate pipe. Tell them the full code, the model, whether it shows at first light-up or after the boiler has run for a while, and what the weather was doing. If your boiler also shows E20, mention it, because two sensor codes together point the engineer towards the wiring or board.