Common problems guide

The most common Alpha CD28 problems

The faults most likely to send a Alpha CD28 into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.
26 documented codes
6 most common
2 DIY-safe checks
4 engineer needed

The 6 most common faults

01
E10 High DIY-safe

Your boiler has stopped working because the water pressure inside the heating system has dropped too low.

What to try first

Locate the filling loop, which is usually a flexible silver hose under the boiler.Open the valves at both ends of the hose to let mains water into the system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (usually 1.0 to 1.5 bar).Close both valves tightly and remove the hose if required.

Est. cost
£0-£120
Parts
Filling loop, Pressure relief valve, Pressure gauge
Full guide for E10
02
E04 High DIY-safe

Your boiler has stopped working because the water pressure inside your heating system has dropped too low to operate safely.

What to try first

Locate the external filling loop, which is a silver braided hose connecting two pipes near your boiler.Open the valves at both ends of the hose to allow mains water to enter the system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (typically 1.0 to 1.5 bar).Close both valves tightly and remove the filling loop if required by your specific model setup.

Est. cost
£0-£120
Parts
Filling loop, Pressure relief valve, Pressure gauge
Full guide for E04
03
Red reset button illuminated High Engineer

Your boiler has stopped working because it failed to ignite, so it has automatically shut down for safety.

Est. cost
£120-250
Parts
Ignition electrode, Gas valve, Printed circuit board (PCB)
Full guide for Red reset button illuminated
04
neon illuminated High Engineer

Your boiler's electronic control panel is experiencing an internal fault, causing a warning light to stay on and likely preventing your heating or hot water from working.

Est. cost
£200-350
Parts
Neon PCB, Main PCB
Full guide for neon illuminated
05
Primary flow switch operated High Engineer

Your boiler is struggling because it cannot detect enough water flowing through the system to safely turn on your heating or hot water.

Est. cost
£150-250
Parts
DHW Flow switch, Diverter valve diaphragm
Full guide for Primary flow switch operated
06
Fan runs at max. speed High Engineer

Your boiler's fan is running continuously at top speed, which indicates the internal computer has detected a safety concern and is trying to vent the system.

Est. cost
£150-300
Parts
Printed Circuit Board (PCB), Fan assembly, Air pressure switch
Full guide for Fan runs at max. speed
Parts most often replaced

on the Alpha CD28

Filling loopPressure relief valvePressure gaugeIgnition electrodeGas valvePrinted circuit board (PCB)

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

  • You can smell gas or see signs of a leak
  • The Alpha shows an Emergency or High severity code
  • The boiler keeps locking out after repeated resets
  • You've tried the DIY checks and the fault hasn't cleared
  • There's visible water leaking from the boiler
  • The flame is yellow or orange instead of blue
See the full 26-code list for the Alpha CD28

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →