Cleaning the combustion side of the heat exchanger

What is going on

Annual service or when combustion readings drift, flue temperature climbs or Loc 16 appears.

Likely causes

• Deposits between the heat exchanger tubes
• Dirty electrodes
• Damaged refractory

Check in this order

1. Remove the heat demand, let post-purge finish, shut off gas and power.
2. Swing the control panel aside; unplug the fan, gas valve and both electrodes. Unthread the gas valve inlet union (keep the fibre washer).
3. When cool, remove the four nuts holding the burner assembly and lift it out without damaging the refractory disc behind the door.
4. Check the electrodes; clean with steel wool if needed. Ignition gap is 4.5 mm.
5. Protect the rear refractory from water; replace it if damaged.
6. Vacuum loose debris, remove the condensate trap and put a drain under the boiler.
7. Wet the chamber with warm water only and rinse between the tubes with a hose until the water runs clear. Keep electrical parts dry.
8. Clean and reassemble the condensate trap, then reassemble the boiler. Replace any gasket that shows damage; the silicone burner door gasket is due every two years.
9. Run a combustion analysis afterwards.

More boiler guides

• Boiler pressure keeps dropping / FILL shows
• Display is blank
• Display normal but no domestic hot water (indirect tank)
• Display normal but no heat to the space