The joy of being a Heating Engineer

Case Removal

Upper module case – Release the one top centre securing screw and withdraw.
Boiler module case – Open the control panel door, release the four captive screws top and bottom, and lift case off the boiler (position of by-pass solenoid may interfere with case removal).

Special Features

This combi is the classic boiler (cast iron heat exchanger) with combi module above. It has a burner on/off switch. DHW pre-heat operating 24 hrs a day, but can be controlled by an external programmer if required. There is a wax capsule by-pass thermostat, on the outlet of the CH flow pipe.

Sequence of Operation: Domestic Hot Water

With no call for either DHW or CH the boiler will fire at low rate to maintain the DHW calorifier and boiler heat exchanger in a pre-heated condition. With the selector set to DHW/CH position, the burner on/off switch, switched on.

• Open a hot water draw off tap.
• The flow of water through the calorifier makes the paddle type flow switch.
• The Honeywell diverter valve remains de-energised.
• The pump starts (check by listening or viewing through the vent plug).
• Mains power is passed through the manual re-set overheat stat to the ignition PCB to start the automatic ignition cycle.Air pressure switch and PCB carry out a NO air check.

Fan starts to run, air pressure switch makes contact.
Spark commences, pilot solenoid (240 Vac) opens, pilot lights.
Flame checked by rectification circuit on PCB (spark and rectification use the same probe on this boiler).
Main solenoid (240 Vac) opens.
By-pass valve (240 Vac) opens and fires at full gas rate.
The burner may fire initially low rate but should increase to max 1-2 minutes. If there is a breakdown in this cycle check:
There is gas to the boiler and the solenoid valves are in order.
The overheat stat is in the closed position. (Try reset button first.)
The spark/rectification probe, lead, and spark gap (3-5 mm) are OK ( £ 1 coin).
The combustion chamber cover is properly sealed.
All fuses and wiring connections on the PCBs are secure.
The tubes from the air pressure switch to the fan. (Trapped, blocked, etc.)
The fan motor, or impeller is not seized or jammed.
Gas on full rate controlled by the DHW thermostat.

Sequence of Operation: Central Heating

Water is not released to the CH system until the temperature has reached 60 ° C. (This is controlled by the wax capsule thermostat.)

• External controls (clock/roomstat) call for heat.
• Diverter valve motors over to CH (no resistance at manual lever).
• The pump starts. The boiler follows the same sequence as for domestic hot water until the main flame is established (see above).
• Gas on full CH rate, controlled by CH thermostat and overheat stat. If there is a breakdown in this cycle check:
  CH control thermostat, solenoids, overheat thermostat.
  Wax capsule by-pass thermostat (ensure that water is leaving the boiler by checking hot water flow into the system.)

Common Faults

Flow switch can get blocked with age resulting in no hot water. Expansion vessel tube can get blocked at the tee piece.