Combination Boilers - Basic Principles of Operation

To be able to fault find and diagnose combination boilers, you must first understand their basic principles of operation.

After a heat or hot water demand is identified, most combination boilers follow a simple process to service that demand. Knowing what this process is can help to diagnose a faulty appliance if you can identify where the process has become interrupted. The flow chart below depicts the typical process that occurs between a combi boiler receiving a demand and the burner firing:

  1. Demand from central heating controls or HW demand

    Central heating controls typically consist of a time clock or programmer and a room thermostat. More commonly, these two items are combined into one and are known as a programmable room thermostat. Hot water demand is normally arrived at through opening a hot tap. This in turn may operate a flow switch such as a turbine, paddle switch or diaphragm valve.

  2. Motorised valve moves to required position

    Depending on the demand, the motorised valve will normally move at this stage to divert water either through the central heating system or through the secondary hot water plate heat exchanger.

  3. Sequence controller / PCB checks primary circuit potentiometer and secondary cicuit potentiometer are in range and continuity exists across overheat stat

    Typical ranges vary depending on the technology used. Any thermister reading either open circuit or zero ohms resistance will most likely be defective.

  4. Power is supplied to the pump and the pump runs

    Not all pumps are 230v. If possible, check for a data badge on the pump to determine what voltage you should expect to find at the pump.

  5. Pump proves using a flow switch on the primary hot water circuit

    Not all boilers prove the pump is running before continuing with the firing sequence.

  6. Sequence controller / PCB checks the fan pressure switch has dropped out

    If the pressure switch has proven before the fan is running it must be considered defective and will result in appliance lockout.

  7. Power is supplied to the fan and the fan runs

    It used to be quite common for fans in combination boilers to use 230v power. Increasingly boiler manufacturers are using DC fans that offer more accurate speed control that is useful with modern pre-mix burners. Due to way multimeters measure DC voltage, budget multimeters can vary significantly in their accuracy - bear this in mind when checking fan operation.

  8. Fan proves using an air pressure differential switch

    The sequence controller looks for the air pressure differential switch to move over from the normally closed position to the normally open position.

  9. Power is supplied to the ignition components / PCB and an ignition spark is delivered across ignition probes

    Many older appliances use a separate PCB for spark generation but most modern appliances handle the ignition sequence on the main board.

  10. Power is supplied to the gas valve and either a pilot gas supply or main burner gas supply at low pressure is established

    Very few if any appliances fire on main burner at full rate. Establishing a pilot burner confirms that the requirements for combustion have been met.

  11. Sequence controller / PCB checks for presence of flame using flame rectification

    Nearly all modern appliances use half wave flame rectification to detect the presence of a flame. An small AC current is delivered to the flame rectification probe. If a flame is present, the AC current will travel across the ionised air above the flame becoming half wave rectified DC current. This DC current is then delivered back to the sequence controller through the burner gound connection.

    If AC current is delivered back to the sequence controller, a short is indicated and appliance lockout will result.

  12. Once flame is proven, gas supply to main burner is established

    Depending on the appliance a number of parameters are then continuously monitored. These may include the flow / return temperature and flue gas temperature.

  13. On demand end, power is removed from gas valve and fan. Power may remain to pump to disperse residual heat build up in primary heat exchanger (pump overrun)

    Where the pump is external to the appliance, a frequent cause of spurious lockouts is the lack of effective pump over run.

Blaney Mechanical are Yorkshire's leading Commercial Gas Engineers, Industrial Gas Engineers and Combustion Engineers based near Sheffield, South Yorkshire covering Yorkshire, Derbyshire and Humberside.

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