The joy of being a Heating Engineer

Vokera 20/80 Flowmatic

Case Removal

Remove four screws at the base. Two horizontal towards rear. Two at the top of the Controls Fascia.
Pull bottom of case slightly forward and push case upwards to disengage top support hooks. Ease lower sides outwards and withdraw from the appliance.

Controls Fascia

The complete unit will now hinge downwards.

Sequence of Operation (Domestic Hot Water)

Only the main on/off neon switch is required to be on, the summer, constant, timed switch can be in any position.

1. On this boiler the fan runs on
   low speed at all times via a 560 ohms resistor to maintain the pilot.
   This is brass coloured and fitted bottom RH side of combustion chamber.
   This resistor will be hot.
2. Open a hot water draw off tap.
3. The flow of mains water changes over the diverter valve and operates the
   treble DHW microswitch assembly. The push rod can be seen operating
   when the tap is opened (high and low voltages).
4. The pump starts. (Check by listening or checking at the pump vent plug.)
5. The movement of water created by the pump makes the flow switch (240 Vac bottom LH side below pump).
6. The fan changes to high speed and makes the pressure switch.
7. Mains power is passed through the safety thermostat (auto reset, lower RH
   side flow pipe) to the gas valve operator (240 Vac) which opens and
   allows gas to the burner.
8. The modulator receives power via the DHW thermostat (Controls fascia).
   Contacts open, the modulator voltage will be zero, high flame
   condition. When the selected temperature is reached the stat contacts
   close allowing full voltage (12.5 Vdc) to the modulator, to give a low
   flame condition. (This works the opposite way round to most other
   combis.)
9. If the DHW stat fails and the temperature of the water becomes excessive the
   safety thermostat will cut the 240 Vac supply to the gas valve operator.
10. If for any reason the safety stat failed to operate, the overheat stat (located in
    a dry pocket just below the safety stat) will operative, interrupting
    the thermocouple.

Sequence of Operation (Central Heating)

External controls (roomstat) call for heat, all other CH controls on fascia calling for heat.

1. Diverter valve stays in rest position.
2. The pump starts. (Check by listening or checking at pump vent plug.)
3. The movement of water created by the pump makes the flow switch (240 Vac bottom LH side below pump).
4. The fan changes to high speed and makes the pressure switch.
5. The gas valve operator (240 Vac) opens and allows gas to the burner.
6. The modulator receives power via the CH thermostat (Controls fascia). With the first stage contacts open, the 12.5 Vdc is diverted through the range rating potentiometer to give a central heating high flame condition. When the preset temperature is approached the first stage contacts close, this allows the full 12.5 Vdc to the modulator. This gives a low flame condition. (This works the opposite way round to most other combis.) If the temperature continues to rise, the second stage contacts (240 Vac) will change over shutting off both gas valve operator and fan.
7. The pump continues to run unless turned off by clock or roomstat.
8. If the CH stat failed in an on position and the temperature continued to rise the same safety controls would operate as for the Operation of DHW Nos. 9, and 10.