Alright .....found some thing out today worcester 24i was constantly sparking after the boiler had fired tried every thing ..ie(flame rectification..P.C.B thermister..the problem was crossed polarity on a fuse spur in the kitchen so lads watch out for this.....cos it got me..
worcester 24i lpg
- Thread starter lee1976
- Start date
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I'm a newbie but i always check the fuse then the polarity, but thats only because i'm not allowed to check anything else.
Is flame rectification on a fan flued appliance needed if the fan is acting as the saftey device to the gas valve? (this is a question).
Is flame rectification on a fan flued appliance needed if the fan is acting as the saftey device to the gas valve? (this is a question).
so flame rectification would be used instead of an air pressure switch not with one - am i correct or out of my depth?
I've been working with gas for around 12months (as an apprentice) so forgive any silly questions to more experianced people (yourself).
cheers
I've been working with gas for around 12months (as an apprentice) so forgive any silly questions to more experianced people (yourself).
cheers
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Different purpose.
The APS checks that the fan's running, the FR checks that the gas is lit. So one doesn't affect the other.
Boilers without APS's have some other means of checking whether the fan's running. Back with ye olde Lynx is was a hot wire which got cooled down, now the electronics senses signals coming back from the fan itself. There have been other boilers with some other means, like thermostats.
The air pressure switch is only used to detect if the fan is running and the flue is not blocked.
The Pcb will check the air pressure switch before the fan runs to ensure it is not stuck on. If OK the fan is then started and the pressure switch checked again. When sufficient airflow is prooved the next part of the ignition sequence can begin.
Low voltage AC is applied to the sense electrode, if a current flow is detected by the PCB it then assumes the electrode is shorted to the burner and the ignition sequence will terminate.
If the electrode is OK, ignition is started and the gas valve modulation coil is fed with the ignition voltage (for the correct ignition burner pressure). The gas valve safety solenoids are then opened. Gas flows and the burner ignites. The Pcb now checks the sense electrode again. The ionised combustion products forming the outer part of the flame will cause a partially rectified current to flow between the sense electrode and the burner. This rectified waveform is detected by the Pcb and the boiler continues to fire.
The currents flowing are very small , only a few microamps and the system relies on good earthing and correct polarity of the phase conductors.
Most premix burners don't have an air pressure switch. As the fan operates it creates negative pressure at the fan inlet. This negative pressure controls the gas valve (zero governor). Hence if the fan does not operate or the flue is blocked the governor will not allow gas flow.
Some premix burners (Viesmann, some Worcesters) do have an air pressure switch. This is probably due to the higher safety standard demanded by their "notified body" overseeing the manufacture of these boilers.
The Pcb will check the air pressure switch before the fan runs to ensure it is not stuck on. If OK the fan is then started and the pressure switch checked again. When sufficient airflow is prooved the next part of the ignition sequence can begin.
Low voltage AC is applied to the sense electrode, if a current flow is detected by the PCB it then assumes the electrode is shorted to the burner and the ignition sequence will terminate.
If the electrode is OK, ignition is started and the gas valve modulation coil is fed with the ignition voltage (for the correct ignition burner pressure). The gas valve safety solenoids are then opened. Gas flows and the burner ignites. The Pcb now checks the sense electrode again. The ionised combustion products forming the outer part of the flame will cause a partially rectified current to flow between the sense electrode and the burner. This rectified waveform is detected by the Pcb and the boiler continues to fire.
The currents flowing are very small , only a few microamps and the system relies on good earthing and correct polarity of the phase conductors.
Most premix burners don't have an air pressure switch. As the fan operates it creates negative pressure at the fan inlet. This negative pressure controls the gas valve (zero governor). Hence if the fan does not operate or the flue is blocked the governor will not allow gas flow.
Some premix burners (Viesmann, some Worcesters) do have an air pressure switch. This is probably due to the higher safety standard demanded by their "notified body" overseeing the manufacture of these boilers.
so flame rectification is a system that tells the pcb etc that there is a pilot before the gas valve opens (just like a thermocouple)?
and a fan tells the pcb that its safe to proceed the ignition sequence?
If in some cases that the fan is linked drirectly to the gas valve, telling it to open - what tells the gas valve to modulate?
and a fan tells the pcb that its safe to proceed the ignition sequence?
If in some cases that the fan is linked drirectly to the gas valve, telling it to open - what tells the gas valve to modulate?
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