BAXI BAHAMA

[gasmanuk]

Hi guys I need some help on a baxi bahama, on central heating it lights for one second then goes out immediatlly then relights again it does this several times each time only lighting for one seconed. After about six times it either lights or goes to lockout. The boiler lights first time everytime in dhw mode and never lockouts. Now I have adjusted the minium gas pressure which is around 4.5mbar but it never stays on long enough to rectify. Both flow switches are OK and have been bridged etc. The weird thing is the fan switches from low to high in time with the burner lighting and going out. Ignition control board and main pcb both changed

HELP

 

[corgigazza]

Check the thermistor and safety thermostat may also be worth checking that the diverter vlave moves to dhw and ch demand also check the ch return filter.

 

[gasmanuk]

cheers gazza checked all those things already this is a real b*****d

 

[gazthepottertonengineer]

sounds to me like its not sensing flame i assume you have checked that boiler has correct polarity ! live and neutral round right way
if thats ok check the air pressure switch !

 

[baxpoti]

Ignition control board and main pcb both changed

So you have the SIT controls and gas valve? (red coloured 'eyebrows' on the fascia panel)
Why did you set 4.5mb low gas are you on LPG?
Low gas is 2.3mb on NG, set it up on DHW. High is 13.6mb

Replace gas valve if unable to obtain these figures

 

[ChrisR]

Not a boiler I know anything about, but, could it be flame lift? Suppose you let air into the CC ? Suppose you put your blowlamp on the detection electrode?

 

[gasmanuk]

no its not even getting to flame sensing stage flame retifacation usually takes about 8 seconds but this is dropping out in less than. Its not aps or electrode because hot water is fine. Its honeywell I meant the controller box but I call it cantoller board. It can't be the gas valve as fan is dropping out at the same time if it was the valve the fan would still run. The min gas pressure on hw is 2.5 but because it just flashes up I only get a reading for half a second but I dont think it is a flame sensing fault

 

[gasmanuk]

yes both those chrisr but its not getting to that stage of flame retifacation

 

[gasmanuk]

Think I shoud point out here that I am a heating engineer so am familer with correct polarity etc. but this has got me so confused that I wondered if anyone else had any thoughts i have fixed loads of these in the past so am not a complete novice.

 

[ChrisR]

I think we all assumed you're an rgi from the name you chose. I take your point about the 1 second and flame rec. Some (eg Classic's) are pretty quick to fail.
I'd look at the APS again. I see its OK on HW but the 'air' pressure could be different with different flame size. Does it switch each time the
fan goes on and off? A Neon can be useful if it's mains, else an analogue meter is better than a digital one.
Apart from the resistances open and closed, you can check the operating pressures, with a digi manometer.
Or use a tube/blow to simulate in time honoured...

Yoiu've probably seen precisely this pattern on an SX20, come to think of it?

Checked the primary flow switch isn't misbehaving - different flow on CH(return filter )?

 

[baxpoti]

The min gas pressure on hw is 2.5 but because it just flashes up I only get a reading for half a second but I dont think it is a flame sensing fault

You said earlier that hw was ok?

If you have a honeywell valve, low is normally around 1.9/2.0mb fixed.

 

[baxpoti]

Checked the primary flow switch isn't misbehaving - different flow on CH(return filter )?

Come on ChrisR, OP says he's bridged the flow switches.
I 'd be suprised if the original installer ever fitted the return filter, certainly worth checking tho.

 

[gasmanuk]

no orig installer didnt fit return filter I have checked that aswell. Chrisr the aps stays on until after the fan cuts out so thats working ok.

If you think about it there is something turning both the gas valve and the fan off at the same time, on the gas valve controller pin 8 and pin 10 control the demand e.g flow switches and thes both have 240v all the time when a demand is called for but even when it tripping on and off 240v on these pins is constant, so there is no outside fault e.g flow switch , eps, filter.

I realise my reading arn't 100% correct on the gas pressure but its not getting that far so its not really an issue its just lighting ang going out straight away but the point is THE FAN DROPS AT THE SAME TIME so it isn't a rectifacation problem???

 

[ChrisR]

You seem to have covered everything. Wiring to thermistor?

 

[baxpoti]

The boiler lights first time everytime in dhw mode and never lockouts.

Is this correct?

There seems to be some contradiction to exactly what is happening?

Do you see the flame lit? or is it the orange neon flame light on?