Bottled gas lines

[Spectric]

The top half of the grate thing is solid over the exhaust with only 8mm clearance between it and the pipe. Presumably it's purpose is to divert the exhaust gas out through the slots on either side,

Going by the link provided by @toolsntat it is removed when running the burner on GAS, plenty of info on this cover and it's removal online.

https://www.motorhomefun.co.uk/forum/threads/water-heater-vent-cover.144507/

https://forums.practicalcaravan.com/threads/truma-water-heater-external-cover.15574/

Any heater that burns fuel and produces CO has been designed to be safe, these devices must not be modified because they then will no longer comply with the certification and can become dangerous so if you have one that is not functioning then there is a problem that requires the right person to rectify and do not start modifying the device because you think the design is wrong.

 

[Kittyhawk]

Yes I do have the cover - all the units come with it and of course it must be removed prior to igniting the burner.

The issue is now, finally, resolved in a way that avoids tinkering around with plastic grating, but first, Truma. I phoned the importer this morning and advised him of the flameout problems together with details of my gas setup, kpa at bottle and burner etc. He told me he had never heard of the issue, there were thousands of units performing faultlessly blah blah blah. I suggested that perhaps he might go on line and check out the NZ motor caravan association forum, the UK Caravan forum and numerous others where he would find literally hundreds of posts on the Truma flameout issue. He declined unwilling to waste his time on what he described as a 'non event.' Reminds me of years ago when we had an H28 yacht with an auxiliary Perkins diesel driving through a TF gearbox which needed replacing. We chose an English PRM box which failed after 30 hours. After much examination of the installation looking for non existent install faults they begrudgingly replaced it. This next one failed after 18 hours. Same scenario again only this time it was replaced with the 'improved and redesigned' model of the exact same gearbox which is still going strong 11 years later with its current owner. I'm adding this to show that manufacturers wil never, ever, ever admit to having an intrinsically faulty product on the market.

Going by the link provided by @toolsntat it is removed when running the burner on GAS, plenty of info on this cover and
Any heater that burns fuel and produces CO has been designed to be safe, these devices must not be modified because they then will no longer comply with the certification and can become dangerous so if you have one that is not functioning then there is a problem that requires the right person to rectify and do not start modifying the device because you think the design is wrong.

I am aware of the risks of tampering with a balanced flue as expressed here and I take the comments to heart.
On ignition, the lpg combustion gasses are cooled down by the metal exhaust manifold and once they exit through the bedding plate they flow downward and are drawn into the air intake extinguishing the flame. This occurs because the design of the solid portion of the plastic grating encourages it and also because cool combusted LPG exhaust gas is heavier than air. When this happens the piezo igniter kicks in but can't relight the burner as the cavity is full of gas. The solution is to squat in front of the air intake and when you hear the igniter ticking, give it a good puff of breath to clear the gas whereupon it will relight. You must be quick before the failsafe thingy shuts down the gas flow, and you have to keep this up for a couple of minutes at every flameout. After the said couple of minutes the exhaust manifold is hot enough that it no longer cools the combusted LPG gas which being warmed, rises instead of falls and the boiler will start to function normally on its own.
And that's how to start a Truma boiler when there is insufficient breeze to blow away the exhaust gas. And I call that a pretty big design flaw by Truma who darned well ought to own up to it.