OK rewardinheaven, the principle is simple: as the seals weaken, the spring washers take up some slack and keep the box sealed up with any luck.
However, the problem is that if the unit is anything like mine, you are only fixing into stupid little clips which are only good enough for car trim if that. My boiler parts guy doesn't even stock them. He suggested Halfords! When you tighten the screws, (and remember on the airbox door there is a solemn message about making sure the firebox screws are tight!), you will be lucky if they don't start twisting uselessly in their seatings. This is what keeps happening to me.
Poxi has a cool idea. You put spring washers on, and then do not tighten beyond the point where the screws start to slip! Sounds as if it might work!
What I have been trying is introducing machine screws with nuts behind, and some spring washers and shakeproof washers to shore it all up. I have managed to fix three of the four fixings this way, but physically working the nuts in and getting the bolt in is a challenge as the thing is deliberately designed to stop you doing anything of the sort. I plan to glue a nut onto the hard-to-reach side of the flange to sort out the final fixing, but in the meantime I tried to hold the nut on by telling my daughter to chew some gum which I could use to hold the nut until it was pulled in by the bolt and held fast. Even the wife is laughing at me for this, and as for Penpusher, he is probably losing the will to live about now!
The culprit of course is Baxi, who should be taken out and shot. How can you supply a firebox with edges so inexactly welded that some are 2-3 mm proud of the others, then demand that the firebox seals be tight, then supply seals that I would not accept in my picnic coolbox, then be surprised when the whole thing fails?
I actually looked at their website this evening and it nearly made me throw up. There were dear little children toddling around basking in the glow of the heat they supply...Well, MY children are COLD, NOW!
If any Baxi people are watching this site, I hope you are ashamed.
If none of you are, it's time you were. In Yankland, there would have been a class action suit against you, and in all probablility you would have been put out of business by now, not displaying websites of happy warm toddlers!
As a matter of fact, there is a mechanism in UK law for something pretty similar, but I don't remember the exact name for it.
Baxi: don't think you are necessarily going to get away with it... this is 2007, we know our rights, we paid our money, and you people sold us crap.
Watch this space!