A lot of modern boilers draw the outside air for combustion in to the boiler body before it enters the combustion chamber. The body of the boiler is sealed at all pipe and cable entry points and the front cover has a continuous seal around it so that room air can't be drawn in.cover of the front boiler what a load of tat.
We had a new boiler about 6 months ago and I asked the plumber about it- he said that you are supposed to be Gas Safe registered (or no doubt considered competent in some other way) to remove the front cover on this type of boiler. He said that as long as you fit the cover back on correctly, there shouldn't be a problem but he couldn't advise you to do this yourself...
Having said that, a different Gas Safe plumber put the cover back on our old boiler a few years ago and didn't seat the seal properly leaving a big old gap, so either way, you take your chances...
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