Currently having new condensing boiler fitted , there's a lot going on (and mess) but only one engineer is doing it all, is that safe ?
If he's a certified installer, assume that it is. If he's certified you have to assume that he knows what he's doing. What possible part of the installation, with the exception of lifting very heavy things, could require the additional person? How would another person make it safer or more legal than just one? By your question I see that you most likely live in a nanny state like the UK, and of course there will be a slew of Health and Safety personnel who all have to come and paw over your new boiler and sign off on it. More people, more hassle, more bureaucracy. Do THEY make it any safer? Don't THEY count as the extra person that is supposed to ensure that it is safe? I can and have installed entire HVAC systems by myself. They were safe. They're still working, and they're still safe.
Yes it is. I have fitted many Gas boilers on my own
The only part where a second person would be an issue from a safety perspective is physically moving the unit. For that, YOU are qualified to be the second person, unless you have limited mobility.
As long as he's Gas Safe registered that's all you have to worry about. He'll do a better job if you're not getting under his feet.
