boiler 20mm condensate pipe

Just to clarify, I obviously mean getting the pipe into central holes in the joists when the next joist along is untouched, forcing it down between 2 joists and into the holes. Simon.

Can I just ask why a condensate pipe needs to be 20mm? I was looking at a condensate pump recently, and the outlet pipe seemed to be about 5mm.

I think 20mm is the "standard". Actually I was thinking of running part in 32mm (!) just in case it ever got blocked (and/or) I could then use it for a basin if I ever wanted one in the room near the boiler. Anyway, 10mm is fine for central heating you might say. The difference is the condensate is dribbling out and has to drain using gravity so it is moving slowly and more liable for deposits to collect (although it should be pure water ?) or even surface tension to have an affect (at 5mm). If pumping you can allow for a smaller bore, but still 5mm seems a bit small. Simon.

No it's got some acid in it from the dissolved carbon dioxide. I'd have thought that would prevent any carbonate build-up even in the presence of air.

It might be more than 5mm. Take a look at picture 2 here

Possibly, otherwise use 22mm PB or PEX - the o/d of overflow is nominally about 21.5mm (not 20mm) and can be joined using push-fit couplings. (Since the pipework isn't under pressure mechanical grip onto the smaller pipe isn't really an issue.)