Hmm. IIRC it was corgi who decided that it was a good idea to run condensate in a larger dia pipe that the 21mm of overflow... Condensate freezes as the droplets are so small they hold very little sensible heat. Once that heat dissipates it will freeze if external temps are low enough - end of. The ONLY point of insulation is to try to reduce the time it takes for the sensible heat to dissipate. However, insulation itself will never stop freezing - only delay it. There are only two ways to stop freezing. The first, and always the best from a scientific perspective, is to install trace heating. The point of trace heating is to replace the heat lost by the condensate as it travels. It also raises the internal temperature of the pipe to above freezing. The condensate will flow but not freeze. One of the things Corgi got wrong was the pipe upscale. Fact is when you upscale a pipe like that you actually introduce additional cooling factors in teh form of internal circulatory air currents. These ADD to cooling going on and so work against keeping the heat IN the condensate. When using trace heating, one should always allow for heat to escape too so you do not use as thick a wall insulation. You also make sure you run it to manufactures specs as it can occasionally overheat pipe. The second way to stop freezing is to supply enough sensible heat in water that over its journey its not possible to drop it low enough to freeze. This is where condensate pumps come into play. Instead of a drip at a time, they pump loads out. That load has enough heat energy in it not to drop low enough over its short journey to freeze. Whilst they are good, personally I'd never use on unless it was an almost vertical run to drain. The last drops will end up freezing so over time even they can block. Lastly, and then I'll shut up I promise, in cold places around the world and of course commercially, trace heating is a no brainer. Britain, bless our cotton socks, thinks it knows so much better... Guess what? We do not... Sorry for going on.