Right, this is the second time this week. I have had the heating on all the time but for the second time, have a frozen pipe. Fairly normal in these conditions but the strange thing is it is the HOT water that has frozen. I appreciate that the water in the pipes gets cold and can freeze, that isn't what's puzzling me. The COLD mains is still running fine, how come. If the cold coming from the mains and into the boiler isn't freezing, and the cold water in the taps is fine, why on earth is the hot water system freezing up?
None of the pipes are outside apart from a 10 foot mains section, but this isn't freezing.

Any ideas?

The only thing I can think of is you have a bit of the hot wter piping in a very vulnerable place so it gets cold easily – eaves / unheated void space? Dunno really tho – what sort of system?

Are you sure it is a pipe that is freezing? Last year it was the condense pipe on my boiler that froze & stopped the boiler from working, so I only got cold water.

Depending on how hard your water is you may have lost some of the salts which could depress the freezing point. The cold won't have and so will have a slightly lower melting point.

hot water freezers quicker then cold.

I found this out as a kid and turned the pavements into the best slides… woohooOOOOOOooooo!

It's only hot water if it's flowing. If it's sat in the pipe and not moving, it very quickly becomes cold water. Very cold in your case.

Mate of mine at work had a similar sounding problem over the holidays. It turned out to be a knackered valve that switched the hot water between the heating system and hot water pipes.

Combi boiler, only about 5 years old. There are polly pipes running inside the house but under the eaves, i guess it might be them but they are inaccessible.

Any ideas on how to locate the pipe 2nd floor sucks air in when opened, as does the first, so i am guessing the blockage is on the ground floor, does that sound like reasonable logic?

Just waiting for the patch of water on the ceiling at the moment!!

sucking air in suggests movement, ie, not a blockage, is it sucking in the whole time the tap is open, or just a gurgle? Is cold pressure ok upstairs?

Damn & blast, my boiler must have overheard me, frozen condense pipe again.. brrrrr