convert single pipe cebtral heating into two pipe system.

[DeltaT]

If you have plenty time just start from scratch as following the one pipe loop with another pipe will give a system with a far greater water content than necessary, therefore bigger bills.
Plan you new system and pipe runs. Have a google for 2 pipe system and work it out to suit your house. Run the new pipes up to front of the rads so they are easy picked up when ready. Follow what Dex says about sizing and reduce to 15mm for last 2 (maybe 3 depending on output) at end of runs. You won't need to bother too much with sizing for lengths if you have a normal size house.
If you are not too good with the copper and don't trust your soldering, use plastic. It is like Lego, even the weans could do it. You need copper for 1st meter or so from boiler and you should use it for any visible runs because plastic looks like a dog phishin in the snow (can still use plastic fittings or Cuprofits look better).
Fire in it is not hard to do.

Ah 'tamz', thanks mate you made me laugh. Not sure our 'little Engerlander chums' know what the weans are??!!
Reading your posts makes my life richer and reminds me why I love being Scottish............

 

[littlepigeon]

Thanks for the info, I'm in Northwich.

bit too far - would have offered to come round for a cuppa and a free bit of advice for you

The boiler at present is at the back of the house so when you say t off do you mean run a flow and return along the downstairs rooms and the same again upstairs?

that was the idea

The house us an old edwardian property so hiding pipes downstairs is difficult due to solid walls.

are the floors solid, since it's under those that the flow and return need to run

At the moment I think all downstairs rads appear to be fed off the single pipe running under the floor boards upstairs.

so each downstairs radiator has a couple of pipes running up the walls back to the main pipe?

Yup downstairs floors are solid, and are def not floors i would want taking up for pipes to run under them. Two parquet floors, one quarry and one minton tiled.

Correct re the the downstairs rads and the two pipes going up. Theres 3 of them at the min but need an extra installing in one of the rooms so would be 4 if i do this.

So kind if you to offer to come over for free advice if closer. but i understand.