how do you remove a radiator but keep the circut?

Fully turn off both valves.

Undo the valve to radiator unions and drain the rad.
I use a paint roller tray a small jug & a bucket to collect the water. Undoing the bleed valve lets the water run out a bit quicker. At the end the water may run black so put towels and plastic sheets down especially if you have carpets. When drained close the bleed screw.

Lift rad from wall brackets (may need two people) and turn upside down to prevent any remaining sludge from dripping out.

i.e. the radiator valves stay on the pipes to keep the water in.

Good luck...

Peccavi
December 2009