i am changing new rads for old they are slightly smaller if i isolate everything will i get much water come out

The best way to do this would be to turn the boiler off, isolate the central heating supply (this will either be the smaller feed and expansion tank in the loft for and vented system or for an unvented system make sure the filling loop is isolated and ideally disconnected). Then drain down the entire system (as you may need to alter pipework as you mentioned the new rads are slightly smaller) using the lowest drain off valve. Make sure you vent all the radiators as well starting with the highest.

By slightly smaller do you mean in length? if they are shorter in length than your current rad's you will need to adjust your pipes, if the length of the rad is the same and the height is smaller all you need to do is turn both valves to the left and right of the rad clockwise until tight and that eradicates the need to drain your system HERE'S A HOW-TO before doing any work turn your boiler of at the mains! 1. tighten valves 2. place a small bowl/pan under the right hand valve 3. slowly undo the horizontal nut on the valve (not the vertical nut) horizontal nut is the larger one in most cases 4. when pan is full nip up the nut and empty it (continue doing this until water stops 5.once water stops keep the pan under the valve with nut loose and undo the rad bleed nut, some more water will escape 6. remove and change rad 7. be sure all nuts are tight and slowly turn left hand valve anti clockwise then do the same with the right 8. bleed rad 9. refill boiler pressure via the filling loop to 1 bar JOB DONE

Only the water in the rads and if your quick and don't open the bled valve you can spin the rads upside down to keep all the water in them , also you can get spacers that will allow you to change the rads without adjusting the pipework . good luck.rob

Dennis. Don't you mean,Old rads for New? Whether you have a sealed system,or gravity,you need to do curtain things,and yes,water will come out. if you just drain one at a time it'll take you all day,and the next day!! To do the job properly,and in the interests of your boiler,and the performance of your system,in performance and efficiency,you need to have a total drain down,fit all new radiators,with new tails into the valves!! Power flush the system thoroughly with chemicals,and add high levels of inhibitor! I am guessing you are changing them as they're not performing,damaged! Post the job, and get a good plumber round! gas safe to! Kieran kelly plumbing Heating & Gas