hello i have a gravity fed central heating system, i want to change my radiator for a towel rail radiator but as it is a different size i need to start again with the valves etc, so what would be the best way to drain the system down
I'm not sure how a 'proper' plumber would do it, but I've just done a similar thing.
Make sure the heating is off, and thoroughly cooled.
Turn off both rad valves, to isolate it, loosened the bleed valve at the top to let air in, unscrewed the valve from the rad, with a cat litter tray underneath (and a spare one in case the rad held more than the cat), and empty the rad. If possible have a big bucket/toilet nearby, and a helper, as you can't put it down until it's completely empty. And they can be very heavy.
Beware as the water in it will probably be black and stain everything it touches.
When it's empty, remove it from the wall and you should then have space to mount your towel rad - having first dressed it with all the necessary fittings (there will be a project here somewhere giving advice on PTFE tape). Take some time making sure it's level/plumb - it's worth it. Also, (and this is a pet hate of mine in my new house) make sure that the bottom of it is higher that your vac - it really bugs me that my vac looks as if it's been chucked off the roof, the rads have no paint left on the bottom corners and if my kids are doing the cleaning, there's a stripe of dust under every rad where it can't reach. Grrrrr.
If you're VERY lucky, the valves will fit straight back on (again with PTFE tape or something similar), otherwise, you'll have to do some plumbing.
Start at the rad end, and make it look neat - you've got to live with it. With as few joints as possible (preferably none) get to a point where you've lined it up with the existing feed and return pipes under the floor (yes you probably will have to lift some boards as it will look terrible with a joint visible). Avoid notching joists to run pipes - you're not a plumber, and secure any loose pipes.
Then ( and this is where you can be glad that we live in the 21st century) invest the best 15 quid you'll ever spend and get a pipe freezer kit (make sure you follow the instructions about distances and times). Using a proper pipe cutter (do not use a hacksaw), make the cuts and use push fit joints to make good (use the ones approved for heating systems).
Then just look over it once to make sure eveything is as tight as it should be and wait for the pipe to thaw (you should have about 20 minutes per pipe - loads of time). Then you can open the bleed valve and fill up the rad (you may have to top up your heating system dependant upon the type). Once full, turn on the heating, and let it get hot for an hour to pressure check it. Put the boards back down (don't nail through a newly laid pipe) and have a beer - you've just saved a furtune.
Check for leaks daily for a week and nip anything up that needs it.
It's really not that complicated, especially if you have all the materials and tools to hand before you start. Start with the mindset that you're definately not going to bodge any bit of it, and anything you do should finish up looking like a plumbers done it
Just one other tip - when you're at the plumbers merchants, buy a pair of 15mm and a pair of 22mm push fit stop ends and keep them to hand. If you slice your finger, or someone falls down the stairs, or you suddenly remember that it's the footy on the telly - you can just bang them on the pipe ends and come back to it later