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Draining central heating system

Postby Ms Marple » Tue Sep 01, 2009 2:22 pm


I am changing a rad. to heated towel rail in upstairs bathroom but unfortunately have to move pipework. so it appears I have to drain entire system. Have checked project page but have combi boiler (direct? sealed?), no tank in loft etc so couldn't follow.. Also appear to have no drainage valve anywhere.
Can I turn off mains, drain downstairs rad, remove it, attach a hose to pipework, then open all valves, stand back and let the upstairs rads drain out?
Then re route pipework, attach towel rail, re attach downstairs rad and refil system?
Have basic plumbing skills but don't understand central heating....(probably obvious) so any help much to understand theory behind things so the fuller the explanation, the better for me!


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Simply Build It

Postby Carl83 » Tue Sep 01, 2009 8:52 pm


You mention you have a sealed system therfore relieving the presure from the sytem and opening a drain will alow you to cut into the system

If you have no drain then remove a downstairs rad and attach a hose to a valve and open it up. This will drain your system ( dont forget to ensure boiler is turned off and you open the upstairs air vents on the rads to allow the water the drain )

Carry out your install and refill the system while venting. Fill system back to a minimum of 0.5 bar but no more than 1.0 bar unless your system runs over more than a couple of floors

Dont forget to use a good quality inhibitor once your finished ( make sure it is aproved by the D.W.T.A as most cheap ones do nothing, I have personally tested many in jars with different metals and found Kamco dm system safe the best )

Any more help just yell.

Hope this helps
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Postby Paul Griffin » Fri Sep 25, 2009 7:59 am

Good information but, without a header tank on a combi system, how do you put the inhibitor back into the system? I also assume that just turning the mains water back on will refill the system; the cross-connection to the boiler is simply to pressurise the boiler diaphragm, yes?
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Postby plumbbob » Fri Sep 25, 2009 11:19 pm

The towel rail is the easiest way of putting in inhibitor, Remove one of the plugs and tip it in before filling the system.

Er, no, the heating system is entirely separate to the hot and cold water. In fact, you don't even need to turn the mains off before draining the system - just the boiler! The cross connection you mention has a valve which you need to open to allow the water to refill the system. As you do this, you must watch the pressure gauge too because the 3 to 5 bar mains water pressure is far greater than the 1 to 1.5 bar needed to make the heating work.
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Postby Paul Griffin » Mon Sep 28, 2009 4:26 pm

Thanks for the information. So, putting it very simply, the only way to get the inhibitor into the system is to remove the 1/2" BSP vent plug in a top floor radiator, use some kind of funnel or tundish and pour the inhibitor into that. Provided I have more than 1 radiator vented on the top floor to allow the air to escape and therefore allow the inhibitor to enter the system more quickly, I should be alright. I assume that, once the inhibitor is in all I need to do is to replace the vent plug, open the cross-connection valve ensuring the pressure doesn't rise above 1.5 Barg and then shut everything once water issues from the top floor radiators; a two man job then!
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Postby acsimpson » Tue Sep 29, 2009 8:13 am

You will probably find 15mm compression joint components screw into the space left by the vent plug, so if the vent does not point up you can screw an elbow in (pointing upwards) and you will have a much easier job.

Refilling doesn't need to be a 2 man job. Just make sure you close the bleed valves before begining the refill. Then once the pressure is up bleed the radiators. if a lot of bleeding is required then you'll need to refill and rebleed a couple of times.
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