Trouble draining heating system through drain valve

10 Jun 2014
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United Kingdom
I am trying to drain my hot water system so I can fix a couple of leaks and replace some radiator valves.

The only drain valve is right next to the boiler (lowest point as is in basement) so I turned off the water to the header tank, opened the valve and loosened the radiator bleed valves upstairs. Water came out but even after 24 hours the downstairs radiators are not drained. Why could that be?

I noticed the drain valve is in the flow pipe out of the boiler, not the return back to it. Is that normal and could it be the problem?
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Closed motorised valves and even the pump itself can slow down draining.
If there is a mid position valve or similar, latch it manually into the open position.

Does the pipework from the downstairs radiators head straight under the floor or is there possibly a rise in level which would trap the water in the radiators? I would try opening their bleed valves to see if air goes in or water comes out.
The radiator pipes don't go straight down, but I can't see any rise.

I've tried opening the bleed valves in the downstairs radiators but water comes out. Upstairs is ok.

Does it matter that the drain is on the flow out of the boiler and not the return?
It wouldn't normally matter because water can also exit via the return pipework, through the boiler and out of the drain.

Provided there are no extra valves. motorised or otherwise, interfering with the flow of water.
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The downstairs radiators must be on a separate drop. In other words the pipes must go upstairs from the boiler and then drop down somewhere else to feed the downstairs radiator circuit. Obviously the water won't go back up and come down to the drain off point by the boiler. Are there any drain off points by the radiators that won't drain. If not you can buy self-cutting drain offs that clamp around one of the pipes feeding the radiators and allow you to attach a hose and drain.
Thanks all! Yes, on reflection I think the radiators upstairs are fed first and then it loops down.

I searched again under the house and found another drain off valve. It was well hidden behind pipe insulation, but once I opened that one the system drained itself in no time at all!

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