Sealed CH drain-down puzzler

[fiveminutejob]

I want to replace a downstairs radiator in hall, needing small adjustments to pipes at each end for new one - so valves will have to come off. (Rads have been extended at different times ; now connected to a newish Vaillant Thermocompact sealed system boiler hanging on integral garage wall). Most rads up and downstairs have TRVs. Have shut off valves, TRVs and lockshield nuts to all rads, then drained down from drain point at base of kitchen rad,leaving drain hose open to outside. Boiler fill loop is set to off. Pressure now zero.

Problem is that hall rad keeps refilling and overflowing clean water from its opened air bleed vent - and I can't figure out a) where it's coming from and b)why it's not finding its way out first via drain hose which seems lower. If I open up hall rad TRV I get about a cupful every 10 mins - for hours!. Have checked all other rads are shut off both ends. Could this just be TRV seepage from other rads? Would decorators caps on all TRVs help? Faulty refill loop? Any thoughts welcome. Thanks

[Bamber gaspipe]

You have drained down the kitchen radiator & the flow & return legs to it.It won`t make much difference to your hall radiator. You need to empty that radiator & the legs to it...Do what you have done to the kitchen radiator, but do it in the hall..

[fiveminutejob]

Thanks Bamber. I have tried to do exactly that - isolating hall rad and draining it but it keeps topping itself up. Only kitchen rad has a drain cock. I thought that would drain down all pipework, just leaving water in shut off rads. That's why I'm puzzled.

[ChrisR]

Have you got a normal HW cylinder? If the coil in it is letting-by, water comes out from the domestic system into the heating system, and it would go down whichever pipe were closest. Half a cup in 10 mins sounds about right for that.
In this weather the trv should shut though!

Hard to prove a leaky coil; turn off the mains input and don't use any tank water for as long as poss and see if the level drops - by seeing if water inlet flows again when you turn the water back on. The level won't appear to drop much.

Only one solution if that IS it - new hw cylinder.

[fiveminutejob]

Thanks Chris. Yes there is a 'normal' hw cylinder in cupboard on landing with a single tank in loft above it. If water was coming from cylinder, I would have expected it to show at drain hose outside. But when hose is dry, hall rad is still topping up. I'm wondering now if the pipework to the hall rad (an add-on by previous occupier) may be the problem, with no route to lowest drain point. All thoughts appreciated. Thanks.

[croydoncorgi]

If you've left all the other rads filled and are relying on all the valves holding 100% and / or parts of the system becoming airlocked, at some point you have to admit defeat, open the drainoff and all the valves and airvents on the rads and drain the system completely. There usually only 10% chance that a system will airlock sufficiently to do work including removed valves with any confidence. And a slow let-by somewhere often creates too much volume to work easily without making a mess. Grasp the nettle - drain the beast.
And don't forget to add inhibitor when you refill.

[fiveminutejob]

Thanks very much cc. Armed with all your helpful responses, I guess a full drain down is the next step.

(This is a great site - so much knowledge and experience willingly shared. Thanks to all.)