Lounge Radiator that only works when bedroom radiator switched on.

Hi all,
Here I have an issue with my central heating and the way water flows through and returns using 10mm small bore pipe work.
Now I realise that what I have is pretty old, and probably pretty sludged up and needs replacing, but despite all this before I replace I need to know whats going on with my downstairs main radiator.
The lounge radiator downstairs is the only radiator downstairs, it’s a double panelled 2000mmx400mm new convector type rad (so pretty big).
The radiator takes an age to completely warm up (a good couple of hours), and the return pipe never really gets hotter then say ‘warmish’.
This would indicate to me that the return pipe at the very least is either blocked with sludge or air, but I assume something must be getting through otherwise the rad would not get hot at all.
Now here comes the killer, this lounge radiator will only work if the radiator in my bedroom is fully on, with both lock shields fully open.
This is also the furthest rad from the lounge rad, being upstairs and at the back, its at literally the opposite end of the house.
This does not effect any of the two other radiators in the house, those being small rads in the bathroom and back bedroom which seem to work totally independently when you switch them on or off, and have no bearing on the rad in my room, nor the rad in the lounge.
Sadly I also have chipboard floorboards, making access a real pain. I have one access hole in my bedroom, and all I have gleamed from this is that the feed is a 22ml pipe that then branches off into the small bore, and the return pipe is 15 ml.
These pipes go off in the general direction of the bathroom and back bedroom, and its under the back bedroom (directly below) that the famous lounge radiator sits.
Additionally the flow return on microbore pipework on the problematic radiator goes at ground level along the floor the length of the radiator (2 meters), inbeds itself in the wall and goes directly upwards towards the back bedroom (at least another 2 meters) and from there I assume it goes into a manifold somewhere, but I doubt if its less then five meters away , and I understand that can also be an issue.
So, the answer it would seem would be to somehow chop away at the chipboard floorboards in the back bedroom, find where the manifold connects it all up and replace microbore with 15mm pipework, which I will get a pro to do.
My fear being that even having done all of that, that rad will still only work with the rad in my bedroom being full on.
Currently when I go to bed in the evening, my bedroom is like a sauna, and after switching it all off I have to open the window to let cool air in before I can get to sleep! Not ideal.
I have tried balancing this radiator , closing both lockshields down to a minimum etc, but still lounge radiator will not work unless both are fully open. Could it be some sort of syphoning effect or something?
Any ideas guys? Am wandering if I need a scientist rather then a heating engineer!
BTW, it’s a Combi sealed system.

I thank you all for taking the time to read my rather too long missive, and i look forward to any feedback, good or bad.
thanks.

Chances are its plumbed in wrong. Rather than hooking into existing pipework in parallel, they have plumbed it in series with the upstairs rad, there for the water must pass through the upstairs rad before it then flows down to the lounge one.

If it was correctly fitted in parallel and the microbore was fed back to the manifold/primary pipes then it would probably heat just fine.

If you turn the lounge rad off all together, does the bedroom one go off aswell?

Thank you ScottishgasMan,
kind of you reply.
i did consider it was maybe plumbed incorrectly, but my thinking on this was that as the other two rads (bathroom and small bedroom), run as they should then this might not be the case.

and in answer to your question , if i turn the lounge rad off altogether, then no, the rad in my sauna bedroom doesnt switch off.
However just to conform this, I will try this now.

regards,
Bertie.

If they are plumbed in series (wrongly) then turning any valve on either two rads off would cause both to stop heating and start cooling.

Thanks again,
In that case it cannot be plumbed in series then, switched off both sides of lounge radiator, and my bedroom stayed piping hot, didnt drop the temperature at all in nearly an hour.
Shame! Like i say, think i need a scientist....

Is it possible that when the bedroom radiator is switched off, the heat demand becomes too small for the boiler to handle (because the rate of flow through the lounge radiator is too low due to blockage or faulty valve) so the burner shuts down? What happens if you switch your "sauna" radiator fully off but have the other two small upstairs radiators fully on? Does the lounge radiator warm up any better than with all other radiators off?

Henry,
No it doesn't.
I can see and feel the pipe work under my bedroom floor, the feed is 22ml and red hot, there is a return pipe (I assume) that is 15ml which remains stone cold until the bedroom is switched on, then it gets hot then the lounge radiator warms up.
I am wandering now that if the flow pipe to the lounge rad is badly blocked with only a trickle going through it, is it possible that the flow and return pipes could have reversed roles? In other words it's being warmed up via the return pipe instead of the flow. Water under pressure will find the course of least resistance won't it, so the above theory is a possibility?
What do you think?
I think I need to start cutting holes in my chipboard bloody flooring.

Mini and microbore are more prone to blockages which is why I don't use it! Before you start cutting up floors, I would dose the pipework up with a good cleaner and let it circulate for a day or two. Drain down and eyeball colour and consistency. If necessary, dose again. See if this helps any before getting the crowbar out

Have a look at .. Armeg solid board cutter . Worth the investment if you use it then put it on eBay.

Do the Bucket Test on both pipes connected to the lounge rad.

1. With bedroom rad turned off;
2. With bedroom rad turned on.

This will tell you if either pipe is blocked and if they are connected to the bedroom rad.

Thanks to Dilalio, Nige and Mr Hailsham for taking the time to reply and try to help.
Looks like its the bucket test, followed by maybe some Sentinel 800, and if all else fails a floor cutter!

Quick question as regards the Bucket test, just to clarify, I remove the Lounge radiator (with both valves closed and still in situ on the pipework). Then i do the bucket test by taking it in turns to open up each valve.
is it an easy job to drain a radiator withou a drain tap? I mean it must hold quite a lot of water i would have thought, whats the best way of getting it all out without sending Mrs Bertie into an apoplectic rage when i drip stuff all over her lovely carperts?
To be fair she seems in a permanent state of rage anyway these last few weeks, so doubt its gonna make much difference.

When draining a rad, I close both valves and use a couple of heavy duty, plastic rubble bags - with no holes (apart from one at the top) - wrapped around one of the pipe tails. Undo the union/nut between that "closed" valve and the rad and pull away slightly to start filling the bag. When bag gets about a third full, tighten Union/nut and empty bag into a bucket then repeat until empty. Loosen air bleed valve to release vacuum to get all water out. Some plastic sheet and a couple of old towels should prevent any black **** escaping onto carpet at the end! If it's a large rad you'll need a helper. When finally empty, close air bleed valve and flip rad off wall, upside down onto dust sheet/towels. Also be worth taking it outside and hosing it through, but watch out for the black ****... It stains everything!

Sounds like you may have a case of reverse circulation going on.