Cold Radiators in Flat Using Open Vented Back Boiler System


Postby Trev37 » Fri Dec 05, 2014 4:07 pm

Need help with a central heating problem (old, but reliable back boiler, open vented system in a flat - only 4 rads): HW is boiling. The 2 rads nearest boiler/pump are boiling. However, the 2 rads furthest from the boiler/pump rads take ages to warm up and then they only get slightly lukewarm.

I've tried a number of things to resolve it:
1) Turning off the 1st hot rad, made no difference.
2) Turning off both hot rads, made a marginal difference (i.e. 3 & 4 rad get very slightly warmer).
3) Took off one of the cold rads, no sludge at all. Also run off some water from the in valve of the rad, also totally clean.
4) Bled all rads - no trapped air at all.

Not sure if its:
1) An airlock, or some other blockage? The system can be quite noisy with a sort of intermittent slithery/gurgling sound (like a babbling brook ;).
2) Or the old 3 port Manual value (HotWater/CentralHeating) with a 'Simplicity' plastic front (a bit like the Rega Diverta valve). This is seized in what looks like the mid (both) position, can't shift it at all. Maybe it's really favouring the hot water?

Any help/suggestions greatly appreciated.
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Postby proptech » Fri Dec 05, 2014 6:33 pm

HI Trev37

If the valve is stuck, this must be the first suspect. The other thing that comes to mind is the possibility of a blockage in the area of the cold feed, indicated by the noises. Very difficult to say any more without info of the system. A photo of the pipework around the cylinder would be helpful.
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Postby Trev37 » Tue Dec 09, 2014 5:14 pm

Thanks proptech,

You're right of course, the stuck valve is the first thing to tackle/eliminate (although the systems been working for a few years now with a stuck valve).

[u]UPDATE in case some gets the same prob.[/u]
I took a closer at the manual valve, and managed to get the circular front of the plastic 'control panel' off by pull it towards me when its in the mid setting.

The spindle looked a bit rusty where it goes into the housing, so hit it with WD40. Put the plastic control back on (didn't want to try turning the spindle direct with a grip, worried I might damage it), and could move it a bit. Investigated a bit further, there was give in the plastic control panel (probably from people trying to budge it), and a plastic stop prevents it being turn too far. I doctored the stop a bit with a hacksaw.

Now I can turn the manual value to favour the CH. However 3&4 rads still don't get hot. But do if I turn off rads 1&2. An improvement. Although its more noise now, can hear the system 'tinkling', at the valve, and at the radiators.

Really considering draining down the system as by next option.
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Postby proptech » Tue Dec 09, 2014 7:00 pm

That photo would be handy. It does sound like a blockage though. Depending on the system design, I'd say in the location of the open vent.
Run a decent magnet over the area where the cold feed enters the system, You'll soon notice a blockage.
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