Isolating CH from HW in an old system?


Postby Jenny B. » Thu Jan 08, 2009 7:16 am

Hi. I have an old and now broken central heating (CH) and hot water (HW) system, which (I googled) is based around an 'open vented indirect hot water cylinder' (exactly like this [url]http://www.diydoctor.org.uk/project_images/domestic_hot_water_systems/Indirect%20cylinder.jpg[/url].

All the cold taps are mains supplied (not just the kitchen, unusually, I read). The gas boiler in the kitchen supplied HW and CH radiators, at least the latter via a pump. The first floor water cylinder (WC) has a backup immersion heater (thankfully). The loft has a water storage tank (WST) and a feed & expansion cistern (F&EC). There are no stop valves anywhere, that I can see :( .

Well, the boiler died of old age, and the Gas Board disconnected its gas supply. I couldn't afford a new one yet, and I'm content with alternative electric heating, and the immersion heater in the WC, for the time being.

But now the old boiler has started leaking water :( . I'd like to just isolate/stop the water flow in the CH part of the system, but it isn't as easy as I imagined. I thought if I turned off the mains stop valve, ran all the taps so that the two loft tanks emptied, tied up the ball-valve in the F&EC, then the F&EC wouldn't refill and the boiler wouldn't have any water to leak.

But when I turned the mains back on, and once the WST had filled from its own ball-valve, the F&EC gradually filled up from its bottom outlet pipe, seemingly ultimately from the WST, since its ball-valve re-opened after the F&EC starting filling and only stopped once the two tanks reached an equal level.

Where's that water coming from? I thought the HW and CH supplies were separate? Does having a pump mean the boiler has its own mains supply, separate from the F&EC? Or does the partial draining and refilling of the WST imply that the source of the water in the F&EC is from the WC? If so, how (isn't the heat exchanger in the WC just a sealed pipe)?

What am I missing here, please?

Is there any reasonably easy way to 'disconnect' the whole CH/boiler 'side' of the system?

Thanks for any help or guidance you can offer.
Jenny B.
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Postby plumbbob » Thu Jan 08, 2009 4:56 pm

The most logical explanation is you have a faulty DHW cylinder coil as in the topic -

http://www.diydoctor.org.uk/forums/view ... hp?p=43970

The best solution temporarily is to fully drain the heating system (which you can't do from the taps), cut the pipes near the boiler and fit two caps to blank the pipes off. If the feed pipes are 22mm you can get Speedfit push on end caps.

If you are in doubt about the size, they may be 22mm, 3/4", 1" or 28mm then you need to cut out a small section and take it with you to a plumber's merchants.
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Postby stoneyboy » Thu Jan 08, 2009 6:24 pm

Jenny B.,
Firstly I think you should get an award for explaining your problems so well. If many of the posts on this forum were as clear then answering would be a lot easier.
I agree with plumbbob's diagnosis that the heating coil in the water cylinder has failed. However, I would suggest that rather than isolating the boiler you isolate the coil in the WCyl. This will make sure there is no cross contamination between the heating and hot water systems. You will need to drain the heating system and leave it dry.
When you replace the boiler you can then decide whether you want/need the WCyl.
end
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Postby Jenny B. » Thu Jan 08, 2009 8:04 pm

Thanks plumbbob. With the mains off, the boiler now seems to have self-drained :( .

Is there any reason I can't cap the pipes near the DHW cylinder (pipes 3 and 4 in the picture I linked above) instead? It's just that the boiler pipework is much less accessible, and amongst a rats nest of other pipes.

Thanks for the note about pipe diameter too - that might have been very annoying!
Jenny B.
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Postby plumbbob » Thu Jan 08, 2009 11:22 pm

Yes they can be capped off there, but the problem of pipe size still applies.
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Postby Jenny B. » Sat Jan 10, 2009 7:36 pm

(Btw stoneyboy, my last message was posted before your's, but held in moderation - I wasn't ignoring you!)

They are 28mm pipes, which made things a little more awkward. But job done, and no more leaks.

Thanks plumbbob and stoneyboy :) .
Jenny B.
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