boiler to hot water cylinder pipe size


Postby lmey » Mon Feb 13, 2012 11:49 am

Hi just moved in to house and i'm not getting any hot water, boilers working ok as heating is working the flow and return to cylinder is 15mm copper the flow into cylinder is hot but the return pipe is cold, a couple of times we have had hot water but this is after about 4-5 hours of boiler being on and some times boiler has been on all day and still no hot water, if i were to re run these pipes in 22mm would this work. Thanks
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Postby plumbbob » Mon Feb 13, 2012 12:37 pm

There are a variety of reasons why the water may be cold. It may be the coil is airlocked. Is there a bleed valve somewhere near?
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Postby lmey » Mon Feb 13, 2012 1:09 pm

Hi no bleed valve I've tried everything fitted new motorised valves pump is working fine had a plumber round and he said ever motorised valve which has now been replaced 15mm pipe flow and return or the cylinder but I've had a look inside the cylinder and it's not firred up so could it be the 15mm pipe as it has a small diameter the hot water has not got enough flow to get to the cylinder.
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Postby plumbbob » Mon Feb 13, 2012 5:15 pm

lmey wrote:......... so could it be the 15mm pipe as it has a small diameter the hot water has not got enough flow to get to the cylinder.


No. 15mm may be unusual, but quite adequate for the purpose. Generally the cylinder circuit is fitted with a gate valve anyway which acts as a lockshield to restrict the flow of water to the correct level. Is there one and is it open?

IF the return is cold then the restriction is in the coil and so you will see nothing amiss inside the cylinder.
If the MV can be discounted (can it; are the electrics ok?) then the coil is restricted in some way. It might be physically blocked but trapped air can be difficult to shift and it is not an uncommon problem.

With the hot water only on, listen to the cylinder carefully. Can you hear water trickling sound?
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Postby bobtb007 » Tue Feb 28, 2012 4:39 pm

Hi Imey

I agree with Plumbob in that it may be an airlock in the cylinder coil and yes the size may be a bit small but if you have a fully pumped system (two pipes from the boiler) then the pump will overcome this resistance.

However if you have a four pipe system, two of the pipes will be to feed the cylinder and will be "gravity" fed, this then will make it more likely for the coil to be airlocked, however you say that there is a motorised valve is that two port or three port, on a gravity system it would be two port (i.e. on/off control from the cylinder stat) and then the two system will be hydraulically separate (but still using the same water but hot water primaries will be gravity and heating system pumped)

If you have the necessary expertise and know what to do in the event of a subsiquent leak (that is only something you know) I, as a professional plumber would loosen the top pipe primary pipe to the cylinder and see if there is a release of air.

If the union weeps straight away, then no air is present and the prob lies elsewhere, however if air escapes first then you have found and resolved the issue, let the air escape and for the union to weep then nip up the connection again and test for flow.

The return pipe should be slightly less hot than the flow pipe.

Also depending on your system you may have a "fixed bypass" (or what used to be called a summer valve). This bypass will have a gate valve in it and if the valve is fully open then all the water may be going round the bypass rather that through the coil, it all depends on how the pipework is configured.

It depends a lot on what system you have

Hope this helps

Russ
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Postby plumbbob » Tue Feb 28, 2012 5:40 pm

bobtb007 wrote: ......I, as a professional plumber would loosen the top pipe primary pipe to the cylinder and see if there is a release of air.


So would I, but as I do it, you'll always hear me mumbling words such as "Our Father who art in heaven......"
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Postby bobtb007 » Tue Feb 28, 2012 5:48 pm

LOL Bob

Them damn cylinder unions, once disturbed never to seal again............

I dont understand why the unions use bsp threads and not taper or even an olive

However I now use what my supplier calls a cap and something, havn't doe one for a year now so I forget teh technical terms lol

Russ
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Postby lmey » Fri Mar 02, 2012 10:48 am

Hi thanks for all your help had to renew the cylinder in the end wasn't an air lock the coil was blocked with something lime scale or something else i dont know but its all working find know nice to have some hot water.
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