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Discuss Gravity fed hot & cold taps - pressure the same?? in the Plumbing Jobs | The Job-board area at PlumbersForums.net

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Hi everyone,

My first post here! :)....

Let me start by saying I am not a registered plumber, but I am an experienced DIY enthusiast, with many successful plumbing projects behind me and keen to learn.

I have a fairly basic question - hoping you can help; I am currently doing work at a flat where both hot and cold taps are on a gravity fed supply, coming from a small cistern mounted as high as possible (the outlet from the cistern is approx. head height). The cistern also supplies a vented indirect hot cylinder, heated by a has Central Heating loop. So - fairly conventional for an older style property.

OK - so my question!; am I right in thinking that the water flow (pressure) at the hot and cold taps should be approximately the same?? Reason for asking is that the flow at ALL of the hot taps is actually extremely low; significantly lower than the cold taps, which have good flow. I would expect a slight loss of pressure for the hot as it goes through the cylinder & fittings, but I think the flow at the tap will be mainly governed by the 'head' between the cold cistern and the tap, which is approximately the same for both hot and cold.

My suspicion is that there is a restriction due to scale/debris in either the inlet or outlet fittings to/from the hot cylinder. I would really appreciate the experience of members on this point - and anything else I could check before taking the plunge and inspecting/changing the cylinder and/or fittings. It's a really tight installation with very little room to move around - and I can tell it's going to be quite a job!

Other relevant info; I estimate the age of the cylinder at about 10 years.

Many thanks in advance.
 
Are all the hot water taps effected? The usual cause is a build up of scale inside the cylinder blocking the cold inlet supply thus reducing the hotwater flow outlet to the taps. Cheers kop
 
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Or the gate valve in the Cylinder cold feed, if there is one, is sheared part closed or part open , whichever way you want to look at it..
 
Hi kop, thanks very much for your reply..... yes; all hot taps are affected; it's only a small flat, but the flow in kitchen, bathroom sink and bath are all very low; circa 1 l/min. when tap fully open.

The cold inlet to the cylinder is quite difficult to get at, but I may be able to isolate, drain the tank and remove the inlet valve for inspection/replacement.... or if the scale build-up is bigger, then I guess I may have to actually replace the cylinder?
 
Thanks snowhead. I will take a good look at it tomorrow; as said; very difficult to actually see the cold inlet to the cylinder because it is at the back of the tank, but I'm sure there is a valve there. Will I be able to do this without draining the central heating loop that feeds the indirect coil inside the tank? Want to avoid that if at all possible!:)
 
If it is indirect yes you can leave the heating alone.

As an addition to the above, if the pipe runs are bad (have high points), there could be an air lock. The pipe off the top of the Cylinder should rise to a vent which terminates over the cold water storage tank and everything else should really fall and then rise to outlets. As long as it can naturally vent, it will be one of the above more than likely. My guess would be to start with the gate valve. If the Gate valve is situated at a height above the top of the Cylinder you will only need to drain the Cold Water storage tank and the pipe that leads to the gate valve.
 
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Thanks Lastplumber - much appreciated.

I looked at the system today and, in the end, did not drain the hot cylinder at all..... When I inspected the cylinder installation closely, I noticed that the valves to and from the hot cylinder have been replaced fairly recently - with new 22mm id ball valves, along with 22mm plastic piping. Both valves open & close really smoothly. The tank also looks quite new. More in hope than anything else, I decided not to tackle any potential scale problem on this visit.

Not mentioned in my first post, but the original reason I am looking at the water system is that the shower pump, which boosts both hot & cold supplies to the bathroom taps, had completely failed recently. I removed the pump yesterday - it's a Stuart Turner Showermate twin 1.4 bar - and discovered that the impeller for the hot side had worn at the hub - meaning it was not being driven by the shaft at all! Unfortunately this model is now obsolete and so it seems are the impellers. So - I purchased a replacement pump today similar but newer model (1.5bar). I noticed the installer who had put the original pump in had used only 15mm pipe to & from the pump (ST recommend 22mm) - and they had also installed gate valves to isolate the pump (which will be introducing undesirable pressure drop upstream of the pump). So - I replaced the 15mm piping with 22mm and the gate valves with new ball valves. On purging the hot side before connecting the pump, I noticed a bit of spurting - probably trapped air - and then the flow significantly increased. With everything reassembled, the new shower pump is working OK, so that is my number 1 priority sorted. Flow from the hot taps is still a bit lower than it should be, but I am going to monitor for now... Hopefully all will be OK until summer, when I can completely drain both the tank and CH circuit to have a proper look at the cylinder.

Thanks again all for your advice, which certainly helped tackle the work I needed to do today.
 
There's probably still air in the hot side but it should even it's self out also I'd use full bore lever valves rather than ball valves
 
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