Negative head shower disaster - help needed!!!

Hi

I really hope someone can help me out here. The story is as follows:-

I have a 1st floor flat which has mains pressure water to all CW outlets and low pressure HW to all hot outets. I also have a direct combination cylinder measuring 1200mm * 450mm which has recently started to leak.

So I ordered a new one which arrived the other day, so I removed the old one and found on removal that it was something called an IMI shower master or similar. The idea of the cylinder is that is provides mains pressure to the shower by utilising a mains cold feed to the cylinder. I have no idea how it does this, but my plumbers merchant says he cannot get one, also a search of the internet this afternoon revealed nothing.

The new cylinder has 3 usable tappings, 1 * 22mm and 2 * 15mm. I am using the 22mm for all the DHW except the shower, and one of the 15mm outlets for the HW supply to the shower.

My problem is this, I have negative pressure at the shower head and since I have a thermostatic mixer control, if I try and run the shower not only do I get no hot water coming out but even worse I have cold mains water back filling the cylinder, causing water to pour out of the overflow and top of the combined header tank.

For the moment I have capped the HW supply to the shower to prevent any more flooding disasters but I do not know what to do with regards to getting my shower working properly again.

I realise that I can fit a non return valve to the HW supply and thus prevent the back flow, but I cant see how a single impeller pump would ever work as surely turning the shower on would not result in the low pressure water supply being able to overcome the mains back pressure, even if the shower head was held at a lower level. I have not been able to find a negative head single impeller pump and am now at a loss as to what to do.

I hope I have explained this all logically and that my understanding of the problem is correct. Any help would be much appreciated.

Thanks
Mike

So, what happened in the end?

The idea of the cylinder is that is provides mains pressure to the shower by utilising a mains cold feed to the cylinder.

Click to expand...

What you have described seems to me to be a thermal store.
Have a look at this http://www.plumbingpages.co.uk/featurepages/HWNpressure.cfm
and this :-
http://www.plumbingpages.co.uk/featurepages/HWdpsThermstore.cfm

I have never encountered an IMI shower master cylinder, maybe someone else in here has, but I suspect that it was more primitive than what you will find in the links above. I would hazard a guess and say that it consisted of a coil similar to those used in indirect cylinders fitted at high level to a normal direct or indirect cylinder. This coil would have mains cold running in to the coil and hot coming out warmed by the store of hot water in your cylinder.

I think you should find that a thermal store as per the links above should provide a solution. I recommend that you enlist the services of an experienced time served plumber who should be able to work out whats going on and reduce the risk of any more floods.

The best but more expensive answer is to get an unvented cylinder but this MUST be fitted by an installer who is qualified and registered as competent They should have an ID card to prove it. Only they can self certify the work for building reg approval.

I'm unable to figure out exactly what your supplier has given you as a replacement but it almost certainly is in breach of the water bye-laws