New mixer shower- 1/2 or 3/4 inch inlet/outlet?

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I'm getting my downstairs bathroom done. Replacing the mixer shower. The system is a gravity system, cold water tank in the loft, boiler and hot water tank in airing cupboard on 1st floor and the shower is on the ground floor. The current 10 year old mixer works fine pressure wise and is a 15mm inlet on the instructions. Shopping around for showers it seems some come with a 1/2 inch inlet/outlet and some with a 3/4 inch one. Does it matter which one I get? Do I need to match them to my current pipes? If so the picture below is taken from the extension loft space directly above the shower. Some are big some are small! Any clues what pipes I've got or do I need to wait till my fitter has ripped out the old bathroom. My Mrs already moans about the shower not being powerful enough to wash her long flowing locks so want to max out what pressure we do have!!

pipes.jpg
 
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Best to wait until you have the pipework exposed in your bathroom Ben, that way you can be sure of the pipe sizes. Unfortunately, being a gravity system you are probably are at the max with regards to pressure and flow though that will be apparent when you fit the new shower.

Only way to really increase that easily is to pump it.
 
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Ok great thanks for your advice. Is that fairly standard to wait till they start before buying the shower etc? If I have 3/4 inch pipes and I go for a 1/2 inch shower am I going to be losing out on available pressure? And vice versa would a 3/4 shower not work as well as with 1/2 inch pipes?
 
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In your picture, the two 22mm pipes on the right hand side are labelled "SH" and "SC". Shower hot and shower cold, perhaps? Each has an isolator, so try turning those off to confirm that they do in fact supply your shower.
 
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There are some specific showers designed for Gravity.
Most now will be designed to work on mains pressure
 
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Oh wow good spot!! Perhaps should have looked at the picture before asking you guys!! If these are indeed my shower pipes would this point to getting a 3/4 inlet/outlet shower and therefore maximising my shower's pressure?
 
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Certainly, if the shower is supplied by 22mm, which is ideal to maintain flow in a gravity system, then it will be a 3/4" bsp inlet shower you'll be looking for.

As gas mentions tho, make sure you are buying a shower designed for a low pressure system.
 
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Sorry Ben, that's a medium pressure mixer, you have a low pressure system.

I thought you said that the current shower has 15mm inlets? If so they that 22mm reduces down to 15mm somewhere so that's the size of shower inlet you will need. I'd wait and see what the inlet pipework is when the old shower's removed.
 
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Cheers. The Crosswater website implies a tank in the loft will provide 'MP' to a shower located on the ground floor. Am I possibly pushing it a bit and will be better off with a low pressure shower? Am I being silly with all this? Will having 22mm inlet/outlet on the mixer really help that much vs just having it as I do now with 22mm pipe work and then switching to a 15mm inlet/outlet just before the mixer? I must stress I have no idea what I'm talking about I just don't want to waste performance and hence incur the wrath of my Mrs!

http://www.crosswater.co.uk/water-pressure-guide/
 
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Phill Peck

On low pressure to a shower if it's 15 or 22 u will still want to get a low pressure shower. That is if u intend to get wet
 
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Ok cool. I think I'm just being clueless and confusing myself having seen the 22mm inlet/outlet on the crosswater ones and getting it in my head that I need to maintain the 22mm all the way through. I presume most inlets/outlets are 15mm these days for a reason so shower will be fine switching to 15mm late in the day. Correct?
 
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Phill Peck

All bar showers will have 3/4 unions but generally fed by 15mm pipe it's just how the fittings work for rear pipe work
 
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And usually pressures for water outlets are defined as -

LP - 0 to 0.5 bar
MP - 0.5 to 1 bar
HP - 1 bar to 2 bar

Some may not say but will give a min bar reading required to work properly. Pressure via gravity is roughly calculated as the distance in M from the cold tank to the head of the shower div by 10. There is loss due to bends and lengths of pipe but if they're minimal then it's wouldn't really affect you.

e.g. 5m/10 = 0.5 bar
 
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