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The Mira mixer shower has variable temperate settings from 1 (cold) through 10 (hot), plus the extra bit for hot water only.

I normally shower at 3 and was thinking there must be a way to let more cold water through at setting 7 or so which would mean a far more powerful shower - since setting it to cold water only is very powerful as it's the hot water supply that's weaker.

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The water supply comes into your home with a fixed pressure from the utility and then is diverted to your hot water heater and cold supply. Assuming everything is in working order and properly, without a tank/pump, you can never exceed the utilities pressure.

If your cold water supply puts off a lot more pressure than your hot water, this likely indiciates another problem like a partially closed valve on your hot water tank, a blockage or inproperly sized pipes. Your shower mixer (assuming 1 handle) balances the pressure so that you won't get scalded if someone flushes the toilet which means it is unlikely you will be able to get different pressures out of the valve when mixing the two supplies.

I would suggest that you verify that all of the valves for your hot water tank are fully open. You might also try turning up the heat of your tank so that less hot water is required, but be careful when you do this as you can make the water dangerously hot.

Your shower valve might have some adjustments on it too.

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I think its pretty common for cold water to be at a higher pressure (or higher flow rate) than hot water as its connected to a combination boiler and so the hot water pressure isn't exactly powerful. But the hot water is hot and the mixer shower reduces the cold water pressure as you turn the heat up (understandably otherwise the water would be too cold) however I feel it could leave the cold water on full pressure for longer in the temperature range while simultaneously mixing in hot water too. – Marcus Jan 3 '12 at 22:35

It depends on how your house is plumbed. In the UK a lot of houses have gravity-fed hot water and mains-pressure cold water, although more and more are going with combi boilers which produce hot water from the mains-pressure cold water (so the hot is higher pressure although the flow rate might not be so great, since it is being heated up as you use it).

IF you have gravity fed hot water - you have a separate hot water tank that isn't pumped - then look into a special type of shower that uses the mains-pressure cold water to boost the hot water flow. An example of this is the Venturi Trevi Boost. I don't have any experience of these showers but am considering one for my own house.

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