Shower problem


I have an en suite and main over the bath shower. The en suite is a mira 88 mixer shower that runs from a hot water tank on the first floor into the loft and down the wall. The cold is fed from a header tank in the loft above the tank airing cupboard. I have replaced the washers (which comes as a kit) in the mira 88 a number of times and also changed the shower head and hose which is fitted on a riser rail. The pipe runs are about 4m for the hot from the tank and 3m for the cold. The mira is about 16 years old and has worked well although not up to a power shower pressure. Problem is that I have to turn the mira control right to the left when in the shower to get any warmth, however if I take the shower head off the rail and dangle down below the unit the pressure increases and very hot water comes out. I then put the shower back on the riser rail and the water gradually starts to cool again. The main bathroom shower appears ok. Do we think the mira is faulty although would there be a suspicion that the main tank is badly scaled up. Alternatively would there be an air lock in the system to cause the problem? I have been badgered by my sons to get a power shower installed on both showers. I understand that this would have to be fitted on the floor of the airing cupboard next to the tank for this to work. Any suggestions of a good make to run both showers. What do I fit at the shower end by way of a control? Basically is it straight forward to install?
Many thanks in anticipation of the groups help.
Simon
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Do the pipes that emerge from their respective storage tanks (H & C) make their way to the shower unit downhill all the way, or is there any part of the pipework which rises before it then falls again? If the latter, then you're sure to have air trapped in the rise, hindering the flow of water.
Rob Graham
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Thanks for your reply. The hot rises into the loft from the airing cupboard tank, lays on the floor of the loft and down into the shower. The cold tank in the loft is elevated by a foot and the shower cold comes out the bottom of that along the loft floor and down into the shower cubicle.
Regards
Simon

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Then it's pretty certain you are getting air accumulating in the high parts of the pipe run. Cut the pipe at this point and insert a T-piece to accept a piece of vertical pipe (with a 180 degree bend at the top) that reaches above the water level in the feeder tank. This will let any air out. The 180 degree bend keeps trash out.
Rob
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which causes more air.
If you get a power shower the above problem because exaggerated and you have to keep the temperature lower than you are used to because the suction side of the pump effectively flashes the hot water off to steam, even at 60C water temperature. Mine has to be set about 55C otherwise this problem causes cavitation in the pump and loss of hot water pumping capacity.
If you fit a power shower it is best to take the outlets separately if you can from the respective tanks...... also for the hot water side from as near the hot water tank as you can otherwise again the pump inlet suction will suck the overflow down and suck in air again causeing cavitation. My power shower delivers 4 gallons a minute and can empty the hot water tank. You may find that this tank will need increasing in size to cope otherwise it will be a very swift shower. Mine lasts 5 minutes before it starts to go cold!
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