I have a shower unit running from my bath taps the cold water pressure is high so i cannot get the right temp. is there a thermostatic .control valve i can fit to the shower pipe outlet

Your supply pressure is too high for the unit, it is too much for the solenoid to shut off, you need to put a pressure limiting valve inline.

If you are using a conventional shower valve of recent manufacture, there is a safety feature to prevent scalding accidents when it is connected to conventional plumbing on a pressurized system. If there is no cold water pressure against the valve, it shuts off the hot water completely. It is called a pressure balancing valve by most manufacturers.

In other words- it is supposed to do that.

There are ways to bypass the valve, but you may want to keep that feature if your storage temp is capable of temps above 110 degrees F or 49 degrees C.

If the water is only cool in the shower, it could be the shower valve. However if water is cool at all taps, something is not working on the water heater system.
Gary

Moen and all shower valve manufacturers follow american federal guidelines. Hold the unit in your hand with the handle facing you - the left side is the hot water inlet and the right side is the cold water inlet.

Probably some pipe rust clogged the exit.
If u have pretty low pressure on one or both, hot and cold water, try keep force closed by hand the water exit at shower pipe, then start both cold and warm water gradually as much as u can keep it closed with the hand.
The pressure of the water will try "escape thru the clogged pipe and hopefully unlock that debris or rust!
The hot water pressure going back thru the cold water pipe might unclog that debris.
Just a simple fix which many times works as a charm..
Otherwise if this is not fixing, try close both water from main service valve.
Then unscrew the secondary shower valves and manually clean them or replace if very damaged.

you need to change the cartridge, it is a temp control and is going bad so it keeps you from burning yourself it is like a mixing valve and is givng more cold water, that way you don;t get burned,,

Sounds like it was installed upside down or the cartridge is installed improperly

The Moen Posi-Temp Valve is a pressure balanced valve and when not used for long periods of time they can stick causing only cold or hot water to pass through the valve.
A New Posi-temp cartridges can be purchased at most plumbing supply houses or big box outlets such as Lowes or Home Depot.
As long as you had cold water before, a new cartridge will solve the problem. In the future it would be wise to occassionally operate the valve at least a couple of times a month in order to keep the pressure assembly from becoming lodged in position. Just follow the instructions included with the new cartridge on the proper removal of the old cartridge and the installation of the new cartridge. a little silicone grease inside of the valve before installing the new cartridge will also help.
Be sure that there is no particles of the old cartridge left in the valve.
sanray

mixing valve must be replaced