After five years I have problem with my shower heater. It is either to hot or to cold. One used to be able to find the comfortable temperature by adjusting the water flow about in the middle position with the shower faucet. Now the heater turns off when it is the position. Either very hot or very cold. Any ideas on this one ? Thanks

Asuming that this is a standard type of shower water heater:-

 

IME the water temperature is adjusted by using the dials on the shower heater, not by adjusting the faucet (which should be fully opened).

 

The heater itself should adjust the flow in order to maintain the correct water temperature.

 

It is possible that the thermostat has failed and this could be replaced, but I would start by cleaning all the filters (in the faucet and in the shower heater) then open the faucet fully and adjust the temperature using the dials on the heater.

 

A photo of the heater, or brand/model might help others to give more specific advice.

Bunged up mesh inlet filter? Always worth a look.

 

14 hours ago, Capealava said:

After five years I have problem with my shower heater. It is either to hot or to cold. One used to be able to find the comfortable temperature by adjusting the water flow about in the middle position with the shower faucet. Now the heater turns off when it is the position. Either very hot or very cold. Any ideas on this one ? Thanks

If none of the other fixes work then a new water heater with better controls.

The hot water is set to medium. But I adjust the water temperature with shower faucet. Turning the faucet all the way to the right very hot water. Turning it back to left adds cold water to adjust the water temperature. Before I turned the faucet a bit right from middle and I would find my comfort temperature. No I turn it back to the left and it gets cooler but still to hot and I turn it more and the shower heater goes off and the water is cold. There is no middle ground. It is either to hot or cold. It is a 7000 watt British REDWING heater. enclosed it picture/

The heater needs flow to operate; switch it to low and don’t use the faucet for temperature adjustment. 

2 hours ago, Capealava said:

The hot water is set to medium. But I adjust the water temperature with shower faucet. Turning the faucet all the way to the right very hot water. Turning it back to left adds cold water to adjust the water temperature. Before I turned the faucet a bit right from middle and I would find my comfort temperature. No I turn it back to the left and it gets cooler but still to hot and I turn it more and the shower heater goes off and the water is cold. There is no middle ground. It is either to hot or cold. It is a 7000 watt British REDWING heater. enclosed it picture/

Step 1 change the setting on the heater from medium to low.

 

Then report if that fixes your problem 

That looks like a multi-point water heater to me - not a dedicated shower heater.

I don't want to give advice as I'm not exactly sure about this type of heater, except that they are normally installed under a sink and not in a location exposed to water.

It does have an Earth Leakage Breaker test button, so I guess it's safe enough.

 

9 hours ago, Capealava said:

The hot water is set to medium. But I adjust the water temperature with shower faucet. Turning the faucet all the way to the right very hot water. Turning it back to left adds cold water to adjust the water temperature. Before I turned the faucet a bit right from middle and I would find my comfort temperature. No I turn it back to the left and it gets cooler but still to hot and I turn it more and the shower heater goes off and the water is cold. There is no middle ground. It is either to hot or cold. It is a 7000 watt British REDWING heater. enclosed it picture/

 

Yeah, that's a multipoint heater, we have that exact same unit. There is no variable heat control other than the fixed power settings (try Low) and no thermostat other than the overheat protection.

 

They are usually connected via a mixer tap on the outlet, control the temperature by mixing in cool water. To get cooler water if you don't have a mixer tap you need more flow.

 

EDIT Reading your last post again. Do you actually have a mixer? The problem may be there, something bunged up.

 

I lowered the temperature to LOW and it seemed to fix the problem. Hopefully the problem will not come back. It is working fine now. Yes it supplies hot water to both  a sink and shower so it is not a dedicated shower heater.