No hot water from shower

[Jacqui1978]

Hello, this is my first go at this so please be kind!!
I have a Worcester Bosch 240 boiler for central heating and water. I don't know how old it is as I only moved in 3 months ago. My shower is connected to my bath taps an is activated by a plunger type switch in the middle of the taps.
In the mornings I can get hot water from the taps no problem but when I activate the shower it runs hot for a couple of mins then goes stone cold.
My hot water is on constant heat but the demand light on the boiler is not always lit.
I can get hot water from both the kitchen and bathroom sinks as well and the central heating is in no way affected. I have also tried different heat settings on the boiler but this hasnt made a difference. Do you think this could be a boiler problem?
Oh! I have also changed the shower head but again no joy on the hot water front!!
Thanks for your help

 

[Nixt]

If heating is off the demand light will only glow if a tap is run. If you have hot water to other taps every time, the boiler is not faulty as it can't tell the difference. The first thing to check is that both hot and cold supply are mains fed (thus giving equal pressures). Then check if your shower/bath mixer has a thermostat built in. Also try if it works with the shower head off ie. just thru shower hose.

 

[gas4you]

As nixt says, see if your shower head is full of limescale, if so put in cup of vinegar to clean

 

[Jacqui1978]

Thank you so much for your help! Ill try all suggestions out and let you know!

 

[Jacqui1978]

Hello!
Got an engineer round in the end to have a look at the Boiler and the Diaphram had gone on it. All fixed now and have got piping hot water! Hoorah!

Thanks for all your help guys