boiler switches off hot water when turning tap to shower

[redhead1]

hi the hot water works fine throughout the house upstairs and downstairs but when i turn my bath mixer tap to shower the hot water turns off and the boiler does too.... any ideas how to remedy this???

 

[denso13]

Maybe not enough flow. Take the shower head off and see it works with just the shower hose connected.

 

[DP]

Let me fetch my crystal ball and see what make and model boiler you have.

 

[Agile]

Is this a thermostatic mixer tap?

Regardless, to keep the hot on you will need to reduce the cold water flow and/or increase the hot water flow.

The boiler if correct will need about 3 litres per minute before it turns on.

Have you measured the flow rate through the shower?

Have you measured the hot flow rate at which the boiler just turns on?

Tony