Shower Bar strange problem.

[lancs_lad]

My son has a thermostatic shower bar fitted above his bath which has separate taps. The shower bar only gives him warm water unless he turns the cold tap running in his bath which then causes the shower to heat up further.
Water is heated by a combi boiler.
Hot taps themselves in the house are fine.
Any ideas would be appreciated

 

[Madrab]

Has it always done this? Is it a new house?

 

[Elkato531]

It could be that the flow through the boiler is too great to heat the water to the shower adequately. Opening the cold tap will reduce the supply to the boiler allowing it to transfer more heat to the shower.
At this time of year, incoming mains water is very cold and the combi can only put a set amount of energy into the water.
A 24kw boiler with a flow rate of 11.5 litres per minute can only raise the water temp by approx 30 degrees.

 

[lancs_lad]

It's not a new house, but it's only the bath cold tap that does this. Turning the sink cold tap doesn't affect it.
Possibly only had the issue since the combi was fitted though.