Shower running luke warm until cold tap in sink turned on

[Bonzo44]

In the last 2 weeks my thermostatic mixing shower has been running cooler than normal. I spoke to someone in the pub who suggested that I could tell if it was a boiler problem or a mixing problem by turning on the cold water tap in the sink and seeing if the water in the shower got warmer. As it did get warmer that suggests that it's a boiler/pressure problem.

I have a Vaillant ecoMax 828/2E condensing boiler which is 7 years old and had a new divertor valve fitted a year ago, although that's probably irrelevant.

The tempreature of the water in the kitchen and bathroom sinks seems okay. Can anyone suggest what the problem might be and how much it would be too repair.

 

[footprints]

If the temp at the other taps seems ok I would suggest a problem with the shower thermostat. It may be that turning on the cold elsewhere simply "robs" the shower of cold water so it feel hotter. Like somebody running a cold tap while you are in the shower and giving you a "hot flush"

 

[Bonzo44]

Thank you for your response, and changing the shower mixer valve has done the trick.