Good afternoon, my name is ***** ***** I will assist with your query today.
Hi Olga, Do you happen to know the model number of your Aqalisa shower, since the majority do not have thermostats to control water temperature but work by a flow control valve.
The two most common reasons for your issue are either a faulty flow control valve (which controls how much water gets diverted to the heater) or boiler\element (which heats up the hot water).
This can only really be diagnosed by a plumber, as you need to take the shower apart, and test the boiler and valve under load\pressure.
If you provide me with your postcode I can recommend a local plumber to come and look at repairing your shower for you, the parts are likely to be in the region of 30-50 depending on the model of shower you have.
Right so its not an electric shower, its just a mixer which takes two feeds (hot and cold).
This will be a most certainly be the thermostatic valve in the mixer tap itself.
Give me a while to pull up the spec of the mixer tap in the shower.
Looks like you need part number 11 on the diagram here which is about £90.
The video here shows what is required.
You may get away with just servicing it but giving it a good clean in hot soapy water, but it will fail within the next 6 months, so depends if you want to save yourself a visit in the near future.
How did you get on today?