The is a thermostatic cartridge which is suitable for shower mixers which use the Bristan CART 23.51.HF cartridge which is used inside some Bristan Fairfax and Aqueous shower mixers.The cartridge is compatible in the following shower mixer valves but not restricted to: - Bristan Fairfax Thermostatic Shower With Fixed Head - Gold Shower - Bristan Fairfax Thermostatic Shower With Fixed Head - Chrome Shower - Bristan Aqueous Thermostatic Dual Control With Diverter - Chrome - Bristan Fairfax Thermostatic Shower Adjustable Riser - Chrome / GoldUnfortunately, over time, the performance of thermostatic cartridge tend to drop. This is because of a deterioration of the inner workings of the cartridge and the destructive force of limescale within it. Frequently asked questions regarding the Bristan 23.51.HF thermostatic cartridge follows:Question: There is a constant leaking / dripping of water out from behind the thermostatic handle. It's leaking about 5 litres per day, which isn't too much of an issue as it's landing on the shower tray, but it's a huge waste of water. Is it the thermostatic cartridge that is the problem?Answer: If the leak is originating from the thermostatic valve where the thermostatic cartridge is, then there is probably a problem with the thermostatic cartridge, so you are correct, replacing the cartridge will probably fix the leaking issue you have.Question: Over the past few month the temperature handle has become increasingly difficult to turn to the point now where when the handle is wet it's difficult to turn it.Answer: This is a common problem where limescale has begun to seize up the handle and thermostatic cartridge. It's a big problem in areas of hard water and therefore in those areas you're more likely to have to change the thermostatic cartridge more often. Replacing the cartridge should fix this.Question: I can't get the shower to output the correct temperature which is a bit dangerous for my kids. Sometimes it's scolding hot, then freezing cold. I spoke with a plumber and he has suggested it's the thermostatic cartridge at fault. What's your opinion?Answer: The first thing to check, which I assume your plumber has already checked, are the water supplies to your shower. Are you getting a constant hot and cold feed to your shower? If you are and you're still experiencing a problem, the problem could be the thermostatic cartridge. The reason we say could instead of definitely is because some thermostatic cartridges don't operate effectively when there is an unbalanced pressure between the hot and cold water feeds, therefore, even though more unlikely, this could potentially be an issue.