Be sure to specify at the contract stage what will happen in the event that repairs are needed to the work, such as a faulty part or improper fitting - we cover the importance of contracts here.

It would be reasonable to expect a plumber to fix a problem in the first instance. This could be down to an error during installation or a faulty fitting. However, persistent failure of a particular fitting would probably warrant contacting the manufacturer to resolve the issue. They may provide a free replacement but you could incur the cost of fitting it.