In my opinion,
Renovating an existing system should always be T&M. Provide a good faith estimate price to do the such work when projects are large. If you are good you keep it to the price you provided.
Installations always are bids set price.
Time and Material billing is generally the best. You should know what you are doing though as far as diagnoses , repairs and recommendations. Don't waste your time and there money. The goal is to be as efficient as possible. Other's do it differently.