We used to use tsp in solution to clean hot water heating boilers.
There is a followup treatment also to prepare the metal surfaces for corrosion resistance. This is rubber glove stuff, and the used solution should be properly disposed. Can you put the head on with an old gasket so you circulate the solution all the way through? The nozzles in the head are another area to look over very carefully.
When all this is over, will you use tap water to add new mineral deposits? No free lunch, that's for sure.
