Determining NFPA 25 20 yr sprinkler head sample

[mjoyner]

I have a client that has requested we do 20 year sprinkler head testing in a building that was built approximately 20 years ago. The issue i am running into is this is a multistory office building that has been renovated many times in the 20 years and during each renovation new sprinkler heads were installed. My question is what determines my testing sample? Would I need to limit my sample to any heads i can find that have the original building finish date that are at least 20 years old? Also, what happens when the heads that were installed during renovations come to be 20 years old? theoretically they could be doing 20 year head testing every five or so years. Has anyone experienced this?

thanks,
M