You do not have that right. Unless your state has no construction codes whatsoever, there are plumbing rules to follow, and backflow prevention is a part of those rules. A dual check device in place near a water meter is meant to be insurance to protect the city water supply from dipsh!t homeowners, but it is not rated as protection from lawn sprinkler systems.
Consider the practical matter of someday wanting to sell your home. The structure will be inspected. Unsafe plumbing gets flagged, and WILL be corrected to code official satisfaction, with fees and maybe fines piling up in the process.
If your sprinkler system is so marginal in performance right now, and you're willing to expend time and money to improve it, spend the effort wisely and install a PVB (if you are on flat ground) or an RPZ, and make the system work with the altered supply performance. There is absolutely no good reason not to do it.
By the way, if you did pull out working Rainbird R-50 heads, you can probably sell them on ebay.
Point taken, and I never said I WOULDN'T install a backflow preventer. But that wasn't my question. I don't have questions on whether or not a BFP is necessary, I asked for thoughts on improving my current design.
You're right, the amount of work I am going to undertake is phenomenal and installing a BFP would be a fraction of that total work. As such, I will install one. But forgive me, it seems a little like you are refusing to offer advice until I do so. If that is the case, then I feel like you are acting pretty immature about this whole situation.
And let's not forget, if I am doing all that work at one time, why wouldn't I do the BFP at the same time? To refuse to help until I do it is being impractical. If someone installing a brand new system came to you and asked you for advice, would you turn him down at first and reply "Install a proper backflow prevention assembly and get back with us"? There are known loss values for these devices so I am having trouble understanding how the design hinges on empirical data the is gathered AFTER the BFP installation...
Also, if you could please be understanding, I am not the original owner of this house and so I would ask that you please don't chastise ME for SOMEONE ELSE'S mistakes. I will make it right when I do the project. For now, I am trying to define what exactly that project is. BFP is at the top of the list, okay? Can we talk sprinklers now?
Lastly, and in hopes I don't hijack my own thread, I don't know what exactly they are looking at in an inspection, but it must be pretty small because there are about 4,000 things the previous owner did that I have serious doubts "meet code". And I'm not just talking plumbing. So your concern about inspection if I sell the home is noted, but I feel it is beyond the scope of the question asked.