SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

Try a 1-1/2 inch connecting line. The 3/4-inch pipe does indeed subtract pressure by way of friction.

By the way, is this a deep well with a submersible pump?

The sizing of your connecting pipe, or any long run of pipe, has to be done with friction losses in mind. It would be best to have flow numbers from the system as it ran from city water. That would make it easy to connect the dots with a pump running from the storage tank, which I will assume, unless told otherwise, is a surface pump (make and model number, please)

By the way, if it hasn't already been done, a ground water interconnection with a sprinkler system requires nothing less than an RPZ backflow preventer in the city water line, at an outdoor location, to isolate the house water from the now-defined-as-toxic sprinkler system water. Keeping an interconnection without the proper backflow can lead to contaminating the house water, and even the city water supply feeding the house. The legal ramifications of that possibility of toxic backflow are expensive far beyond the price of proper plumbing.

Then you need to consult friction loss tables, along with your flow data, and establish the proper plumbing for the connecting line. Total pressure loss in the interconnect should only be one or two psi, in order for it not to be getting in the way of system performance.

As for your un-named backflow preventer, I will once again stress for the wider audience that any city water interconnect to ground water has to be done right. In fact, some localities flat-out prohibit the practice, with the laws squarely on their side, because a homeowner can have his Certificate of Occupancy revoked, and his house subject to being emptied of its occupants and the doors padlocked, until the offending plumbing is remedied and inspected, and the CO restored.