When any one zone of the sprinkler system turns on, the water pressure inside the house drops to zero (no water coming out of the faucet) for a 3-7 seconds before returning to normal. After that drop, both the sprinklers and inside faucets/showers work fine until the next zone turns on, then the water pressure inside the home drops again. PSI measured at the main valve is 55 (after the pressure regulator). I have low flow sprinkler heads installed, and fairly small zones. Any ideas on what could be the issue?
I don't think you have a leak, what I believe is your pressure regulator is set at a high enough pressure that when the zone turns on and has a high surge until the pressure builds this is causing the no flow in the house for a few seconds. Once the flow stabilizes the pressure builds back up and you have pressure in the house again. To keep this from happening install a regulator on your sprinkler system and set it to a slightly lower pressure than what you want in the house. This will make set sprinkler lines take slightly longer to fill and if 2 psi lower will slightly reduce the water spray distance but your pressure would not drop as it is doing now.
You could have a large leak in the sprinkler piping or an in-line valve. There are probably several control boxes with the valves inside them.
Since it happens on every zone, there could also be a leak in a run of piping common to all zones. Turn on the system to manual and start looking where the sprinkler piping connects to your main control box. From there, the piping will branch out to other zones or other control boxes. If there's a leak, you will see bubbling or one section of the ground might be much wetter than the rest. It might even be as obvious as water shooting up from a control box. If you find that there isn't a leak, you might consider purchasing a Booster Pump.