Primary-secondary piping systems call for the tees that go off to the secondary circuit to be close together, ideally not more than six inches apart. Think like water and you’ll see why this is. Flow down the primary main and imagine you’re entering that first tee. You have two choices: You can stay in the primary loop and travel along the run of the tee, then six more inches before you enter the tee that’s returning the water from the secondary. That’s easy. Or you can enter the bull of that first tee, go all the way through the secondary circuit, and then come splashing back out through the bull of the return tee to enter the primary flow. Hey, that’s hard work! You’re not going to do that unless the secondary pump comes on. And that’s why we keep those tees so close together.