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

Because there's just so many variables, there's no way to so for sure.

I can say that with a 3/4" pvc (sch 40?) pipe, you are going to be losing 6psi of pressure for every 100ft of pipe that 5.5ish gpm is flowing through. Given that you're already down to 40psi (an you don't say if that's 40 psi at the source, or the heads) it sounds like you will be cutting things very close.

The best suggestion I can offer is that you go ahead an tie into the zone and setup three test heads connected with some temp pipe. If the other two heads still work ok with the three test heads, you'll be good to go if the pressure losses from water flowing from the point of the test to the final spots don't lower your pressure too much going to those heads. But when you add the additional length of pipe, the pressure going to the original two heads after the test can't get any worst than what they were doing at the time of the test.

Of course, then there is the future to consider. What might change your water pressure in the future? If you're one of the 1st houses in a new housing developement, you can be assured that once the neiborhood is built out, your water pressure will be lower.

If the test goes ok, then you will know the "worst-case" performance for the existing heads.

But for the new heads, with a run of 100 feet, you're going to lose a fair amount of pressure.

From what I've seen, there's not a huge price difference (not for a small project) between 1/2", 3/4" and 1" PVC pipe and fittings. The serious price jump starts when you get into the 1-1/4" and larger pipe fittings because they are used less frequent.

So I would suggest that you use 1" PVC for the 100 foot run as a minimum. 1" pipe will have only about 1/4 as much pressure loss as 3/4" pipe. Why? Do the math. A 1" circle has about has twice the cross sectional area as a 3/4" pipe. That means a given amount of water flows twice as fast in a 3/4" pipe as a 1" pipe. Friction losses in pipe are a function of the water speed squared. So when you double the water speed, you quadruple the losses.

You can check out the results here http://www.hunterindustries.com/resources/pdfs/technical/domestic/lit091w.pdf. If you look at the page for PVC pipe, you can see that at a flow of 5gpm, a 1/2" pipe will lose over 8psi/100ft, a 3/4" pipe will lose a little more than 2pis/100ft, and a 1" pipe will lose a little more than 1/2psi/100ft.

And even if the existing pipe you are connecting to is only 3/4", you still will get better performance in the new heads by using the 1" pipe. Water is relatively a non-compressable fluid. That means it doesn't mater if the pipe is 3/4", 1", or 2' in diameter. The pressure will be the same everywhere except for pressure losses. Basically, the water moving through the pipe creates friction as the water moves along the pipe surface. This friction expresses itself as a pressure drop down the length of the pipe. So using the example of a 100ft long pipe, if the water isn't moving, the water pressure in the pipe is the same everywhere. But if the water is moving at 5gpm, then the pressure at the end of the pipe is 8psi lower at the end of the 100ft pipe compared to the start of the pipe (assuming the pipe is level, because gravity adds or subtracts about 4psi/10ft of elevation change).

Why not add a zone? Overdrawing the water supply is a very common mistake. Extra work and expense for an additional zone, but on small water supplies, it's worth it.

The reason early morning pressure might be better is because of lower usage by the rest of the community.

As I've somewhat stated before, water flowing through a pipe produces friction on the pipe walls, and this friction expresses itself as a drop in pressure, and the faster the water is flowing, the greater the loss of pressure.

So if you check your water pressure at 10:00 AM, other people who obtain water through the same pipes you do will be flushing toilets, running dish washers, pressure washing their driveway, washing their car, etc, etc.

But if you check your water pressure at 4:00 AM, most everyone will be asleep. Any dish washers that were started the previous evening will be finished, no one will be flushing toilets or washing their car or driveway, etc. As a result, the pressure losses from everyone else's water needs will be gone and you'll have a higher water pressure.