Quote Originally Posted by mas985 View Post
I deleted my original response because I realized that I could make this real simple.

If you look at the head curve of a 3/4 HP two speed pump, you will notice that at 0 GPM it reaches a maximum head of about 20 feet or so. This means that with a pipe 20 ft in the air, no water will come out of the top, 0 GPM. There is a good chance you will burn up the pump in the process too.

If the vacuum valve requires 2 PSI to close, then the maximum head loss possible is 15 feet (20-5). If your roof is lower than 15 ft, you may have a chance at closing the vacuum valve. If not, you are out of luck. Its that simple.
I completely agree with this. 100%.

Even if the valve does close, you need quite a bit of flow to push the air downwards to completely prime the pipe. So in reality, your are limited by much less than 15 ft.
I see. If the flow is too low, you could end up with air permanently stuck in the pipe. This I believe. In fact, if you make the pipe wide enough, that will happen at any flow. You probably need a turbulent air-water mix to remove all the air. Which implies some minimum flow.

But why is it bad to have air permanently stuck in the pipe? Mmmm... maybe I see now. With air stuck in the pipe, the potential energy of the water as it falls down is wasted instead of contributing to the suction. So you don't get the pressure reduction at the top. Ah ha.

So that's the issue. You want to get rid of the air from the pipe to make the system work more efficiently. And that requires the high speed. Now I believe you.

However, if the pressure near the top is relatively low, then you don't care, because in any case you never want to have negative pressure. So the 1-speed pump works fine, even with air in the pipe, as long as it barely makes it over the hump. Which is probably not a healthy situation.

I think maybe I will stick to my 3/4 HP pump!

Thanks again!