two-speed with solar panels

[mas985]

OK it looks like I failing misserably at explaining this so I will try again. Let's start with fundamentals and then I will answer your questions below.

Head loss in a pump and plumbing system consists of several factors.

Static head: This is the net elevation change from the suction port to the return port. In a water filled pipe that goes up 100 feet and loops back to the ground, there is 0 static head. There is only static head in plumbing systems such as water towers where the water ends up at a different elevation. However, there can be temporary static head while priming piping that goes up to your roof. Once the pipe is filled, the static head gets cancelled out.

Dynamic head: This head loss is due to the water movement inside the pipes and is sometimes called friction loss. A pool's plumbing system will experience dynamic head loss on both the suction side of the pump and return side of the pump since water is moving through pipes on both sides.

So total head loss = Static Head + Suction Dynamic Head + Return Dyanmic Head

Return dynamic head can be estimated from a filters PSI reading with this equation:

Return Dyanamic Head = 2.31 * Filter PSI

Suction dyanmic head can be estimated from a vacuum guage at the input to the pump, if you have one, with this formula

Suction Dynamic Head = 1.13 * Inches Mercury (vacuum guage reading)

Also, it is important to understand the pump affinity equations, when going from high speed to low speed:

RPM Low Speed = RPM High Speed / 2
GPM Low Speed = GPM High Speed / 2
Dynamic Head Loss Low Speed = Dynamic Head Loss High Speed / 4
Static Head Loss Low Speed = Static Head Loss High Speed (this is why high speed is required for priming)
Filter PSI Low Speed ~ Filter PSI High Speed / 4

So I think most of the confusion is due to the priming of the solar panels. Initially, the panels will have air in them and no water. So when the pump is first turned on, it must first lift the water to the top of the roof before the static head can be cancelled out. It is this intial lift which requires the the high speed of the pump. Once the water has been lifted to the roof, it will start to drop on the other sided of the panels and that is when the static head gets cancelled out. At this point, you can immediately switch over to low speed and the flow will continue with a drop in PSI but not so much as to trip the vacuum valve.

If you turn off the pump, allow the panels to drain and then turn on low speed, there will not be enough power to reprime the panels. However, if you switch over fast enough, there is not time for the panels to drain so low speed will be ok. Automatic controllers have this feature which runs the pump at high speed for 3 min and then switches to low speed automatically.

So with the numbers you gave me,

Filter PSI no Solar High Speed = 16 PSI
Filter PSI with Solar High Speed = 23 PSI

Solar is adding 7 PSI.

using the pump affinity equations at low speed you will get

Filter PSI no Solar Low Speed = 4 PSI
Filter PSI with Solar Low Speed = 5.75 PSI

Note that 5.75 PSI is the equivalent of 14 ft of static head which is not enough to prime onto a two story roof. Perhaps a one story but I doubt it.

On to the questions:

OK. But can this work if the high point in the loop is higher than 13 feet over the pool water level, and there is a valve at that point that opens at negative pressure? (Taking atmospheric pressure as the origin.)

Yes, because after the pipe is filled on the pump's high speed, there is no more static head loss and you are left with only dynamic head loss which should be less than 14 feet when the pump is at low speed.

If you think this works, then it means that the pump at low speed generates enough head to get the water all the way to the top, and there is no need for the high speed.

As I explained before, you must prime the pipes with high speed before switching over to low speed. Low speed will probably not have enough head to overcome static head but does have enough for the dynamic head without static head so the panels must be filled with water before switching to low speed.

More specifically: you say the low-speed dynamic head is 13ft, or 5psi (by the way, 13ft is closer to 6psi).

No, because I also included suction head. 5 PSI * 2.31 + 1.3 in.mg. * 1.13

So on the way up to the panels, the pressure drops, both because of friction and height.
Yes

What's the pressure in the pipe at the top? Is it 2psi? 1psi? Or -1 psi? (The reference point is atmospheric pressure, about 15psi at sea level, so -15psi is the lowest value.) If you give me a positive number (or even 0 as a limit case), I'll tell you that in this case 5psi is enough to get the flow started, not just maintained. If you don't agree, please explain why. If you give me a negative number, I'll tell you that that's not possible, because the valve will open. If you don't agree, please explain why.

Again, this depends on what stage of priming and high or low speed. Pressure will drop the further away from the pump you get until the pool which is 0 PSI.

So let's assume that the pipe is fully primed with the high speed. If we assume 2/3 of the 7 solar PSI loss is from the filter to the vacuum valve, then the PSI drop is about 4.5 and the PSI at the vacuum valve is 18.5 PSI. The rest is lost in the remaining plumbing on the way to the pool return.

For low speed, the calculation is similar. But all the PSIs are 1/4 the value at high speed so the PSI at the vacuum valve will be 4.25 PSI. This is more than enough to keep the valve closed.

To ease your mind, people with pools and two speed pumps have been operating on low speed with solar for years so no worries.

I am sure it doesn't hurt to have the higher speed available. I still don't see why it's necessary. I agree completely that the two-speed pump makes sense if there is a siphon effect. But the valve prevents that effect. What's wrong with this reasoning?

Again, you are ignoring the intial priming of the pipes. Head loss is high while the pipes are filling with water which requires high speed. After the pipes are filled, then you switch to low speed. When the pump is off, all the water drains back out again so the next time you turn on the pump, you must use high speed again.

By the way, I just took readings of the pressure at my filter. It's 16 psi without solar. When solar kicks in, the reading rises to 24 psi, then drops down to 23 psi. That's well over the static head for the panels, so I am positive I could run at much less power and flow.

Remember that at low speed, the PSI is only 5.75 or 14 feet of static head.

Hope this helps and let me know if I am still not clear.