two-speed with solar panels

Hi,

I just installed photovoltaic panels on the roof and had to switch to time-of-use metering. This is going to make my pool recirculation more expensive, since I also have solar pool panels and I need to run the pump at peak hours (summer afternoons).

So I am considering a two-speed pump, but I am not sure if it will work for me. The current motor rating is 3/4 HP. I get plenty of flow---my pool is small (about 10,000 gallons) and I only need to run the pump 5 hours/day or so. The pump is 8 or 9 ft below the pool water line. The solar panels are about 20 ft above the water line. I am positive I can get by with a lower flow, but I still need a lot of head.

Does anybody have advice for this situation?

Thanks!
Luigi

Well you should only need the head to initially lift the water to the roof. Once you have flow established the height of the roof has no bearing as you effectively are only lifting to the surface of the pool. You create a siphon pulling down from the roof which cancels out the lift.

As for time of day pricing you will probably find that if you put a cover on the pool at night you will have no need for the solar panels in the peak summer months and will be able to run the pump at night as you have done in the past.

Why 2 speed in your case? Why not just wire in the pump so that it kicks on for solar demand, then kicks off when no-demand. You are probably going to want the full 3/4 hp to run solar anyway.

My solar controller has a relay for a booster (110V only, unfortunately) that could do easily handle the 3/4 hp pump (its a Goldline). If your pump is 220v, you could use that relay to trip a 220v relay to run the pump, or get a controller that has a built in 220v relay (I'm pretty sure some of the more expensive pumps do this).

Or, even cheaper, just set your pump timer to run for the 4 or 5 peak sun hours and then at night. No need for a relay!

Thanks tphaggerty, good suggestions. I already run the pump with the basic Intermatic timer for the 5 peak sun hours---and that's enough. It's an indoor pool, it doesn't get much dirt.

With time-of-day metering, electricity at peak sun hours costs about 50c/kWh. The pump probably uses the full 3/4 hp, which is about 600W, so that's 3kWh/day and $45/month. There is enough sun here (Oakland, CA) to take advantage of solar heat May, June, July, August, September. You're right, maybe I shouldn't bother, but still. According to the literature, I can get 1/2 the flow for 1/4 of the power, so I would save around $150 each summer, plus about 1/2 of that in the winter.

Cleancloths: thanks for the comments. I thought of the siphon effect too. but then I wondered. There is a valve at the top of the panels that lets air in when the flow stops. I believe this is done so that the panels do not get scrunched by the vacuum produced by gravity pulling down the water. I would think this means that the water must still be under pressure when it gets to the top. Siphoning implies negative pressure, but the valve would not allow that (unless it allows SOME negative pressure---but can it be that smart?)

I don't know how the valve works so I cannot tell you. I don't think "scrunching" would be an issue, as they would be filled with water as would the pipe feeding them when the pump stops.

Why did you switch to time of day? What is the savings off-peak that makes it worthwhile? I have peak / off-peak here in NJ as well put the rates are something like 18 cents and 9 cents versus a flat rate of around 15 cents.

I did it because I have a 10KW PV array on my roof which generates about 12,000 kwh a year almost all during peak times. I always used to run my pump at night as I have a heatpump to heat the pool so it made sense. But after much thought I am now running the pump during the day as the heatpump is much more efficient when it is 80 degrees out then when it is 30-40 degrees at night. Also, I added a pair of 2' by 20' panels to supplement the heatpump so I need to run during the day for that too.

The solar valves release when there is a vacuum in the line without any pump pressure. However, what cleancloths was describing is simply negative head not a vacuum. The pipe will still be pressurized throughout the whole run. Head is added with a pipe rise but then is subtracted with a pipe fall. Net net is zero head gain due to the fact they are on the roof (the panels and pipe runs will add head). You just need the high speed to fill the pipes but after that, low speed is sufficient. So after priming, the total head loss for solar is the same if they were on the roof or on the ground, assuming the same lengths of pipe.

I think I have a valve because after the pump stops I can hear water coming down from the panels for a minute or two. When the pump restarts, for the first couple of minutes only air flows down from the panels and into the pool through the jets---and there is no doubt about that! The explanation of why the valve is needed also makes sense, so I have never questioned this.

I switched to time-of-use because that was required by our power utility (PG&E) in order to sell them energy. We just installed 30 Sharp 170W PV panels (which by the way appear to produce a peak of 4300W, according to the inverter).

I like the cost of electric energy in NJ! But at that price I don't think I would have installed the panels.

Is there really a difference between "negative head" and "vacuum"? As I see it, unless there is still positive head at the top of the panels, the water on its way down is pulled by gravity and produces a "suction" at the top. By suction I mean a pressure lower than atmospheric. If the system is sealed, that negative pressure "sucks" the water up from the inlet. But if there is a valve, the suction will make the valve open. As I said, it's possible that the valve will stay closed with a small amount of suction, but eventually it will open, and I would imagine the margin to be too small to be of use.

Thanks!
Luigi

Quote:

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Originally Posted by semenzato

I switched to time-of-use because that was required by our power utility (PG&E) in order to sell them energy. We just installed 30 Sharp 170W PV panels (which by the way appear to produce a peak of 4300W, according to the inverter).

I like the cost of electric energy in NJ! But at that price I don't think I would have installed the panels.

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That is interesting, as I think I am the only one in NJ that has time-of-use with solar panels - they actually had a hard time getting my bill right for the first six months because of it. My system is comprised of 54 Sharp 185W panels and we get a similar yield to yours, the max I have seen on the inverters is about 8600W.

You probably would have done it in NJ even with our "low" electric rates. That is because the state has a very aggresive program to promote solar. At the time I installed mine they were offering rebates of up to 70% or $5.50 per watt installed. Add to the the solar renewable energy credits which you can sell each year. Those credits currently sell on the open market for $225-$250, and are set to double in two years. It makes the project very attractive. I have had the system for about two and a half years and figure my full payback will be reached in just under 4 years. Unfortunately for late adopters, the rebates are now 50% or less.

I don't know what kind of solar panels you have, but take a look at this link that shows the change in efficiency at different flow rates. It doesn't drop nearly as fast as your power consumption will drop at lower flow rates. The tradeoff will be somewhat slower heating of the pool with much larger savings in electric cost. Very roughly speaking, in a variable speed pump, the GPM is roughly proportional to the RPM while the power consumption varies as the cube of the RPM. You can see from the solar panel efficiency graph that going from 4 GPM at 80% efficiency to 2 GPM at 70% efficiency isn't a huge drop in efficiency, but that the power consumption will drop by a factor of 8 (a little less than that due to "fixed" losses). With a 2-speed pump, you may not be at the "sweet spot" of reasonable efficiency and low power consumption, but you just might.

Richard

Quote:

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Originally Posted by semenzato

Is there really a difference between "negative head" and "vacuum"? As I see it, unless there is still positive head at the top of the panels, the water on its way down is pulled by gravity and produces a "suction" at the top. By suction I mean a pressure lower than atmospheric. If the system is sealed, that negative pressure "sucks" the water up from the inlet. But if there is a valve, the suction will make the valve open. As I said, it's possible that the valve will stay closed with a small amount of suction, but eventually it will open, and I would imagine the margin to be too small to be of use.

Thanks!
Luigi

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Yes there is a difference between negative head and a vacuum. The primary difference is the reference. Vacuum usually refers to pressure below atmospheric or negative PSI. Your solar valve on the roof will not release until the pressure in the pipes is below atmospheric which requires the pump to be off and water to be draining out of the pipes. This causes suction.

However, with the pump on, there is a net positive pressure within the pipes usually between 10-20 PSI above atmospheric pressure (i.e. your filter PSI reading). This keeps the valve closed. So while the water going down the pipe offsets the water going up the pipe, this is called static head, there is still a substantial positive pressure to keep that valve closed. There just isn't any head added to the system because of elevation. The only pressure difference is the dynamic head loss experienced in pipe and solar panels. The water falling simply makes it such that less pressure is needed to push the water up the pipes.

The easiest way to see this is when the solar is first turned on. You will see the PSI rise substantially and then after the pipes are filled, the PSI will drop slightly but still higher than without solar.

So in formula terms:

Filter PSI Solar on = PSI Solar off + PSI elevation Rise - PSI elevation fall + PSI Dynamic Head Loss Panels and Pipe

PSI elevation Rise - PSI elevation fall nets to 0 so there is no PSI impact due to elevation.

I have been trying to learn a little more about solar options and have a couple of basic questions.

Can you use PV solar panels to heat your pool? I guess I thought that you either had a PV system that generates electricity that can create units that can be sold or you have a "water heating" system that just creates hot water. Is that accurtate?

It seems like having solar for heating the pool is beneficial early and late in the year to extend the swim season, but does anybody use their system for anything during all those other months? I know during peak summer it seems like you can use it to cool your pool, but what about all that sun during the summer and winter months? Just curious.

Some of you that have a PV system. What size system do you have and how much energy are you able to generate over the year?

Thanks,

Riles

Quote:

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Originally Posted by rilesworld

I have been trying to learn a little more about solar options and have a couple of basic questions.

Can you use PV solar panels to heat your pool? I guess I thought that you either had a PV system that generates electricity that can create units that can be sold or you have a "water heating" system that just creates hot water. Is that accurtate?

You could use them to generate electricity and then run an electric heater but that would be very inefficienct. The better bet is to use the electricity to run the pump and then use passive solar for heating.

It seems like having solar for heating the pool is beneficial early and late in the year to extend the swim season, but does anybody use their system for anything during all those other months? I know during peak summer it seems like you can use it to cool your pool, but what about all that sun during the summer and winter months? Just curious.

Really depends on where you are. Most of the year, I use the solar to keep the pool between 85-90 degrees. There is onlyabout three weeks in the summer where I don't need it. Also, you need to add more heat if you don't use a solar cover. Although our days here in CA can get quite hot, the nights seem to be fairly cool and that is when you lose a lot of heat.

Some of you that have a PV system. What size system do you have and how much energy are you able to generate over the year?

Thanks,

Riles

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Some thoughts.

A PV system generates electricity, what you do with that electricity is up to you. CA like NJ has some very good incentives to install PV. PV provides electricity year-round - so I find it a much better use of my roof space. My 54 panel system generates over 12,000 kwh per year.

I second mas985. My controller stays on all summer. After it rains or we have a few cold nights, it will kick on, even in July & August.

Extending the season isn't anything to sneeze at, though. We opened about 3 weeks ago, the water in now 85+. We will swim until almost the end of October in 80+ water. No one else, and I mean no one, will do that around us (unless they get solar!!)

Quote:

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Originally Posted by tphaggerty

I second mas985. My controller stays on all summer. After it rains or we have a few cold nights, it will kick on, even in July & August.

Extending the season isn't anything to sneeze at, though. We opened about 3 weeks ago, the water in now 85+. We will swim until almost the end of October in 80+ water. No one else, and I mean no one, will do that around us (unless they get solar!!)

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I've done it for years in Northern NJ with my heatpump. The key issue is more of using a solar cover then how you heat. The heat loss in April/May and Sept/Oct at night is HUGE if you don't use a cover.

Quote:

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Originally Posted by cleancloths

I've done it for years in Northern NJ with my heatpump. The key issue is more of using a solar cover then how you heat. The heat loss in April/May and Sept/Oct at night is HUGE if you don't use a cover.

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True enough. We also use a "solar" cover religiously. When we first got the pool, we bought a very expensive, very ugly commerical reel. We were going to return it because it is very big, but after getting used to it (we can roll it over the diving board with no problem and can reel the cover up without bending over), we decided to keep it.

Many people we talked to about their pools complained about the price of heating, but didn't bother to use the cover. Sort of like heating a house and leaving all the windows open at night when it's still getting down into the 40s.

Oh, I see what you mean. The "suction" from the falling water reduces the pressure at the top, but not quite enough to make it negative (that is, below atmospheric). The term "negative head" confused me. I would think of it more in terms of "head reduction".

However, I am still not sure this works. You're saying that I would use the pump at high speed to fill the pipe, then I can switch to low speed. But you also agree that even with low speed there is higher-than-atmospheric pressure in the pipe at its highest point (where the valve is located). That means that at low speed the pump still has enough power to fill the pipe, and I don't need two speeds, just the low speed.

Anyway, thanks for your help and advice. I will have to do some measurements and calculations and see if I can get by with a smaller pump.

Quote:

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Originally Posted by semenzato

Oh, I see what you mean. The "suction" from the falling water reduces the pressure at the top, but not quite enough to make it negative (that is, below atmospheric). The term "negative head" confused me. I would think of it more in terms of "head reduction".

However, I am still not sure this works. You're saying that I would use the pump at high speed to fill the pipe, then I can switch to low speed. But you also agree that even with low speed there is higher-than-atmospheric pressure in the pipe at its highest point (where the valve is located). That means that at low speed the pump still has enough power to fill the pipe, and I don't need two speeds, just the low speed.

Anyway, thanks for your help and advice. I will have to do some measurements and calculations and see if I can get by with a smaller pump.

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Not quite. Static head, which is basically the rise in elevation, does not depend upon the rate of water flow (GPM). Dynamic head (pipe friction loss) does depend on the rate of water flow. A pump at high speed has a pump head curve such that it can take care of both the static and dynamic head together (i.e. high head curve) quite easily. See here:

http://www.poolplaza.com/WhisperFlo-techspecs.shtml

The pump at low speed, which is basically half speed and half flow, only has enough power to handle the lower dynamic head (notice the head curves are much lower). So at low speed, the flow rate is 1/2 of the high speed so the dynamic head is 1/4 of that at high speed. Since the pipe is primed with full speed, at half speed there is no longer any static head so it will have no problem with the head loss that remains.

Here is what would happen in a typical pool.

No Solar with full speed filter PSI = 15, head loss about 45 feet
With Solar at full speed filter PSI = 20, head loss about 52 feet
With Solar at low speed filter PSI = 5, head loss about 13 feet

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


Here are a few more links you can learn about pumps and hydraulics.

http://www.powermat.com/pools/index.html
http://poolplaza.com/pool-pump-sizing-2.shtml
http://www.zoeller.com/zep/techbrief/JF1article.htm
http://www.eere.energy.gov/industry/.../pumpintro.pdf

and I have much more technical version when you are ready.

OK. I will indulge myself by being doggedly stubborn and incapable of understanding this.

The pump at low speed, which is basically half speed and half flow, only has enough power to handle the lower dynamic head (notice the head curves are much lower). So at low speed, the flow rate is 1/2 of the high speed so the dynamic head is 1/4 of that at high speed. Since the pipe is primed with full speed, at half speed there is no longer any static head so it will have no problem with the head loss that remains.

Here is what would happen in a typical pool.

No Solar with full speed filter PSI = 15, head loss about 45 feet
With Solar at full speed filter PSI = 20, head loss about 52 feet
With Solar at low speed filter PSI = 5, head loss about 13 feet


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.)

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.

More specifically: you say the low-speed dynamic head is 13ft, or 5psi (by the way, 13ft is closer to 6psi). So on the way up to the panels, the pressure drops, both because of friction and height. 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.

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?

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.

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:

Quote:

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Originally Posted by semenzato

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.

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

mas985 - that was an excellent explanation and it would be great if that could be made a sticky or a FAQ for the board. You did a much better job of simply stating what I was trying to say. Thanks.

Just one foggy thought left for me. When my sand filter pressure goes up, needs a backwash, the panels do not stay full with water. You start to see bubbles in the returns and can hear the vacuum break opening/closing. This is my audiable alarm to backwash!

Is this an indication how a low speed pump would work on my pool? With a low speed at 1/2 gpm, is there enough flow to keep the vacuum break closed, with a clean filter? Right now I have a 1hp pump and the panels' highest point is 20'.

Joe

Each pool is unique in operation and your results may vary. First, at low speed the GPM is 1/2 of the high speed value not 1/2 GPM. So if your normal flow is 70 GPM, at low speed it will be 35 GPM.

When a filter gets dirty, the pressure ahead of the filter goes up and the pressure after the filter goes down. So, the problem you are experiencing could be due to this pressure decrease when the filter is dirty. You do not have enough pressure to fully prime the solar pipes. It sounds like you are waiting too long before cleaning your filter.

Another reason could be that your vacuum release valve is not operating properly but difficult to prove. Normally, they only require a few PSI to close and negative PSI (vacuum) to open. So if it is not closing and you have enough pressure after the filter (hard to determine) then it could be a malfunctioning valve.

Of the two, my guess would be the first problem. So the solution is to keep your filter clean. With a clean filter, there should be no problem with low speed but I would need to know the pressure readings that you currently have.

Pump model, HP, Service Factor
Clean Filter Solar Off PSI
Clean Filter Solar On PSI
Dirty Filter Solar Off PSI
Dirty Filter Solar On PSI

With these numbers, I can assess if a two speed would work for you.

Mark,

I meant 1/2 gpms of the high speed pump not 1/2 gpms total. My bad for not clarifing that. I was guessing a low speed rate may act like a dirty filter.

I understand your explanation, and believe the dirty filter is the problem.

My system is a Fafco Sun Saver 5- 4x12s panels.

pump- Waterway svl56 1hp 1.25sf Clean Filter Solar Off PSI 14psi
Clean Filter Solar On PSI 16psi(17psi at first, then drops to 16 when all air is purged)
Dirty Filter Solar Off PSI 20psi, time to backwash
Dirty Filter Solar On PSI I need to verify this reading.

Thanks for looking at my numbers.

P.S. I agree with Cleancloths with the STICKY recommendation.

Joe

If the highest point of your solar panel is 20 ft above the pump, you will need at least 9 PSI to get to the top of the panels, 2 PSI of friction loss and another 2 PSI or so to close the valve for a total of 13 PSI. By my estimates, you will have about 12 PSI after the dirty filter or 1 PSI at the top of the panel which may not be enough to close the valve. I suggest cleaning after 4 PSI rise instead of 6.

As for two speed, as long as the pressure does not drop below zero PSI when switching to low speed, the valve should stay closed. But this does require a proper prime at high speed so all of the air is purged from the system. If not, when the pump is switched to low speed, the valve may open.

Thanks for the excellent explanation. It's quite clear and I agree with everything except one part: the high-speed priming to fill the system. To be fully honest, I think that either what you say is wrong, or there is some factor at play that we haven't discussed.

It all boils down to this. To keep the valve closed, the pressure near the valve must be positive. I believe that pressure and flow completely describe the state of the system near the valve. To be concrete, suppose we have 2 psi and 40 GPM.

Suppose there is a diverter at the top that when turned, instead of sending the water to the panels, it sends it into a wide pipe going straight up for 4.6ft, open at the top. The pipe is wide so dynamic loss is negligible. We divert the flow. What happens?

In my book, the water goes up the open pipe and overflows, at an almost identical rate. The pressure near the valve is still about 2 psi (2 psi x 2.31 = 4.6 ft) and the flow is about the same. At the pump, there is no way of telling if the water is going through the panels or falls out of the pipe extension.

So it would seem that our pump (at low speed) is capable of lifting water to the valve, actually a bit higher than that, with plenty of flow. This pump, according to you, is not able to prime the system. Why? Sure, there is a period of transition as the pipe fills up. But the pressure at the pump can never be higher than when the pipe is full. Surely there is dynamic head, but that is also always less than when the pipe is full.

Then there is no need to prime at higher speed or power.

Joe's (joenj) contribution is revealing and made me realize that in fact there is one more variable we can play with: the jet diameter.

Just one foggy thought left for me. When my sand filter pressure goes up, needs a backwash, the panels do not stay full with water. You start to see bubbles in the returns and can hear the vacuum break opening/closing. This is my audiable alarm to backwash!

As Joe's filter gets more and more clogged, the after-filter pressure drops, the pressure at the top drops, finally becomes negative (or near there, depending on how the valve is built) and the valve opens. What happens then? The water on its way down (which is "responsible" for the negative pressure) flows at a higher rate that the pump can supply, and air is sucked in. As the amount of air in the pipe increases, there is less water in the pipe, consequently less weight. The suction drops (that is, pressure increases), and the valve closes again. The frequency of opening/closing depends on the hysteresis of the valve, and possibly other factors.

But the flow of the water on its way down can be reduced by narrowing the jet outlets. If you can live with the lower flow, that will make the system work again.

This also means that as you get close to zero pressure at the top, the dynamic loss on the down pipes is no longer relevant when computing the savings due to a lower flow. On the up pipes, it still is. (Sorry, I should explain this more clearly, but it's dinner time.)

I should say that it's VERY helpful to be able to have this kind of detailed discussions. I cannot thank enough all those involved.

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.

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.

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Originally Posted by mas985

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.

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I completely agree with this. 100%.

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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.

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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!