Question about pressure, backflow, and solar bypass/return

AbsoluteTruth

Member
Apr 6, 2020
8
Sarasota, FL
Hi all. I recently installed a few solar panels to heat my above ground intex 12'x30" pool. My solar panels lay flat on my deck. I have an intex 2500gph pump that pushes water to a three way valve that I have at full open (all 3 sides open). The right side of the three way valve pushes water to the solar panels. Then the output of the solar panels is run to a PVC "T" junction and returned to the pool. A pic of my setup is here:
My question is this: how is the pressure at the "T" junction not going to push cold water BACK into my solar return line when I have the three way valve completely open? I'm having a hard time wrapping my brain around that one. Im worried that the straight flow between the three way valve and the "T" junction will outweigh the pressure of the return hose, i.e., forcing water back into the solar panels, intead of letting the warm solar water back into the pool return line. Can someone explain to me how my setup works in terms of pressure, or if there are any glaring problems with the way I have it set up?

Thanks!20200514_110314.jpg
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
13,486
Pleasanton, CA
The short answer is that water will seek a path of least resistance. So by having the 3-way open to all sides, the bypass path has the least resistance so most of the water will go in that direction. A little may still make it to the panels and back but it will be proportional to the square root of the head loss in each path. So if you want more water to go in the direction of the panels, you have to increase the head loss of the bypass. You do this by closing off that path by a bit. But since you want high flow rates through the panels (0.1 GPM/sqft), you really want to close off the bypass by a lot if not completely. You don't want much of the water to bypass the panels for efficiency reason. The only reason to open the bypass is because the filter pressure rises too much because you are forcing too much water through the panels. However, you want at least 1-2 PSI rise in the filter.

Are the panels on the ground or elevated?
 
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AbsoluteTruth

Member
Apr 6, 2020
8
Sarasota, FL
The short answer is that water will seek a path of least resistance. So by having the 3-way open to all sides, the bypass path has the least resistance so most of the water will go in that direction. A little may still make it to the panels and back but it will be proportional to the square root of the head loss in each path. So if you want more water to go in the direction of the panels, you have to increase the head loss of the bypass.
I have already tested the setup with the return line from the solar being manually placed into the pool through the pool cover, instead of routing the line back to the pool return. I was getting sufficient flow and heat at that point in time. So I know that the water pressure going to the panels is fine. My pump is 2500gph and my panels aren't rated for that kind of pressure so I dont want to run full solar, hence the three way valve being in the full open position instead of diverting all water to the solar. My question is with respect to the warm water being routed back into the return instead of just being an independent hose that sits over my pool, and whether water is being forced back into the solar return by the pressure.

You do this by closing off that path by a bit. But since you want high flow rates through the panels (0.1 GPM/sqft), you really want to close off the bypass by a lot if not completely. You don't want much of the water to bypass the panels for efficiency reason. The only reason to open the bypass is because the filter pressure rises too much because you are forcing too much water through the panels. However, you want at least 1-2 PSI rise in the filter.
See answer above, with running all of the water to the solar panels from my 2500gph pump, I believe it will create too much pressure. 2500gph/60 minutes = 41gpm which, by my understanding, is far too much for the solar to handle. Hence why I run the three way valve in full open.

Are the panels on the ground or elevated?
The panels are on the ground, lying flat. It is two 2x10' panels in series (I plan on making them parallel as soon as the adapter arrives).
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
13,486
Pleasanton, CA
Again, you will get approximately the correct flow rate through the panels if you close the 3-way valve just enough for a 1-2 PSI filter pressure rise. That is not too much for the panels to handle. Most panels can handle 30 PSI without issue.

Also, a 2500 GPH pump does not actually produce 2500 GPH on pool plumbing. That is the run out flow rate (i.e. no head loss). The actual flow rate is dependent on the head loss through the plumbing so as head loss increases, flow rate decreases. Basically, the head curve of the pump varies from 0 GPH to 2500 GPH dependent on the head loss.

Also, you don't want a large temperature rise in the panels as that reduces the efficiency of the panels. Ideally, the exit temperature of the panel water should only be a few degrees above the inlet temperature.
 

PoolNewb2020

Well-known member
May 7, 2020
163
Lathrop, CA
I have already tested the setup with the return line from the solar being manually placed into the pool through the pool cover, instead of routing the line back to the pool return. I was getting sufficient flow and heat at that point in time.
Running the return from solar straight to the pool eliminates a lot of restriction from that tee connection. Once you connect the solar return to the tee, the water coming from solar has to "fight" with water coming from the by-pass, so there is a lot more restriction. But we cannot know for sure just how much.

I would suggest, if you have an infrared thermometer, measure pipe temperature at each point and see if you are getting desired temp rise.
 

AbsoluteTruth

Member
Apr 6, 2020
8
Sarasota, FL
Again, you will get approximately the correct flow rate through the panels if you close the 3-way valve just enough for a 1-2 PSI filter pressure rise. That is not too much for the panels to handle. Most panels can handle 30 PSI without issue.
I have no way of testing PSI on my current system. What is the best way to accomplish that?

Also, a 2500 GPH pump does not actually produce 2500 GPH on pool plumbing. That is the run out flow rate (i.e. no head loss). The actual flow rate is dependent on the head loss through the plumbing so as head loss increases, flow rate decreases. Basically, the head curve of the pump varies from 0 GPH to 2500 GPH dependent on the head loss.
Interesting, that's a good point that I was not aware of.

Also, you don't want a large temperature rise in the panels as that reduces the efficiency of the panels. Ideally, the exit temperature of the panel water should only be a few degrees above the inlet temperature.
Yeah this was something I realized when I was setting up the solar. The differential between the intake and outlet temps should only a few degrees, and is. Right now the differential between the pool temperature and the pool return (with the solar plumbed into the return) is about 0.5 degrees. When I had the solar return straight to the pool, routed through the cover, the differential was usually about 2-3 degrees.

Running the return from solar straight to the pool eliminates a lot of restriction from that tee connection. Once you connect the solar return to the tee, the water coming from solar has to "fight" with water coming from the by-pass, so there is a lot more restriction. But we cannot know for sure just how much.
Yeah this was my question. I dont know how much the water coming from the solar is "fighting" the water coming from the bypass. How can I determine this? I was thinking about getting a check valve for that junction so that I can ensure no water is backflowing from the bypass into the solar return. Do you think that would be helpful?

I would suggest, if you have an infrared thermometer, measure pipe temperature at each point and see if you are getting desired temp rise.
I do not have an infrared thermometer. I have a digital thermometer that came with some pool part, which is an LCD readout coming from a long wire with a temp sensor on the end. Ive been putting that wire into the pool return to measure water temps. If I were to purchase an infrared thermometer, where would the best place to test the temperatures be? On the hoses or on the PVC parts? Sorry, I am very, very new to all of this.
 

PoolNewb2020

Well-known member
May 7, 2020
163
Lathrop, CA
Here are some infrared thermometers from home depot. A $20-$30 one should be more than enough. They can come in handy in variety of situations.
Try pointing it at similar colors, as color does affect the reading a bit, so in your case on the white couplings or valve housings. They have a laser dot showing where the reading is taken from.


 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
13,486
Pleasanton, CA
There is no way that pump can over drive the panels. It is just not powerful enough. If I were you, I would just redirect all flow through the panels to get maximum efficiency.
 

PoolNewb2020

Well-known member
May 7, 2020
163
Lathrop, CA
Yeah this was my question. I dont know how much the water coming from the solar is "fighting" the water coming from the bypass. How can I determine this? I was thinking about getting a check valve for that junction so that I can ensure no water is backflowing from the bypass into the solar return. Do you think that would be helpful?
You don't need to worry about backflow so no need for any check valves. The pressure at the inlet hose will always be greater then outlet, even if it is a very small amount. That pressure difference ensures no backflow. When I said "fight" I meant which side flows through the tee and back to the pool, not the direction of flow. The rule is that the side with higher pressure wins. Solar panels will provide a significant pressure drop, so there could be very little flow actually making it back to the pool. Most of it could be naturally blocked by the higher pressure water coming from the by-pass.

That is why most suggest to close that three way by-pass valve.
 
Last edited:

AbsoluteTruth

Member
Apr 6, 2020
8
Sarasota, FL
You don't need to worry about backflow so no need for any check valves. The pressure at the inlet hose will always be greater then outlet, even if it is a very small amount. That pressure difference ensures no backflow. When I said "fight" I meant which side flows through the tee and back to the pool, not the direction of flow. The rule is that the side with higher pressure wins. Solar panels will provide a significant pressure drop, so there could be very little flow actually making it back to the pool. Most of it could be naturally blocked by the higher pressure water coming from the by-pass.

That is why most suggest to close that three way by-pass valve.
So if I close off the three way bypass valve, such that 100% of the water from the 2500gph pump gets routed through the solar, it wont create too much pressure on the pump and/or solar panels?
 

PoolNewb2020

Well-known member
May 7, 2020
163
Lathrop, CA
So if I close off the three way bypass valve, such that 100% of the water from the 2500gph pump gets routed through the solar, it wont create too much pressure on the pump and/or solar panels?
As mas985 mentioned already that 2500gph, which by the way is about 40 gallons per minute, is the absolute maximum this pump can deliver, which assumes no back pressure. The greater the back pressure the slower the flow.
You could do a quick and dirt flow check by filling a container you know has certain number of gallons and measuring the time it takes to fill it. If your pump truly delivers 40gpm at the end of the solar outlet, it would take about 8 seconds to fill a 5 gallon bucket.
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
13,486
Pleasanton, CA
So if I close off the three way bypass valve, such that 100% of the water from the 2500gph pump gets routed through the solar, it wont create too much pressure on the pump and/or solar panels?
Pressure in the panels will be less than 8 PSI which the panels can easily handle. Folks with AG pools run this way all the time with solar.