Question about pressure, backflow, and solar bypass/return

Jump to latest Follow Reply
A

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:
https://imgur.com/w6jX0Dc

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
 
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?
 
  • Like
Reactions: PoolNewb2020
mas985 said:
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.
Click to expand...

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.

mas985 said:
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.
Click to expand...

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.

mas985 said:
Are the panels on the ground or elevated?
Click to expand...

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).
 
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.
 
AbsoluteTruth said:
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.
Click to expand...

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.
 
mas985 said:
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.
Click to expand...

I have no way of testing PSI on my current system. What is the best way to accomplish that?

mas985 said:
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.
Click to expand...

Interesting, that's a good point that I was not aware of.

mas985 said:
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.
Click to expand...

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.

PoolNewb2020 said:
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.
Click to expand...

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?

PoolNewb2020 said:
I would suggest, if you have an infrared thermometer, measure pipe temperature at each point and see if you are getting desired temp rise.
Click to expand...

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.
 
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.


Infrared Thermometer - The Home Depot

Get free shipping on qualified Infrared Thermometer products or Buy Online Pick Up in Store today.
www.homedepot.com
 
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.
 

Enjoying this content?

Support TFP with a donation.

Give Support
AbsoluteTruth said:
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?
Click to expand...
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:
PoolNewb2020 said:
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.
Click to expand...

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?
 
AbsoluteTruth said:
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?
Click to expand...
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.
 
AbsoluteTruth said:
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?
Click to expand...
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.
 
You must log in or register to reply here.
Thread Status
Hello , This thread has been inactive for over 60 days. New postings here are unlikely to be seen or responded to by other members. For better visibility, consider Starting A New Thread.
Top Bottom
Back
Jump to latest Follow Reply