Flow Rates and Flow Balancing Explained

Recently one of our competitors has been telling prospective solar pool heating clients that plumbing, valve, and panel header size don’t matter. In fact, they are even saying smaller is better. Does that make sense? It’s just nonsense, and we want to set the record straight.

We use all 2 inch PVC pipe, valves, and full 2 inch inside diameter panel headers. They use 1-1/2 inch plumbing and panels. It doesn’t sound like a big difference, but a 2 inch pipe holds 80% more water volume than a 1-1/2 inch pipe! More importantly, a 1-1/2 inch pipe has 3.3 times the friction loss of a 2 in pipe!

All new residential pools are built using 2 inch plumbing (or larger) for good reason. Ask yourself: Does it make sense to use smaller plumbing for solar panels? The answer should be obvious. Of course not!

But why does this all matter?

Delta T

The most important concept in heat exchanger design and operation is Delta T (∆T). Delta T is the difference in temperature between the fluid being heated and the heat source, in our case, the surface of the solar panel. The greater the Delta T, the faster the heat transfer.

To put it in simple terms, if the water inside the panel is colder relative to the solar panel surface, more heat will be transferred to the water. This is science. This is physics. There is no denying this fact.

So how do you keep the water in the panel colder? You increase the flow rate! Keep the water moving faster and it will absorb more heat. This sounds counterintuitive to some, but it’s true. You don’t want a large temperature rise. You want a small temperature rise with a large volume of water flow.

So why does size matter?

Friction (Head) Loss

It is common sense that trying to force water through a smaller pipe will result in a lower flow rate. Just like more cars on a busy road causes traffic jams, more pressure in a smaller pipe means slower speed. The reason water flows slower in smaller pipes (and solar panels headers and tubes) is friction loss. The head (pressure) increases because more water molecules are subjected to the surface roughness of the inside of the pipe.

A smaller pipe equals more friction equals less flow. And as we learned above, less flow equals slower heat transfer to the water inside the panels!

Our competitors, if they are being truthful, will give us this point. Since they have no leg to stand on with this issue, they turn to another tactic…

Balanced Flow

Solar Pool Heating Panel Systems are Z-Type Heat Exchangers
Solar Pool Heating Panel Systems are Z-Type Heat Exchangers

An array of solar panels is essentially a “Z-type” heat exchanger with an inlet and outlet manifold. In any heat exchanger, balancing the flow so each small tube has the same flow rate is certainly important. You would not want some tubes with high flow, some with average, and some with low flow rates. Ideally, each tube has the same high/optimal flow to gain the most heat.

Other solar dealers will try to argue that their system balance flow rates better because they have more restriction. They argue that since our panels have very low restriction with larger tube openings and large headers, that our panels don’t balance flow rates adequately. However, they fail to understand the mechanics of manifold physics.

The most important factors in determining the effectiveness of flow balance are the ratio of the header to tube size and the overall flow rate. We have larger headers, so we can have larger tube openings and maintain balance. Higher flow rates also promote more balance. Since our system is built for higher flow rates, we have this aspect covered. The argument that our panels doe not have proper balance is totally false.

Any system that uses opposite end feed and return plumbing is inherently more balanced than one using same side feed and return. Increasing overall flow and using proper header to tube size ratios takes care of the rest of the issue. One competitor actually uses same-side feed and return plumbing (U-type heat exchanger) because their restriction is so high that they claim they can balance their flow with this design. However, that comes at the expense of overall flow, which we know decreases Delta T. Another competitor boasts about artificially/purposefully restricting flow on return plumbing to balance flow. That only makes sense if the inherent design of the system makes it necessary, and again it decreases Delta T!

There are some cases where increasing back pressure in our panels would be advantageous, particularly in very large systems. However, we prefer to balance flow in these cases with center feed/return strategies or balancing valves. It’s all about the flow when talking about heat exchange.

So what does this all really mean? Why does it matter?

Less Heat or More Energy?

When confronted with the facts, the science, the physics, competitors might start talking about test ratings. Solar panels are tested by a variety of agencies and labs using a variety of methods. One of the best rating methodologies is employed by the Florida Solar Energy Center (FSEC). It is a composite rating to indicate how a particular solar panel will perform over different seasons under test conditions. There is only one problem. Test conditions do not exist in the real world.

Every dealer should tell you that all solar panels of similar construction should perform within about a 10% margin under test conditions. Some panels with separated tubes perform very well in summer (when you don’t need pool heating) and very poorly in winter (when you need it most). Separated tube panel FSEC ratings have shown a 15% lower performance rating. But all of these test ratings lack usefulness in the real world where all else is not equal.

You see, the test ratings assume a specific flow rate per panel. The problem is that for a given single speed pool pump, all of our competitors’ systems will have lower flow rates. This is because they either use smaller plumbing and valves or their panels have high restriction to flow or both. This is undeniable, and we know that more flow means more Delta T which equals more heat transfer.

The issue is even more disturbing if you have a variable speed pump, which are fantastic energy savers usually. Since our competitors’ systems have more resistance, they need to set the pump speed higher to achieve the same flow rate as our system. This means you pay for more electricity to achieve the same (or similar) performance, have more noise, and run the pump harder than necessary.

So take your pick… systems with smaller plumbing and more restriction either transfer less heat or use more electricity. That, too, is undeniable.

They’re All The Same

When confronted with the facts, you will see other solar dealers revert to the old school tactic that all solar panels are going to heat your pool just fine. If you are the low price leader, this is a natural reaction to a competitor with a superior product. In fact, this was largely the case in Southwest Florida until we came along. All of our competitors’ systems are pretty close in performance and quality, with each having some good and some bad characteristics. Price alone was often a deciding factor, save for some small differences in features. But we are different.

How are we different?

The manufacturer of our panels took all of the good features of the existing products on the market and incorporated them into one panel while eliminating or mitigating the negative aspects of each. As a solar contractor, we took a similar approach and improved upon the materials and methods used to install solar pool heaters. Solar pool heating options are no longer all the same!

The biggest advent in Solar Pool Heating recently is the move toward more efficient pool plumbing, smaller pumps, and variable speed pump technology. All of the other solar panels on the market were developed decades ago. We consider these options obsolete now. We are able to achieve high flow rates, balanced flow, and low energy use in one purpose-built solar panel design paired with efficient plumbing and smart installation practices.

More Analogies

In case you are still questioning whether water should flow faster or slower, here are a few more analogies that might help explain the physics (and the common sense) behind Delta T.

Air Conditioning

Florida homeowners are very accustomed to air conditioning. Imagine if an AC company came into your home and told you that they could improve your air conditioning performance by installing a slower fan in your air handler. You would get less air out of the vents, but the air would be colder.

Does that make any sense? Of course not. The concept is exactly the same as solar pool heating, only in reverse. The faster the air flows over the air conditioning coil, the more overall cooling you get. This is because the difference in the air temperature versus the refrigerant temperature is greater. More heat is absorbed by the refrigerant at a higher Delta T.

Ice Water (or Tea)

I like to call this the Delta Tea analogy. You have probably noticed that adding ice cubes to a glass of room temperature liquid results in the ice cubes melting very quickly at first, but then they tend to linger and melt at a slower rate. This is Delta Tea in action! When the difference in temperature between the liquid and the ice cube is high, heat is transferred quickly and the ice melts faster. As the temperature of the liquid gets colder (closer to that of the ice cube) the Delta T decreases and heat is transferred more slowly.


We like to tell the truth – the whole truth. Don’t listen to anyone that says smaller plumbing is better. It’s just physics!





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