Solar Pool Flow and Pressure

How much pressure and how much flow should go to solar? This is seldom a concern except on large commercial projects because as it turns out the ideal flow rate to filter a typical residential pool is about the same as the ideal solar flow rate.  But what is the ideal flow rate for solar?

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  If we put the water through too fast will it not get a chance to be heated?  True of false?  We hear that surprisingly often. The answer is false.  Another version of this same misconception is the idea that you can use a solar collector ten times smaller  and therefore ten times less expensive by simply running the water through it much slower.  Wouldn’t it be great if all we had to do was pee in the pool and it would suddenly become body temperature?

Understanding the ideal solar flow conundrum requires a change of thinking. You must think in terms of energy not just in terms of temperature. What’s the difference? Energy is what takes your pool from 75F to 85F.  A lit match is 2000F. Its fire. Fire is hot. Obviously a lit match under your pool is not going to raise the pool temperature.  Heating the large volume of water requires a large volume of fire. Energy is volume and temperature not just one or the other.   Put the idea of temperature out of your mind for a moment and consider some basic science. If we put the flow through the solar panel very very fast then what we have is a solar collector that is the same temperature as the pool water entering it. The temperature of the big black solar collector is as low as possible. It feels cool to the touch.  If it was hot, that would tell us there was no water flowing through it. In fact this is how we can check for correct solar pool plumbing. We feel across the width for hot spots. A hot spot is an area of no flow.  Anything in the sun is seeing the full power of the sun. The sun has about 1kw of energy in it per sq m exposed directly to the sun’s rays. If the collector is cold it is not losing as much heat as it would be if it was hot. A hot solar collector surface in the sun is at a high temperature because the energy is not being taken to the pool. The collector is heating up not the pool. The collector temperature rises until it is hot enough that the heat the collector is losing to the air equals the heat being gained from the sun.  

 

The ideal solar flow rate is as high as possible. 

 

The upper limit is not flow. It’s pressure. If we put too much flow through solar we will load the pool pump with higher and higher pressure. Higher pressure costs more electricity and causes noise and wear on all pool equipment in the circuit including the most vulnerable piece of pool equipment, the lungs of the system, the solar pool heater. We admit it! The thin black tubes making up the heat exchanger with the 5000 degree sun is the weakest link. 170F is entirely possible on a black thing in the sun with no water taking heat away. Every solar pool heater out there achieves maximum solar setpoint in the summer because it’s sized for the spring. It will sit stagnant in the hottest summer sun getting very very hot. We better not combine that stagnation temperature with excessive pressure. But there is no flow in the off condition. How can there be pressure?  Understanding pressure in the pool system and the solar system and how they relate  is key to understanding the optimum or ideal operating condition.  Pressure is also known as head and can be expressed in distance units. The pressure 10 feet underwater is 10 feet of head. The pressure at the bottom of a 10 foot tall pipe full of water is 10 feet. The pressure at the top of that water column is zero and the pressure half way down the column is 5.

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Let’s start with a pool that has no solar heater.  The pressure on the filter is actually the pressure in the pipe between the pump and the filter. This is always the highest pressure in the system. Downstream as we go through more restrictions like gas heaters and pool inlets, the pressure drops. There is a pressure drop of 5 psi through a gas heater. This pressure drop or restriction to flow is there on purpose as a safety to make sure the gas heater doesn’t fire when there is no flow. There can’t be pressure drop through a gas heater unless there is flow, good flow, safe flow.  Installing  pressure gauges throughout a pool mechanical system to see the pressures is no different than teeing in pipes that run vertically and measuring the height of the column of water. This type of pressure gage is called a manometer. One foot of head equals 0.433 psi.

If we turn up the speed of the variable speed pump all the pressures in the above system will rise. The water columns will all rise. Next let’s add a solar pool heater but let’s not turn it on just yet. If we install the solar system such that it is underwater then it will be under head or under pressure. This is with solar off. The pressure is caused by the restrictions downstream of where we install solar in the  pool plumbing. It doesn’t matter how we set the solar bypass valve because there is no flow to bypass. Solar flow is zero.  This is the most important pressure condition because this is the pressure with solar off and this corresponds to when the solar temperature can be very high.  Its not nearly as risky or damaging to the solar collectors to see pressure when solar is on because solar temperature is close to pool temperature when solar is on.  This is the primary consideration, not solar flow. We haven’t turned solar on yet. We’ll get there.

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If we install solar 20 feet below pool level we expose solar to 20 feet of head. That’s too much. It just is.  It would be nice if we could just pretend that black super hot plastics can take 20 feet of head but the harsh reality is that regardless of what the manufacturer thinks their pressure rating is, they are going to have a problem at 20 feet of head. We’ve seen this hundreds of times. Its what we always check for when we have a service call on any system, ours included. In fact we do consider 8 psi or 20 feet of head (0.433 psi equals one foot of head) to be our upper design limit. If the bottom of solar is 20 feet below the top of the pool then the pressure is too high for a solar pool heater regardless of the flow condition which can only serve to increase that pressure.

If solar is under pressure before we divert the flow through the solar panels an interesting phenomena can occur. If solar is above pool level and the pool circulating pump is off solar will empty and fill with air from the vacuum breaker. The vacuum breaker lets air in when the solar pressure is below zero to prevent damage from negative pressure. Negative pressures can be just as damaging as positive pressures. Your hot water tank can collapse if it is under negative pressure. With an empty solar system the pool pump will turn on and the air in the solar system will compress. Now you have a compressed bubble of air in the solar panels and since air is  water the pressure in the solar panels will be same as the pressure at the bottom of the water column. Pressure is not reduced by the height of solar above the pool until the bubble of air is released.  The biggest problem in solar pool heaters leaking is this pressure pulse as solar starts up and this is one of those freak accidents that is seldom understood by the diagnosticians. It only really shows up when a system is elevated maybe 3 stories and if the return line check valve doesn't seal. This is why we put a pressure gage on the roof as part of the system check but we can also just look at the filter pressure gage. If the pressure pulses upward a lot as solar starts up we know that pressure pulse is seen on the solar panels as well. Its not usually an issue but its worth checking.

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The minimum solar flow rate is 2 gpm for every 100 sq ft of solar panel but you don't need to measure that with a flowmeter.

 

As we close the bypass valve or turn the 3 way valve to divert flow to solar we are resisted by the fact that the air bubble does not want to flow down the return pipe to the pool. Water is forcing the air bubble down and air is trying to float. There can be a significant pressure pulse in the solar heater as the water is trying to force the air down and out.  This can mean that we have to close the diverter or the bypass so far that when solar does push all the air out the flow is so high that we develop a different kind of pressure known as the dynamic pressure. This is the pressure caused by the restriction to flow through the solar system itself.  Too much flow means we create too much pressure in the solar panels again loading the pump excessively and stressing the components.  Consider that a pool filter might be 30 inches in diameter. At its maximum rated pressure of 50 psi we can calculate that the force trying to blow the filter apart is the area times the pressure.  Inches squared times pounds per square inch is pounds force. In this example the force is 35,000 pounds. When we see a filter operating at 50 psi we duck! We have seen many a filter leak at its rated maximum pressure. Pressure is our enemy in many respects especially when that pressure translates to pressure (stress) on the sensitive solar panels.  So it is important to pay attention not just to static pressure, the pressure caused by the pool mechanical system, but also the dynamic pressure, the pressure caused by the flow through solar. Relative to the solar system, pool system pressure is considered static pressure. Its always there when the pool pump is on. Static pressure in the solar system is actually dynamic pressure in the pool system.

The maximum flow allowable is the flow that causes restriction to flow through solar to become apparent as a measurable pressure drop.

 

You can usually divert a lot more water or speed the pump up before the system pressure even reacts.  You can add as much as 5 psi but its usually easy to keep it below 3 psi. You have to look at the pressure pulse upon start up as well.  If that isn't below 5 psi then schedule the pump to be on before solar activates. Its possible to create a worse pressure pulse upon solar start up if the flow is too low. You want the flow as high as possible but not so high you create significant dynamic head. Flush the air out, then see the pressure come down, then keep increasing solar flow until you see the pressure start to go up. You can do this just by looking at the pressure gauge on the filter.