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The Marriage of a Solar Heater and a Swimming Pool's Mechanical and Plumbing System


If you're installing a pool or changing a pool pump don't make the mistake of thinking you need more pump power to accomodate solar. The opposite is in fact the real concern here. Up until very recently, pool pumps have been getting larger and larger as pool builders strive to outdo each other in an attempt to sell their pools. To be fair, pool building is a trade and it is competitive. Nobody wants to talk about what you can't do and if you size the pump properly for filtering it means that you can't run the vacuum at the same time you are skimming the surface of the water. It means you can't have a big dramatic waterfall from the spa to the pool at the same time the inlets are shooting water across the surface of the water. So instead of talking compromise or how to work around these very minor issues, the pool industry has simply used large enough pumps to do everything at once... and then some!! Our view is that operating cost is a much more significant issue and should be the starting point of any pool mechancial system design. Skip Phillips is past NSPI president, and one of the founders of the prestigious Genesis3 design schools for pool builders. He teaches the philospohy that you want a low horsepower pump. Pentair, one of the largest pool equipment manufacturers in the world now professes this same philosophy. Years ago there were very few of us promoting this idea but suddenly in California, with Governor Schwarzenegger's energetic attack on global warming, we're starting to see low HP and variable speed drive pumps becoming mandatory. It's all good for everyone involved and its good for solar because low HP means oversized pumps working against undersized pipes are no longer going to be overpressuring solar panels. But this is all just California and all just starting now. Meanwhile, we have very tricky pool mechanical systems we have to find a way to marry solar to and even with VFD pumps, these suckers are still capable of 3HP so we still have to be very careful how we add solar.

How to size your pool pump is a brief discussion of how to specify the size (HP) of your pool pump. This is essential reading for anyone having a new pool built because you do not want to oversize a pool pump for any reason. Contrary to popular belief, solar does not require a larger pool pump.

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Understanding Pressure

Pressure is what this document is all about. If the pump has to create too much pressure to raise the water to the solar panels then we're loading the pump too much with the solar heater. If the pump has to create too much pressure forcing the water through the solar panels then that pressure represents stress that we need to avoid. If the solar panels are stressed with too much pressure from the pool system then we have a problem too. It's all about pressure.

The limitation to solar pool heating that nobody wants to admit is that solar panels can not handle pressure over the long term. Solar heaters last 30 years but only when set up without the pressure. We've been called out to replace/repair hundreds of systems 10-30 years old. We always check pressure and we can tell you with certainty that pressure plays a role. It's obvious, really. Pressure causes stress. Nobody in this business understands what pressure is in the first place and negative talk like this is a sure way to lose a sale. Solar panels can get very hot when they are sitting idly in the Hot Sun, especially if they are full of water. Collectors are purposely designed with a thin wall of plastic or rubber. If we made them a quarter inch thick they wouldn't operate very efficiently. We need a jet black surface and we need it to be evenly pool temperature. A cold black thing in the sun is a perfect solar collector. If we're taking a large portion of the flow from a pool mechanical system and diverting it through dozens of feet of pipe and spreading it out over a huge thin black plastic area on the rooftop we're really asking for trouble if we pressurize that system too much. It takes next to no pressure to get the water to move through the solar collectors because the collectors don't restrict flow very much so don't make the mistake of thinking we need pressure to make this system operate properly.

All the pool pump has to do to activate solar is raise the water to the solar panels. From there the water falls back to the pool by gravity. We don't try to make a closed loop where the water falling back down pulls the feed water up creating a syphon effect meaning the pump doesn't have to do any work. No No No. That would create negative pressure in the solar collectors and a negative pressure (a vacuum) can collapse pvc plumbing when it gets hot and hot it will get.

To be completely honest we shouldn't even be using pvc plastic plumbing in a solar heater because the temperatures can get hotter than pvc can handle. Even if we go to a metal collector and accept the large efficiency penalty we still shouldn't operate the collectors under pressure because we're still dealing with PVC pipe. We get away with exceeding the normal temperature capability of PVC by taking the pressure out of the equation. We can't avoid PVC by the way. Your pool is plumbed in PVC and that becomes part of this system.

In designing anything with any plastic any competent engineer knows that you can design for pressure or temperature but not both. That's what metals are for.

First you must understand what pressure is and where it comes from. You don't really need to know this. You just need to know that your solar designer does so here it is for the sake of completeness. Pressure is sometimes called head and measured in feet instead of psi. That's because pressure and height are interchangeable. The conversion is 0.433 psi equals one foot of elevation difference. The pressure 10 feet under water in the ocean is 10feet x 0.433 equals 4.33 psi. It doesn't matter if the ocean is the size of the Pacific or the size of your backyard pool or the size of a 2" PVC pipe, the pressure 10 feet under water is still 4.33 psi. When we're looking at a 2" PVC pipe filled with water the pressure 10' down is 4.33 psi. Therefore a great way to measure pressure is with a column of water. We simply tee in and see how high the water sits above that point. In the animation here you can see that when the pump shuts off the water level drops right down to pool level. Zero pressure with the pump off. When we turn the pump on we see that the pressure is highest right after the pump, before (upstream of) the filter. Restriction to flow downstream is what causes pressure.There is restriction to flow through a filter as well as through a gas heater as well as through the inlets where water enters the pool. If the plumbing is too small for the flow then you'll also have enough restriction to "drop pressure" through the piping and fittings. If we change the pump to a smaller HP model we reduce the flow and that in turn reduces the restriction to flow through everything in the system and brings all the pressures down.

What we want to do is tie solar in where the pressure is lower than the height of the collectors...

If we tie solar in where the pressure is lower than the height of the collectors then we can do a conventional solar installation in which case the collectors drain down at least somewhat when solar is off. If the collectors are draining down with solar off and the pump still on then we know for sure there is no pressure in the collectors. The only way we'll have pressure in the collectors is if we send too much flow through them. Our designs all call for a bypass valve so that we have full control over the flow rate. We never have to worry about too much flow causing pressure . We adjust our solar bypass so that we don't send so much flow to solar that we create pressure through the solar plumbing. With solar tied in where the pressure is lower than the roof height and the bypass valve adjusted so we aren't sending too much flow to solar we have a properly balanced smooth operating solar heater than can operate freely without stress. That's good design practice. That's the difference between a 30 year old solar heater that has paid for itself 15 times over and one that requires regular "maintenance" and is abandoned after 5-10 years.

Note that if the collectors remain full of water when solar is off there can be a tendency for the water in the collector to migrate. For example if the sun is out and solar is off the sun heats the water in the collectors and causes the water to rise (hot water rises). That induces a flow forwards heating the pool that we don't want. Conversely if this situation occurs at night an induced flow can occur backwards cooling the pool. We've discovered this effect can be quite significant taking away as much as 30% of the daily gain. Induced flows can be controlled somewhat with one way valves and positive sealing valves and even by shutting the pump off at night so this isn't a really big issue. The important thing is eliminating the possibility that a pvc pipe will overheat and with pressure, empty the pool. 10 psi is 22 feet of head. 10 psi inside a pool filter that is 30" in diameter translates to over 7000 pounds force trying to blow that filter apart. 10 psi on a 2" removable coupling at the end of a bank of solar collectors is 49 pounds force trying to blow that rubber connector off that pipe. 10 psi is too much pressure to subject any solar collector to long term in real life. All our products have passed all the testing agencies pressure tests with flying colors but those tests are all meaningless. They don't test real life conditions. Manufacturers come up with their pressure ratings by doing short term tests indoors without raising temperature. Manufacturers sometimes believe it is less expensive and more profitable overall to just provide the necessary warranty collectors rather than solve these problems up front. Manufacturers like selling through Hot Sun because the warranty rate from our systems is much lower. Real life conditions are the test bed I've worked in for decades. I've seen what happens to collectors when they are under too much pressure. Solar service is big business but it shouldn't be. Set up stress free, solar heaters are very reliable and almost never need any service over a very long life.

If there is no place where we can tie solar in where we can get the pressure down to an acceptable level and for some reason we don't want to change the pool pump then we go to a completely different solar implementation plan. We actually use the pressure in the pool system to drive the water to solar.

We don't have to divert water to solar. The water is already at the height of the solar panels and then some. All we have to do is allow the flow to rise up to the solar. We use valves that seal when closed so that we isolate solar from the pressure and flow when we want solar off. When we want solar on we allow the flow to go to solar. The tricky part to undestand is that we can't just let that solar heated water go back into the pressurized regular plumbing. Why? Because that pressurized regular plumbing is under pressure. The water will not flow from low pressure to high pressure unless the low pressure becomes a pressure higher than the high pressure. Therefore we need to deliver the solar heated water to a place where the pressure is low. This place can be a separate pipe to the pool or spa that a builderr brilliantly provided for you because he read this document. This separate dedicated return for solar can be the bottom drain of the pool if it isn't needed normally. It can be a line for a vacuum that you don't use often. You can only use it for solar or vacuum, not both at the same time. That may not be a significant compromise.

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If the pool has in-floor cleaning:

Cyclical stresses are far more damaging than constant stresses. If you absolutely must have the auto clean function then do yourself a favor and add a second pump/filter/skimmer and return to the pool so that the pool can be simply filtered with a smaller pump and that alternate filter system can power the solar heater. If you don’t want the second pump/filter and want to use the same pump that powers the autoclean then you need a separate place to deliver the solar heated water. It’s a good idea when installing an in-floor cleaning system to also have regular pool inlets and a 3 way positive sealing valve that can choose flow to auto clean or to regular inlets. Then we can tie solar in where the pressure isn’t so high and the only negative becomes that you can’t run auto clean and solar at the same time. You can automate the 3 way valve that chooses one or the other so solar is on when it wants to be and auto clean activates whenever solar isn’t on. That means your oversized pump is on practically all the time and it’ll cost you. Its also unlikely we won’t still have too much pressure in any case because the pump is too powerful for the pipe size when not restricted by the auto clean device. Smartest way to go is scrap the auto clean idea in the first place and stick to a nice simple pool system that will be reliable and won’t cost you an arm to operate. If you must have the auto-clean feature then have a separate pump and filter and skimmer dedicated to it. If you don’t want that big extra expense then the solution is a separate pipe to the pool dedicated as the solar return.

Set it up like this and the excess pressure in the pool plumbing will drive part of the flow thru solar when the motorized valve opens. You don’t have to divert any flow. It’ll flow because the pressure where solar is tied in is NOT lower than the height of the collectors.

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If the pool has a built-in spa:

If you do manage to find a builder that will do a two pump system try to get them to install the same plumbing size, the same number, size and type of inlets into the spa as in the pool. That way the pump sees the same restriction whether you're in pool mode or spa mode. The pressure is the same in either case and solar can tie in conventionally and operate in spa and pool modes harmoniously without any disfunction or marital trouble.

If the pressure is too high in spa mode and in pool mode, sometimes all we have to do is install solar after the 3 way valve that chooses pool or spa on the leg going to pool.

That way solar can only heat the pool and only when the flow is to pool (whenever you’re not heating the spa with gas). The higher pressures seen when the valve turns to spa are isolated from solar and all we have to be concerned with is restriction to flow downstream of this valve (on the pool leg) being too high (due to a pump that’s too big). Enlarging pool inlets often solves the problem. .

If the pressure is still too high then as an absolute last resort we can return the solar heated water to the spa. When we’re in pool mode there’s no pressure on the pipe to spa so tie solar return in there and we’re in

business. Just turn solar off before going into spa mode. Failsafes can be done electronically.

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If it isn't built yet:

In the case of a built in spa where we're going to have more pressure than the roof height (one pump system), a really cool effect (and solution) is to run a separate pipe through the spa wall during construction. This will be the dedicated solar return.

In pool mode some water goes to solar and returns to the spa and overflows to the pool. You can knock the flow down if you like and deliver warmer water to the spa. With this trick you can actually maintain the spa at a higher temperature than the pool at the same time. You can't even do that with your gas heater. In pool mode let the water flow at top speed and you keep the collector efficiency maximized and spa and pool stay at the same temperature. You do deliver more water to the spa and the waterfall to the pool in this case but you can plumb it so solar on or off IS the flow to the spa when you're in pool mode. Note the original spa bypass valve is now closed. This is such a great solution and great feature that I like to see it done on any pool/spa combination.

If you have any questions about pool/solar compatibility issues please fill in the form you'll find under Sizing. That way we'll have your relevant pool information along with your question.

Now that you've been on the home page and learned all about which collector type is best for your situation and learned all about plumbing and auto controls, you're ready to go directly to the pages where you can communicate directly to me the system particulars so I can finalize a price and answer any questions you might have.

The above link does include enough information that you can size and price the entire system yourself but its easier for you and more accurate if you let me help by filling in the form you'll find there.

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