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Pipe Size For Swimming Pools

Pipe Size For Swimming Pools?
Does pipe size matter for swimming pools? Bad news guys! As it turns out pipe size does matter for swimming pools and filtration systems. All innuendo aside, the smaller the pipe size, the faster the velocity of water for a given flow volume. 1.5" pipes are more efficient than 1.25" pipes, just like 2" pipes are more efficient than the 1.5" ones. On pool systems you might only see 1.5" and 2" pipe sizes used however when you look to larger pools and with commercial pool systems you will find pipes a lot bigger than this.


What is the point of diminishing returns on oversizing your pool plumbing system? That is a hard question to answer in a general sense but when you calculate efficient flow rates and apply them to accepted industry standards for filtration you start to see a repeating pattern with residential swimming pools. Residential pools, more often than not, have poorly designed hydraulic systems which are needlessly inefficient, and the pool is operated such that established filtration minimums are not met. But your pool stays clear pretty much all of the time and honestly you hardly even notice the pool pump electricity when it gets mixed in with all the other bills you pay. Does all of this really matter? It really does. Pipe size makes a huge difference for pool systems.





One of the very worst offending mistakes that pool owners make is to buy a pool pump with too much horsepower. Why would someone do such a thing? The answer is because they have a "bigger is better" mind set. They large size their combo at McDonalds. Their car has the bigger motor upgrade option. They want an impressive, quality pool pump and they don't mind paying a bit more to get it. The only problem is that a bigger horsepower pump is actually specifically un-useful for pool filtration. A well designed single speed pool pump on a low head resistance pool would probably be a high efficiency 0.5 HP pump. Now you would for sure use a variable speed pump, but traditionally speaking if you actually did all of the math on flow rate and turnover and sized the pump to the filter accordingly, easily a 0.5 HP pump is going to do the job well and for little money.


too many horsepower
So knowing the true sized pump is only 0.5 HP why would the pool have been built with a 1 or 1.5 HP pump? Probably because that is what pool owners expect to see, and the people who built the pool did not calculate flow rate, or explain turnover rate, or size the filter properly to the pump. They just know that a 1 or 1.5 HP pump will make most customers happy and keep most pools clean. Then when that pump fails the new pool owner wants an even better pump so he buys a 1.5 HP or 2 HP pump to really impress the neighbors. The filter sustains damage from too much flow and the pump costs more money than the last one to run.


The mentality of this person who bought the larger pool pump would have been entirely better off had they upsized the pool plumbing instead of upsizing the pump. The pump is already big enough on your pool system for more than you will ever ask from it. If you have much larger pool pipes you would have a system that moves way more water than other pools and it will do it for way less electricity cost. Like a car that goes a million miles per hour and runs only on air. You would want that, right? Then you should want bigger pipes for your pool. More flow potential, less electricity used, lower system efficiency losses.


I think it warrants mentioning that guys really missed an opportunity here. Instead of buying more horsepower the right move all along was bigger pipes. How is this a thing that we missed out on? Every dude out there should be bragging about how big their pipes are but instead here we all are with tiny 1.5" pipes and a hot rod engine that you can't get out of second gear. I'm disappointed guys, we can do better than this. Next spring I want you all out there making your pipes as big as they can be on your pool - no more puny pipes!


Efficient flow rates - You can move water through a pipe up until a certain point, past which there is a sharp loss in overall movement of water versus energy it takes to move the water. At slow speeds there is not much in the way of turbulence or friction that prevents the water from flowing. Past the velocity of approximately six feet per second (6 fps) there is a sharp increase in system efficiency loss. The water begins to swirl and create vortices and turbulence, all of which counteract smooth flow of the water. Friction especially is a concern and measured as friction losses in a plumbing system. As water velocity increases, friction and losses within the system increase. Take a look at these values for maximum flow efficiency at 6 fps for various PVC pipe sizes:


1.25" = 28 GPM

1.5" = 38 GPM

2" = 63 GPM

2.5" = 90 GPM

3" = 138 GPM

4" = 238 GPM

6" = 540 GPM


These numbers should stick out in your mind as looking funny. Look at how much difference pipe size makes to total potential efficient flow. Just going from 3" pipe to 4" pipe raises your total potential for efficient flow from 138 to 238 gallons per minute. It does not seem that way intuitively when you think of such a small change from 3" to 4" pipe size. This is the nature of the flow equation and why pipe size is so important in your swimming pool. Consider that you should filter the volume of your pool at minimum three times every day, but you want to only use efficient flow, not more expensive and wasteful flow numbers. Look at the maximum pool volume that you can adequately filter using efficient flow values only:


1.25" = 13,440 Gallons

1.5" = 18,240 Gallons

2" = 30,240 Gallons

2.5" = 43,200 Gallons

3" = 66,240 Gallons

4" = 114,240 Gallons

6" = 259,200 Gallons



These numbers assume that you run your pool pump 24 hours per day. If you only run your pump eight hours per day then you can divide these volumes by three to find out the maximum pool volume you can efficiently filter. As you can see these numbers do not line up in the slightest with real world swimming pool applications. How many pools out there larger than 18,240 gallons are built using 1.5" pipe? A lot. A whole lot. So why does this happen? Simply because nobody is taking the time to calculate flow rates or to pursue efficient plumbing systems.


When you visit a "high end" pool installation this is one of the things you might notice about a pool someone has paid a lot of money for, in that it will definitely not be built using 1.5" pipes and having 2.5" to 4" pipes for the plumbing system is normal even on "normal" sized pool installations. When you design with hydraulic efficiency in mind you start by designing a plumbing system that can handle three times the volume of the pool through the filter every 24 hours. Better still is a system sized to have four turnovers per day instead of three which nets 98% of all of your water being filtered instead of only 95% with three turnovers per day.


Should you upgrade pipe size on only some parts of the pool? - This is a very common question from existing pool owners who have come to realize that their pool system was not designed for efficiency. If you re-plumb your 1.5" pump, filter and heater with 2" pipe will you have any benefit since you will still have buried sections of 1.5" pipe that you are not able to access to replace. In the event that you can not facilitate a full pool renovation to increase the pipe size for your whole system you can still benefit by upgrading what you have access to on the pump pad. Most of the valves, unions, twists and turns, as well as flow disrupting equipment are all located on the equipment pad. Yes you will have limitations from the sections of pipe that you can not replace, but you can slow down the water velocity and limit your efficiency and friction losses through the section of your plumbing where the vast majority of friction losses are. There is a net positive result in re-plumbing even existing systems where you can not remove all 1.5" pipe sections completely.


Yes you can move a lot more than the volumes of flow listed here through a PVC pipe. A single 1.5" pipe only moves 38 GPM efficiently but can easily move in excess of 90+ GPM when you hook up a large 1.5 HP or larger pool pump to it. This is how so many pools meet filtration demands with pipes that really are too small. You just drive extra water through the pipe and ignore the efficiency losses. At least that is what pool owners used to do. Now with variable speed pool pumps becoming law for filtration systems on all swimming pools it is suddenly much more important for you to know how much water your system is moving, because you can (and should) change the amount to be an efficient number. A flow meter is more or less required equipment for a modern day swimming pool, because this is the tool that allows you to know how much water your system is moving at any given time, and when you apply the information you learned in this article you now know how to vastly increase the performance of your pool while reducing the cost you pay to run it. That's a good deal.





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Swimming Pool Steve

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