Variable Speed Pump Schedule
More and more I am being contacted by pool owners who have upgraded their equipment pad and want to know how to schedule a variable speed pump for the most possible efficiency. The problem is that this is anything but a straight forward question to answer. It might seem straight forward enough - what RPM should you set the pump to run at? In order to answer this question it would be best to give an example of a real world situation. So here is an email communication with a pool owner who contacted me looking for information on the best way to program the run schedule for their new variable speed pump. Here are the pertinent details of the situation:
pool volume 13,000 gallons
pipe size 2" suction & 2" returns
pump Hayward TriStar 950
filter WaterWay 150sf cartridge
heater 100,000 BTU electric heat pump
peripheral salt chlorine generator
This pool owner wanted to know a simple question - "what RPM should I run my variable speed pump at?" - The answer is surprisingly long desipte sounding very straight forward.
In order to answer this question in a meaningful way that actually helps you with this problem there is a lot of information we need to cover. Your pool system (all pool systems) are more dynamic than you are currently appreciating. It is not technically possible to answer your question with a sentence or two. That being said, I will try to provide some context and useful information for you here.
How much water do you need to filter - You have some hard figures to work with so let's start there. Your pool is 13k gallons. By commercial standards you need to turnover your pool every 6 to 8 hours. By commercial standard this means the commonly accepted commercial swimming pool filtration requirements. For example you need three to four turnovers of water every 24 hours. For this example we will use three turnovers for our calculations. This means 39k of water pumped through your system every 24 hours in order to filter all of the water in your pool at least one time daily (95% of all water has been filtered at least once on the third turnover). By the "average" residential pool standards (not a real rule, but what most pool owners actually end up doing at the end of the day) you will need to pump at least 13k gallons through your filter every 24 hours, which is equal to one turnover.
All of that seems really confusing...so how much should you actually filter the water? That depends on what kind of person you are, how much use the pool gets, how well the chemicals are balanced, if there has been or will be rain...lots of factors. Ultimately if the water turns green, you are probably not filtering enough. If you go bankrupt, you were running the pump too much. But lets try to hone in on an answer more useful than that!
The "real" way to know how much to run your pump is by monitoring flow rate. I don't believe that model of pump has a built in flow meter readout and this is needed to actually calculate anything. Without actual flow rates we can only guess. Informed guesses, but ultimately still a guess.
Even if you perform an unnecessarily complex calculation to determine resistance to flow in your system, you would only be taking a momentary snapshot of the resistance to flow forces in your unique plumbing system, as the changing filter conditions will invalidate those static readings almost immediately.
How to actually monitor flow rates - At the end of the day the entire industry approaches this the same way. Install a flow meter, which maybe less than 1% of residential pools have, or simply go with conventional standards and try to get within the ballpark of the right amount of filtration. If you want to add a flow meter then this is undoubtedly a useful item to have. Possibly the most useful item that most pools don't have. If you want more information about these flow meters you can read this article that explains how to choose a flow meter for your pool. Without a flow meter there are going to be a few assumptions we need to make. Let me walk you through how I would do this if it were my pool.
1) What does the suction manifold on your system look like? Specifically how many suction pipes do you have, and what size pipe is it?
The pipe size in your pool will determine the flow you can achieve more than any other factor. If you have 2" pipes instead of 1.5" pipes then you will probably achieve double the flow under maximum flow conditions. For this example we will consider 2" suction and 2" return lines, with multiple suction lines and multiple return lines entering the suction and return manifolds.
2) How many return pipes go back to the pool? What size are they?
The number of suction pipes and return pipes you have will also make a difference to the total flow rates. Multiple suction and return pipes will have better flow dynamics than a system with only one suction and one return pipe, even though there is a limiting factor of all of the water channeling through a single pipe restriction on the equipment pad. More pipes equals more potential for flow.
3) I like to aim for 1 turnover per day. I keep my pool at 80, clean, and basically nobody uses it. If you keep yours at 90, or tons of people use it every day, then you should be increasing it above one turnover per day. So the question is what is your pool temperature and how much use does your pool get? Do you want to try to run your pump the absolute minimum amount or do you want to be well within the accepted amounts to ensure you never have a water quality problem?
The more use a pool gets the more it needs to be filtered to help keep it clean. While chemical sanitizing is a different process from filtration of physical debris, these two things work hand in hand to keep the water clear. If you introduce factors that add debris or bacteria load to the water, like lots of swimmers, then you also need to increase the chlorine you are adding and the amount you are filtering the water.
So what do the answers to these questions tell you? It gives a general perspective as to what type of pool system you have. There are not that many pipe configurations or pipe sizes to choose from so you will fall into one of a couple different categories. Above ground pools have one pipe in, one pipe out systems in the vast majority of cases. They will be 1.25" or 1.5" pipes. Inground pools will typically have 1.5" pipes for older pools and more likely to have 2" pipes on newer pools. Smaller pools will have one or two pipes in and out, where more complex pools will have three, four or five pipes coming in and out of the suction and return manifolds.
The point of these questions would be to help better understand more about what type, and quality, and age, and efficiency your pool is. Most specifically I will be able to estimate whether your pool has a low amount of resistance to flow, a medium amount of resistance to flow, or a high amount of resistance to flow. I apply this when estimating flow rates. Pump curves show approximate flow rates for given system resistance, measured in feet of head. Low, medium and high resistance to flow might equate to 20 feet, 30 feet and 40+ feet of head resistance when reading a pump curve. All of this falls well outside the definition of scientific, but does help to glean a better understanding of your pool system as a whole when making an estimate for expected flow rates in a pool system without an actual flow meter.
Pool Pump Programming Schedule
Moving ahead using a single turnover of 13,000 gallons daily we can divide by 24 to determine the hourly flow rate. This in turn is used to calculate the gallons per minute flow rate that you would need to meet a single turnover of your pool water. You definitely do not need to filter your water 24 hours every day but you certainly can say that it is optimal for the pool if you do so.
13,000 gallons / 24 hours = 542 gallons per hour if you run the pump 24 hours per day (or 1084 gallons per hour if I only want to run the pump at night for 12 hours)
542 / 60 = 9 gallons per minute of flow needed if the pump runs 24/7 (or 18 gallons per minute if the pump only runs 12 hours per day)
Remember this is all for a single turnover daily. If you want to meet commercial standards then triple these numbers, if not quadruple, but most residential pools are in the one turnover daily range, for better or for worse. To put it another way, as a pool professional I want to say filter the whole volume of the pool three times every day, four even, but if you only filter the volume of your pool once every day then the water probably still won't turn green.
Why you can't just run your pump on low RPM all the time - Before we go further we need to recognize something here. If you ran your pool pump on a super low RPM 24/7 and made 9 GPM of flow, the pool might get 13k gallons turned over every 24 hours...but the heater would never fire up, and your skimmer would be largely ineffective and you would have debris floating all over your pool. Also probably the flow switch on your salt cell would not close and you would have trouble making enough chlorine for your needs. This is why a proper variable speed pool pump schedule includes periods of time where the pump will run higher than the minimum RPM values.
If you had a flow meter you just dial it in to 9 GPM and set that as your base operating speed. Without one you can make an estimate. A 2" PVC pipe can move around 80 GPM total. Your pump can move around 160 GPM if you had a perfectly designed hydraulic system, which you don't because it would take pipes in the 4" and 6" size minimum to achieve these flow rates, but you could estimate that under max conditions your flow rate would exceed 80 GPM by a bit. Assuming you have at least one section of pipe somewhere in your system which throttles down to a single 2" pathway, which all systems do, you will not likely exceed these soft maximum flow rates by very much regardless of how powerful your pool pump is.
80 GPM x 60 minutes = 4800 gallons per hour, theoretically with your pump running on full. This means you could make a 13k turnover in as little as 2.7 hours or so. This is how all "those people" with older pools and 1.5 horsepower single speed pumps get by only running their pump an hour or two per day. When a pump is running on full speed it moves loads of water, quickly, just not very economically.
Balancing savings with performance - So here is the crux of the situation, when you turn the speed up on your pump you easily have more than enough flow to keep your pool clean. Unfortunately this costs a lot of money. You could run your VS pump on 1100 or 1200 RPM for around 64 hours for the same electrical costs as running it on full speed for one hour. This is why variable speed pumps are so great, and save so much money, but misusing one can basically invalidate all the savings potential.
So how to apply all of this to your pool? First, let your pump run 24 hours per day on the lowest RPM setting that results in water moving, and at least a few PSI registering on your pool filter pressure gauge. Now we know essentially that you are going to make your filtration demands because something like 9 gallons per minute, which is what we calculated you need for a single turnover for your pool, is EASILY going to happen even on low RPM settings. In reality water flows very easily at low speeds and low turbulence...far more efficiently than at high speeds where inefficiencies like turbulence and friction heavily reduce how easily the water can pass through your plumbing system.
Establish your unique minimum RPM setting - Not all pools can run on the absolute minimum RPM setting that a pump can technically operate at. Flooded pump systems (pumps situated below the height of the water level of the pool) are the most efficient and can operate on the lowest RPM settings. The further your pump is from your pool, and more importantly, the higher your pump is above the water level of your pool, the higher your RPM will need to be to actually get your system moving water instead of just vibrating and making noise. Look in the pump for moving water, feel in front of the returns for moving water, and finally, use the pressure gauge on your filter. If it says flat zero then you might need to jog up the RPM a little for your minimum value.
Running Pump On Low Speed Constantly
If you only need to consider filtration and turnover rates then you already have all the information you need to adjust your minimum flow RPM values. However most pools have peripheral items like gas heaters, electric heat pumps, ozone systems and salt chlorine generators and all of these items require flow before they can operate. But this is a good thing.
If a salt system, or a pool heater, were to continue to run when there was zero flow in your system then this would almost certainly result in an explosion. As a safety mechanism these items all have built in flow meters which detect flow rates or systems pressures before allowing the appliance to turn on. What this means for programming your new variable speed pump is that you will need to adjust your run schedule to match the needs of your unique pool system.
Each pool system is completely unique - The amount that you need to run your pool at higher RPM's, and just how high those RPM's need to be, will be unique to every single pool installation. No two systems will be exactly alike, so I can not simply tell you 1700 RPM so your heater fires. Maybe your heater takes 1400 RPM to fire...or maybe it takes 2700 RPM. This depends on your plumbing system, your pipe size, equipment orientation, total dynamic head, flow rate, your elevation above sea level - so much more than you might expect looking at this for the very first time. So how the heck do you figure all of this out? Easy, induce a call for heat (turn up your heater) and slowly turn up the RPM of your pool pump until the heater fires up.
Variable speed pump schedule for gas heaters - If my heater fires at 1500 RPM I usually would add some buffer into that, like 1750, and call that my heater RPM. maybe even 2000 just to be safer and account for changing system flow rates and pressures. You can simply program your pool pump to run for a few hours per day above this RPM level, and at that point the heater will fire up. The rest of the day even if the heater is calling for heat, the pressure switch will not be activated by the low flow situation and prevent the heater from firing until flow increases. Preferably you would have an automation system to act as an intermediary between your pump and your heater and this would allow your heater to call for heat, and your pump to respond in kind by ramping up RPM to allow for the heater to run for a while, but without automation you will need to use the internal pressure switch on your heater as your form of control over when the heater lights up. I would not run my pump anywhere near to the RPM needed for the heater during most of the day - you don't want your heater trying to light up all day long.
Variable speed pump schedule for salt systems - If you have a salt system, an electronic chlorine generator, this variable speed programming issue becomes harder. A natural gas pool heater can heat up a heck of a lot of water in an hour or two of run time, but a chlorine generation system more or less needs to be running most of the day in order to manufacture enough chlorine for your pool. This can severely hamper your ability to get the most electrical savings from your system as you may be filtering enough to keep the water clear, but not enough to generate the chlorine you need. This would result in running your pump more often at higher speeds in order to close the flow switch in the salt system just in the same way as the pressure switch in the heater detects flow.
Variable speed pump schedule for heat pumps - A heat pump takes a long time, specifically, in order to generate heat for the pool. This means that you will not likely be able to run the pool at super low RPM most of the day. This heater will require many hours of sustained higher RPM in order to run continuously throughout the day and slowly add heat to the pool (similar to how a salt system needs to run all day to slowly make chlorine). With electric heat pumps and salt systems what this means is that you will never realize rock bottom electrical costs for your pool...but that is OK because you don't have a regular pool. You have an upgraded pool. It is okay that you will have to run your pump a little higher RPM to get your heater running. The fact that you are running at ANYTHING less than full speed, all day long, means you will be saving money versus pools of even 10 years ago.
The logic is simple. You don't buy a Porsche and run the engine so lean on fuel that you break something - all in the name of trying to get better gas mileage. You really have the wrong pool for "good mileage" on your pump if you have an electric heat pump and a salt system. You have pretty much the worst case scenario for trying to run the pump on a lean schedule. Every part of your pool more or less needs lots of flow. The bottom line is you will save on your electricity bill, but just slower/less than a person with a pool with less bells and whistles who can get away with running their pump a little less or lower RPM speeds.
In total, I am sure you appreciate more now how it is not really possible to provide a definitive answer about what RPM to run your pump at, but rather this long explanation about how it all kind of comes together in a general sense. Again, installing a flow meter takes a lot of the guess work out of this, but does not change hard values like how much RPM your heater will require before it turns on and stays on, or how many hours per day your salt system needs to run to maintain your free chlorine levels.
Example 24 hour variable speed pump schedule
2 hours @ full RPM (80 gallons per minute, but could be as high as 100) = 9600 gallons
10 hours @ 2000 RPM = (50 gallons per minute) = 30,000 gallons
12 hours @ 1000 RPM (28 gallons per minute) = 20,160 gallons
This real world run schedule of two hours at full speed for heater firing and vacuuming, 10 hours at 2000 RPM for the salt cell to be working, followed by 12 hours of minimum RPM running which costs almost nothing, but adds an extra 20,000 gallons of water filtration to your 24 hour schedule. In total this represents 59,760 gallons filtered in 24 hours. While these numbers are just examples this should still give you a good idea as to how you go about calculating your flow rates, filtration requirements, and how to program your variable speed pump to allow for your peripheral items like heater and salt system to operate.
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