What Size Pump Do I Need For My Pool?

What is a Pool Pump?

You’ll see a variety of equipment in your swimming pool equipment area.  The long, almost rectangular thing with a clear lid and a basket inside is your pool pump.  A pool pump consists of multiple parts.  They are typically broken down into two main parts:  The “wet end” and the “dry end”.  The dry end is the motor.  No water ever touches this part.  You will see a label on your pool pump with multiple numbers.  One is the frame size, one is the shape of the flange and one is the horsepower.  We refer to horsepower when we talk about the “size” of a swimming pool pump.  The part that holds the basket and has the clear lid houses the water and is often referred to as the housing.  The volute is also a technical term for this portion.  It contains a seal plate, a pump seal, an impeller, and a diffuser.  These components all work together to work your pump.  When we connect the dry end (the motor) to the volute (the housing) viola!  We have a pool pump.

What does a Pool Pump Do?

All swimming pool pumps, regardless of brand and size, work in the same way.  They are what creates the water flow needed to evenly mix the chemicals needed to sanitize our swimming pool.  You see, the water is drawn out from the pool through the open box area of your pool known as the skimmer.  Water can also be drawn by the drain (or drains) at the bottom of your pool.  The large one at the deep end is known as the main drain. 

Once in the plumbing system, the swimming pool pump pushes the water through the system, making many stops along the way.  For example you may have a salt system or a heater.  All swimming pools require a filter.  After the filtration process, the water is returned to the swimming pool vessel through the return lines.  Those are the nozzle holes that go around your swimming pool.  If you put your hand by one while the pool pump is running, you will feel the water coming out.  If you have a spa on the same circulation system, you will see this in the spa as well.

What Size Pump Do I Need?

As mentioned above, size is referring to the maximum horsepower of the pump. Pumps can come in many different horsepowers.  The smaller typically being used for fountains or water features.  Main circulation pumps for a residential, in-ground swimming pool would typically be in the .75 HP to 3 HP.  

Most people think “The Bigger the Better!”  But this is far from the truth.  Pool pumps are kind of like shoes.  Too big or too small and there will be issues.  By buying too large of a single speed pump, you will not only get higher energy costs, but can place strain on your plumbing and filter.  Both of these can only handle a certain amount of gallons of water at a given time.  

When sizing a pump we need to take into consideration the size of the plumbing, which typically comes in 1 ½” or 2” pipe sizes.  We also need to take into account the size of our swimming pool filter.  Each filter is only designed to effectively filter at a certain gallon per minute or gallon per hour per square foot of filtration area.  Most people think, big pool, big pump.  This is not always true if the plumbing and/or filter are of a smaller size.  For the purpose of this topic, we will assume the swimming pool builder took these things into consideration when building your pool based on the gallonage.  If you had a big pool, say 25,000 gallons, but only 1 ½” plumbing, you can’t throw in a 3 hp pump.

A simple way to look at it in respect to single speed pumps is how much horsepower (HP) will be needed to turnover the water in an 8 hour period.  A turnover is when every bit of swimming pool water passes through the filtration system.  We want that to happen once a day in cold months and possibly twice a day in the hotter months.  This can get expensive which is why more people are upgrading to Variable Speed pumps which are energy efficient.  They are even required by law in some states.

Knowing the gallonage of the pool is important.  There are formulas you can use to determine this.  Once you have the gallonage, you can determine the gallons per minute. You take the gallons divided by your turnover rate.  Then take that number and divide it by 60 (for minutes) and you will get your required gallons per minute.

Then you will need to know the average feet of head.  Total dynamic head is basically the resistance and length your pool pump needs to push the water through.  The more resistance, the larger the pump and/or plumbing required.  A pump’s flow rate is dependent on the feet of head.  You will need to measure all the length of your pipes and take into consideration 45 degree and 90 degree angles plumbed in as well.  

All of this is extremely confusing and can get very technical with math.  The simplest way is to replace your pump with what was there before, if it worked for you.  If you are building a new pool, ask the builder to make a suggestion. You can also go to your local pool supply store for advice.

Variable Speed Pumps

These are great in so many ways.  You can run it on any HP up to its maximum.  What I mean by that is a single speed pump only runs on 3,400 RPMs.  A variable speed pump (VSP) set to 3,400 RPMs will have a certain HP limit.  The beauty is you can go a little bigger with a variable speed pump because with a VSP you run it on lower RPMs, thus saving money!  Variable speed pumps are newer technology and save you a ton of money.  The good news is, they don’t have to break the bank.  If you have determined that you plumbing and filter can handle 1.5 HP max or 2 HP max, the American household name of BLACK + DECKER has you covered.  My favorite part is they are about half the price of other name brand pumps.  https://poolpartstogo.com/collections/in-ground-pumps/products/bdxbtvar200 .  See you poolside!

This article provides information on what pump size you need for your pool. And if you want to know the ideal schedule for your pool pump, read this article.