Understanding the Pool Circulation System

Your pool’s circulation system is the elements of your pool that take water from the pool, filter it, heat it, and return the water back to the pool. We’ll go over each of these elements and describe how they function and what you need to do with them.

Water outlets:

Water outlets are any pool opening where water goes out of the pool for circulation, some examples are:

a.) Skimmer: The skimmer is used to circulate water from the top of the pool and to “skim” debris off the water surface. It is critical that the skimmer be in good operating condition and functioning properly; the skimmer basket must be in place at all times and free from damage and the skimmer weir, (flap in the throat of the skimmer that maximizes skimming action), is in place and working freely. The skimmer is the place we recommend that you use to vacuum the pool to insure any large debris that may be vacuumed up is caught in the skimmer basket thus avoiding a potential plugged circulation line.

b.) Main Drain: The main drain is typically located at the deepest point of the pool and is used to lower the water level, however its primary purpose is to circulate water from the bottom of the pool. It is recommended that about 1/3rd of the circulated water come from the main drain. Some residential pools do not have main drains, instead relying on a deep water outlet to drain the pool and for extra circulation.

c.) Deep Water Outlet: A deep water outlet is located on the lower portion of the side wall and is used to circulate water and to lower the water level. A grate insert fitting must be installed to prevent suction entrapment.

d.) Vacuum Outlet: A vacuum outlet is installed on some pools and is intended to be used primarily as a suction port for vacuuming the pool. Extreme care must be taken when vacuuming a pool through the vacuum outlet so as not to vacuum up large debris that may become lodged in the circulation plumbing. We recommend you vacuum through the skimmer using a vacuum plate and skimmer basket if large debris is a concern.

e.) You may have other suction outlets designed into your pool for a unique purpose such as to feed a booster pump for a waterfall. The possibilities for this are endless and far to numerous to cover here. If you have any suspected water outlets beyond what we have discussed here consult with a reputable pool builder or service company for operating procedures.

Circulation Pump:

Your pool has a circulation pump that is responsible for moving pool water throughout the circulation system, (this is not to be confused with auxiliary pumps such as cleaner booster pumps, jet pumps, waterfall pumps, etc.). Water is drawn in from the pools outlets into the front of the pump, through the hair/lint trap, through the pumping chamber and out the top of the pool pump on its way to the filter. Two important things with pool pumps. First never run the pump without having the hair/lint trap basket securely in place. The trap basket collects all large debris in pool water before it can go into the pump impeller plugging it. Inspect the trap basket carefully looking for cracks or other damage that could allow debris through, replace if necessary. Secondly, never run the pool pump without first having filled the hair/lint trap with water, (“prime ” the pump). Most inground pool pumps are self priming pumps, this means that the pump is capable of pulling water up to the pump above the pool?s water level; However you still must fill the hair/lint trap first to give it an initial prime. Above ground pools and in rare cases inground pools have flooded suction pumps. These pumps need to be located below the pool water level because they are not capable of “pulling” a prime on their own and depend on gravity to keep them primed. The only maintenance for a circulating pump is to clean out the hair/lint trap basket as necessary and to lubricate the hair/lint trap lid gasket or o-ring with a non-petroleum based lubricant about once a month, (we recommend teflon or silicone based lubricants).


Pool water comes from the circulation pump into the filter where small debris particles are removed, and then goes on to the heater or back to the pool. There are three types of filters, sand, cartridge, and diatomaceous earth. Sand filters are the most common due to their ease of cleaning and low maintenance. A filter works by forcing pool water to go through some type of sediment trap thereby removing debris particles; A sand filter passes the water through a special filter sand, a cartridge filter passes water through a fibrous element, and a diatomaceous earth filter passes water through a thin layer of diatom that is coated onto a grid. Water is directed through the pool filter either by a simple valve type setup or more commonly, a multi-port-valve (mpv). We will discuss filter cleaning and valve use in the pool operation section. Located somewhere on your pools filter is the pressure gauge which gives out a reading of the ambient back pressure or resistance in your filter system to water flow. It is essential you have a properly functioning filter gauge as it is used to judge how plugged the filter medium is; this relates to knowing when your filter needs to be cleaned, (cleaning the pool filter is the only maintenance necessary), and will be explained in detail in the pool operating section. The water comes out of the filter and then enters the pool heater.


The typical pool heater is fueled by either natural or propane gas. Electric is an option but is costly and better suited for portable hot tubs. The pool water passes through the heat exchanger which is located in the fire box above the flame bed, thereby collecting heat from combustion and transferring it into the pool water. Pool heaters are lit either by electronic ignition or a pilot light and are controlled by a thermostat. A pool heater thermostat operates similar to an oven; you set the thermostat to the desired temperature and the heater turns on all the way and continues running until the desired temperature is reached, and then turns off, (note: a pool heater will only operate when the pool circulating pump is turned on). There is no routine maintenance for a pool heater other than to be sure no combustible material is on top of or leaning against the heater. Refer to the owners manual or contact a reputable service company for operating instructions and other safety precautions. The pool water then goes back to the pool via the returns.


Pool water returns are places in the pool where water comes back in from the circulation system. A typical pool has two or three returns depending on the pool size. The return is usually a 1 1/2″ threaded opening that may have a directional eyeball that screws into it, (directional eyeballs are used to “aim” the water thereby enhancing proper water circulation within the pool).

Chemical Feeders:

Your pool may have an automatic chlorinator/brominator attached to the circulation system. Pool water is directed through the feeder where sanitizer is eroded into the pool water and sent back to the pool. The feeder should have a metering valve that adjusts the rate sanitizer is added to the pool. Refer to the owners manual for operating instructions. Caution: During normal use chlorinators and brominators will build up concentrated fumes within the tank body. After opening the feeder for servicing, back away and allow these concentrated fumes to disperse before continuing. The only maintenance is to lubricate any cover o-rings with a non-petroleum based lubricant and add chlorine/bromine tablets as necessary.

Mineral Purifiers:

Your pool may have a mineral purifier, (Nature 2, Vision, Frog, etc.), that is plumbed in line within the circulation plumbing. Inside the purifier vessel is the cartridge that contains the purifying minerals; the cartridge must be replaced every 4-6 months. There is no maintenance with these units as the minerals sort of maintain themselves, however, there is a start-up procedure for new cartridges that must be followed using the manufacturers directions.

Cleaner Booster Pumps:

You may have an automatic pool cleaner that uses a booster pump to power the cleaner, the most common brand is Polaris. The pump draws water from the pool circulation plumbing and pushes it back to the pool via the dedicated cleaner line to which the automatic cleaner is connected. There is virtually no maintenance with this pump however it must not be turned on unless the pool circulation pump is running; the booster pump is not self priming and depends on the circulation pump to prime it and keep it primed.


Some pools have an ozonator which adds ozone to the pool water somewhere in the circulation plumbing. Ozone acts to destroy organic waste and break down chloramines and bromamines. Some ozonators have a metering valve to control the air flow or ozone production. Follow the manufacturers directions for setting the flow rate. The ozone generation cell will eventually burn out and require replacement.