When it comes to pool filtration systems, potential and current pool owners love to look at the big ticket items; heaters, salt water generators, lights, and all the great products that add value to the swimming experience. However, there are two easy to miss but very inexpensive items that every pool owner should be made aware of. Missing these two items can void your manufacturer warranty and lead to early failure of your pool equipment in many cases.
The pool industry has not done enough to promote and raise awareness of the importance of check valves and sacrificial anodes. When planning a filtration system for your pool, always talk to your contractor about check valves and sacrificial anodes. Any contractor that is unaware of these two items and why they are important may not have the right skillset to build you a pool and filtration system that will last. This brings us to the important questions - when do I need a check valve, and when do I need a sacrificial anode?
Any pool that has a heater and a chlorinator will require a check valve.
Any pool that has a salt water system will require a sacrificial anode.
Any pool that has both a heater and salt system will require both a check valve and a sacrificial anode.
Check valves (figure 1) will be the single most important piece of your filtration system for protecting your heater from the corrosive backflow of your chlorinator. Whether you have an inline chlorinator, offline chlorinator, brominator, or salt water chlorine generator, all these pieces of equipment are the main source of chlorine/bromine levels in your pool. When the pump is turned off, the super chlorinated water that has just been produced by your chlorinator will backflow into your heater. This super chlorinated water will sit inside the heater and corrode the heater exchanger. Most above ground pool heater exchangers are copper, and this corrosion can greatly reduce the life of a copper heat exchanger. Heat pumps and inground heaters often have cupro-nickel or titanium heater exchangers which makes them much more corrosion resistant, but super chlorinated backflow can still shorten the lifespan of these heat exchangers over time. Therefore, preventing backflow is central to protecting the heat exchanger in your heater.
Check valves work by only allowing the water to flow in one direction. If the water can only flow in one direction, namely towards the chlorinator, it cannot flow backwards towards the heater. A spring loaded mechanism inside the check valve will extend with the flow of water, and recoil back to close the valve when the pump is off. The arrow on the check valve indicates the direction of the water's flow. It can be installed vertically or horizontally.
By installing a check valve, you are staying compliant with manufacturer installation specifications required for warranty. Any heater that is experiencing heat exchanger failure that does not have a check valve installed, will not be eligible for warranty replacement from all major heater manufacturers. Although the use of check valves has been required by manufacturers for a long time, many pool contractors and home owners are unaware of how important they are. Check valves can cost between $30-40 for a simple Praher 1.5" check valve. Don't spend thousands of dollars on a heater only for it to fail prematurely due to a lack of a $30 check valve.
Sacrificial anodes have even less representation in the pool industry than check valves, although they too provide an essential benefit. Sacrificial anodes are required on all salt water pools. When a salt water chlorine generator is used on a pool, the generator uses a process called electrolysis to separate the chlorine from the salt in the water. Salt, or sodium chloride (NaCl) is made of sodium (Na) and chloride (Cl). By introducing a 12v charge to the water inside the salt water chlorine generator, the sodium can be split from the chloride, thus generating chlorine from salt. This is how a salt system works. A byproduct of this process can be stray electrical current. Although this current is small, it can lead to chemical corrosion of metals, known as galvanic corrosion.
Sacrificial anodes are made of zinc, which has a higher active voltage than the other metals in your pool or filtration system (such as copper, cupro-nickel, or steel). Through a process known as cathodic protection, the zinc in the sacrificial anode will corrode and deteriorate before the other metals in the pool can. Just as the name suggests, the sacrificial anode "sacrifices" itself to prevent damage to any metal in the pool (such as the walls) or the filtration system (copper/cupro-nickel heat exchangers). Over time, you will notice the zinc become covered in green or white scaling, as well as the anode physically getting smaller as corrosion takes its toll. Due to the sacrificial nature of the anode, the zinc will degrade down to basically nothing over time. This means that the zinc anode will have to be replaced, typically every 1-2 years depending on the type of zinc anode used.
Anodes come in two formats, known as "inline sacrificial anodes" (figure 2), and "skimmer sacrificial anodes" (figure 3). Inline sacrificial anodes are the more expensive, but more robust option. An inline anode consists of a "tee" fitting, with an anode in the top of the tee. Water will pass through the tee, surrounding the zinc anode, and return to the pool. Due to cost and complexity, inline sacrificial anodes are more common with inground pools. The cost of an inline anode is typically between $100-150. As the fitting takes up space within your filtration system, it can be hard to find a place for the anode on smaller above ground pool filtration systems. The anode must be plumbed with the anode facing vertically to avoid leaking. The inline anode also offers a bonding lug on the top of the anode, which is fantastic for tying the anode into your existing bonding grid. The inline anode is the better choice for inground pools and filtration systems with adequate space. Thus, the inline anode is typically used by pool contractors rather than DIYers.
The second version of the anode is the "skimmer anode". The skimmer anode is very simple to use, as you simply place the anode in the skimmer basket, and you are ready to go. For the more adventurous pool owner, they can mount the anode vertically inside the skimmer basket. We do not recommend laying the skimmer anode flat in the skimmer basket overtop of the skimmer hole, as this can impede flow to the pump. By using a skimmer anode you can simplify the installation of your filtration system, as you have one less fitting to find a place for. This is especially useful if you are tight on space. As the skimmer anode typically costs between $20-30, it is very cost effective. Unlike the inline anode, which is replaced every 2 years, the skimmer anode is typically replaced every year depending on how worn out the anode is.
Real World Applications
Below is a real world example of how a check valve and anode should be installed with your above ground pool system.
In order to stop the backflow of chlorine from the salt generator from damaging the heater, the check valve has been installed after the heater, but before the salt system. This can be seen in the bottom left corner of the filtration system.
The anode has been installed between the filter and the heater. This is the most common location on installations, as it is typically the connection with the most room. An anode can technically be installed at almost any point in the filtration system and stay within manufacturer specifications however.
Using this guide, any potential or present pool owners can understand how to best protect their pool. By using inexpensive options such as check valves and sacrificial anodes, pool owners can stay warranty compliant and have confidence that their pool will be safe and sound for years to come.