What you need to know about chemical automation (water chemistry) and the spa bypass as it is has been defined in the past and is currently being installed today.
We will start with the pool circulation water as it enters the system via the skimmer. The water then travels through the pump, filter and lastly through the heater if there is one. Upon exiting either the filter or the heater the water is sent through the pool/spa return usually a three way valve. In most cases this valve, while in pool mode, will be in the off position to the spa return line allowing all of the return water to be returned to the poo/spa through the pool side of the valve. The action of returning the water through either side of the return valve allows for the Spa to function separately from the pool while in spa mode.
A portion of the water returning through the pool return line will now be diverted to the adjacent spa return line by way of the spa bypass.
The spa bypass is usually a 1" pipe that tee's off the pool return line then crosses over to the spa return line allowing a portion of the pool return water to return to the spa through the spa return line. A ball valve is used to regulate the amount of water returning to the spa return as well as a check to keep the spa water from draining back into the pool when the circulation pump is turned off.
In the past there have been many different methods used to add chemicals to swimming pools and spas. The single most popular would be the tab feeder. Chlorine tablets would be stacked into a cylinder, water from the pool circulation system is passed over the tablets and returned to the pool circulation system and returned to the pool and spa ahead of the spa bypass.
Recently the trend has been to install some form of chemical automation. Like the earlier forms of chemical automation such as the chlorine tablet feeder. These devices also employ the same method of injecting chemicals into the pool return line ahead of the spa bypass.
Here in lies the root of the problem. As heavily chemical treated water returns to the pool and spa the amount of water returned to the spa is disproportionately higher in comparison to the volume of water in the pool.
The calcium fallout in this instance was directly attributed to the large volume of the chemically treated water being returned to the spa through the spa bypass to satisfy the amount of water that was needed for the large spa spillway to function as designed. The circulation system on this pool has a salt chlorine generator installed without the aid of an acid injection unit or an ORP system. Not that this same or other chemical related issues wouldn’t occur if an acid ORP system were installed. The issue is returning heavily chemical treated water directly to the spa through the spa bypass.
In most cases where this type of damage has occurred in the spa but not in the pool is caused by the spa bypass being incorrectly located. This damage is not usually due to the location where chemicals are being injected into the pool return line, usually just ahead of salt chlorine units either. Much of the damage to the plaster in spas today is caused by poor water chemistry brought on from the introduction of salt chlorine units. Look carefully at your Spas and you can see where calcium has fallen out of solution due to high pH and in the spa caused by the disproportionate amount of salt chlorine generated water being returned to the spa.
Where should a spa bypass be located? The correct location to install the inlet to the bypass is directly ahead of any chemical injection, chlorine generators or acid injection. The reason is simple. You only want to return chemically treated Pool water to the spa. The spa bypass should be installed on the affluent side of the heater before the chemically resistant check valve and any chemical injection system. This will insure that only treated pool water is returned to spa.
It is our opinion that no salt chlorine generator should installed or be operated without an accompanying acid injection system or ORP controller.
We suggest to anyone who has installed a salt chlorine generator to immediately have a licensed contractor reconfigure the existing spa bypass and install some kind of acid injection unit.
Thank you for taking the time to educate yourself on this very important issue and if you have any questions regarding the issue please feel free to contact us.
Sara Howard is Owner and President of Ultimate-Pool.com. She can be reached by email at firstname.lastname@example.org