Shared Pool & Spa Systems
Shared pool and spa systems are the most economical way to have both a pool and a spa installed as the filtration and heating equipment is shared between these two bodies of water. If you did not share the pump, filter and heater between the pool and the spa then you would have more control over each body of water, but your equipment and installation costs will double versus a shared pool and spa system. If you have the money I think pretty much all pool installers would agree that keeping the swimming pool and hot tub completely isolated and independent from each other is better...but if you are able to make just a few concessions about the hot tub you might be able to share the equipment. If so this will dramatically reduce the cost of your equipment and installation.
While shared pool and spa systems can be a real cost saver for those looking to have both a swimming pool as well as a hot tub installed they will not be for everyone. Essentially a shared pool and spa system is great as long as the specific limitations of this arrangement do not interfere with your priorities for using your spa. Once you know more about how a shared pool and spa system works, and the limitations of sharing your pump, filter and heater between two different bodies of water, you can make an informed decision about whether this is something that appeals to you or not.
Spa water heating time - The most critical difference between a shared pool and spa system and isolated pool and spa systems is that when you want to use the hot tub you will need to turn on "spa mode" to heat the water up to 104 degrees (or your preferred spa temperature). Under day to day operation the system will be in "pool mode" which has both the pool and spa water around pool temperatures. Let's call that 80 degrees Fahrenheit for this example. When you want to use the spa you will need to switch your system to "spa mode" which will temporarily change your suction and return manifold valves for your equipment such that water is only being drawn from the spa, and only being returned to the spa. During the time in "spa mode" the pool is essentially stagnant as no water is being sucked, filtered, heated or returned to the pool at all. By allowing the spa to operate isolated like this your system can now begin heating the water from 80 degrees up to 104 degrees.
To heat water from 80 to 104 requires a 24 degree spread. Assuming a 2200 gallon spa (10' x 10' x 3') and knowing that water weighs 8.3 lbs per gallon, this would take a 400,000 BTU pool heater just over one hour to accomplish.
2200 x 8.3 = 18,260 BTU to heat this spa by one degree
18,260 x 24 degrees = 438,240 total BTU's needed to heat this spa from 80 to 104
If you want more information about heater sizing you can read this article about what size heater do you need for your pool.
In this example, this fairly large hot tub that holds about 2200 gallons would need over an hour for a very large 400,000 BTU heater to bring the water up to spa temperature. If the numbers were repeated but this time with a 250,000 BTU heater it would now take 1.75 hours. Would you be willing to wait one hour and 45 minutes just to be able to use your hot tub? Most pool and spa owners are not. In the above example we address the single greatest drawback from shared pool and spa systems in that you will need to wait to use your hot tub, or more specifically you will need to plan a little in advance from when you want to use it. We also answered the question about why shared pool and spa systems need such large heaters...it just takes smaller heaters too long to bring the water from pool temperature up to spa temperature.
Location of the hot tub - When you have a shared pool and spa system then you need to locate the spa either inside the pool, or directly adjacent to the pool such that the water from inside the spa will be able to overflow and fall directly into the pool. This is an integral aspect of how shared pool and spa systems work and if you want to have a hot tub in a different location than the pool, or you want the hot tub in the pool area, but not overflowing into the pool, then a shared system will not be viable.
So why does the hot tub need to be located right next to the pool? This is because in order to "share" the equipment between the pool and spa in such a way that the spa does not keep turning green the spa needs to overflow (a small amount) into the pool. When you have a shared pool and spa, conventionally speaking, the system will be in "pool mode" over 99% of the time. All day and all night, except for the few hours per week that you are actually in your hot tub, the system will be in "pool mode". During these "pool mode" hours there needs to be a mechanism for delivering clean, filtered and chemically treated water into the spa, or else you would very likely find that it went green the next time you go to use it.
To solve this a small bypass is installed in the plumbing system, which consists of both a one way check valve, as well as a small manual valve. When in pool mode this manual valve is cracked open just slightly which allows a small amount of clean pool water to escape into the spa return line (usually). This brings clean, filtered and chemically treated water into the spa system which in turn causes the spa to (intentionally) overflow into the pool, usually via a small overflow trough. This small amount of overflow sent to the spa, over the course of many running hours during the day, is more than enough to cycle the full volume of the spa over such that it will be clean (as clean as the pool) all the time.
Basic Pump And Filter Installation
Start with a simple example of a pool with a main drain and skimmer. This pool only has a pump and a filter. This is the most basic installation you can have for a swimming pool filtration system.
Add A New Heater
After a while everyone got tired of being cold so a new pool heater was added to the system - Yay!
Basic Shared Pool And Spa System
Now getting more complicated, we have added a spa suction (picture a set of main drains in the spa) and changed the suction side manifold to have pool main drain and skimmer on one side, and spa suction on the other. The return side manifold has the pool return on one side and the spa retun on the other. Additionally there is the bypass installed that includes a manual valve as well as a check valve. A check valve only allows water to flow in one direction, which is denoted by the arrow over the valve.
Automated Pool And Spa System
Where tee junctions were shown you now see MVA1 and MVA2 which are motorized valve actuators. In these locations three way valves are used which allows flow control to be closed in any direction. MVA's are controlled typically with an automation system, and with a shared pool and spa system such as this both MVA1 and MVA2 will change position in unison. If you have trouble picturing this, an MVA is just like a hand that turns a valve handle. MVA1 and MVA2 turning in unison is the same as someone holding a valve handle in each hand and turning them at the same time.
Shared System On Pool Mode
Shown is "pool mode" where water sucks from the pool main drain and skimmer, goes through the filter and heater, and returns to the pool. Note that a very small amount of water is siphoned away through the spa bypass which allows a tiny amount of water to go back to the hot tub when in pool mode. In this arrangement the hot tub would continually overflow into the pool allowing it to stay clean and clear during long periods of time when the pool is in this most common state.
Shared System On Spa Mode
When you turn your system to "spa mode" this activates MVA1 and MVA2 to turn (at the same time) and now both the pool suction ports as well as the pool returns are closed off from flow. As indicated with the red arrows the pump is now able to draw water from (only) the spa, heat it, and return it to (only) the spa. No water can travel backwards through the bypass to the pool return because of the check valve.
If you are paying close attention you might notice that when in "spa mode" there is no flow to the pool. Wouldn't this result in the pool turning green if the system was left in "spa mode" for an extended period pf time? Yes, it would. Unlike "pool mode" where the spa receives a constant flow of clean water, when the system is in "spa mode" there is no water going to or from the pool, essentially making it stagnant. The idea is that the system is never intended to be left in spa mode as this would be impractical, and once you are done using the hot tub you would switch the system back over to "pool mode". It is not intended that you will leave your hot tub running at 104 degrees other than the periods of time you intend to use it with a shared pool and spa system.
Advantages of shared pool and spa systems can be boiled down to cost and space savings. If you have a large budget and you have enough room (for extra pumps, filters and heaters) then undoubtedly a system that is wholly independent is superior. With shared systems you have limitations on waiting to heat the water, location of the spa, but also the consideration that when the pool is closed then the hot tub is also closed. This is a major deal breaker for many people.
If you live in a cold climate area where the pool will need to be closed and winterized then you will also need to close and winterize the spa if they share equipment. For some people that is no problem, for others, like myself, the cold weather is the best time to use a hot tub, and closing mine for the winter would be out of the question.
A shared pool and spa system is somewhat of a compromise, but depending on what your priorities are it is a viable way to get both a pool and a spa by utilizing the equipment that you needed to buy for the pool anyway. Perhaps just a larger heater than what you might need only for a pool system to keep your waiting period as short as possible to bring the spa water up from pool temperature.
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