Hot Tub Water Level Drops
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Do you have a shared pool and spa system, which means you have one set of filtration equipment which is shared between both the pool and the hot tub, and notably an overflow from the spa where the water spills over into the pool? Is your hot tub giving you a problem where the water level drops any time the filtration pump turns off? Obviously any problem with your pool (or spa) is not a good thing but in this case there is some good news for you as I can help you to understand exactly why this problem is happening and how you should go about fixing it.
It should be mentioned that pool and spa systems are largely unique, and not all of them operate the same way. This is especially true for shared pool and spa systems which are significantly more complicated than a regular pool filtration system without a hot tub. I made this page to help people better visualize what is happening with a shared pool and spa filtration system. After viewing that page it will be a little easier to visualize theses systems as ones that have dual suctions and dual returns. Each one with one from the pool and one from the spa. When on pool mode, which is the default mode, the suction is 100% from the pool, and returns 90% to the pool while 10% of the water flow is diverted to the spa. The spa the spills over through the spillway and back into the pool. This is generally how you prevent shared spa systems from being stagnant.
So what happens in the case where the water in the spa drains every time the filtration pump turns off? Most likely, in almost every case, this will boil down to a failure (or fouling) of the check valve on the return side of the system. The make up valve, which is a pipe configuration consisting of a valve and also a check valve, is how you control and regulate the flow of water between the pool side of your system and the spa side of your system.
Pool mode - In pool mode 100% of water draws from the pool and 90% returns directly to the pool. The remaining 10% returns to the spa which then overflows into the pool.
Spa mode - In spa mode the pool is entirely stagnant. 100% of the suction comes from the spa and returns 100% to the spa. This allows you to adjust the heater setpoint to 104F and heat only the water currently in the hot tub.
When the pump for your system turns off, the check valve in the make up between the pool and spa sides is supposed to hold the water in the spa (which is physically set higher than the pool water level). However, the seal on the check valve can foul or fail over time and allow water to leak in the opposite direction. A check valve is a valve that only lets water pass in one direction. So if you have a check valve that allows water to pass in the wrong direction, then that is something that needs to be replaced. Fortunately many brands of check valves are serviceable, meaning it was anticipated that they would eventually foul or fail, and you can replace the innards of your check valve to resolve the problem of your spa water level dropping when the pump is off.
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