Bad Valve or Check Valve?

I have a spill over spa that is 18 inches above my pool. My plumbing setup is attached. When the return water from the the filter is directed to the spa and the pump is turned off, the spa will drain down to the spa drain level. This happens whether the spa is getting water from the pool (spillover mode) or isolated from the pool (suction from the spa - return to the spa).

I do not have any check valves installed on the plumbing. I'm wondering if this is; a) normal, b) due to a bad 3-way valve c) due to the lack of a check valve d) some other problem.

Any ideas?

Your plumbing diagram does not show a spa make up loop for fresh water in the pool mode. My guess is that you are using the return three-way valve for the spa make up water and when you turn your system off then the spa drains down to the pool.

If that is not the case then you may have trash caught in one of the three-way valves or one of the three-way valves may have been damaged by PVC glue when installed.

If your system is automated then maybe one of your actuators needs adjusting to close all the way.

You could also install a check-valve on the pool suction side of the three-way valve and another check-valve on the spa return side of the three-way valve.

Hope this helps