I have an attached spa and pool, where the spa water spills over into the pool. When I want to heat the spa, I have the suction line valve turned to "spa" and the return line valve also turned to "spa," so I am taking water from the spa, heating it, and returning it to the spa (rather than taking pool water).

But when I do this, the water level in the spa goes down significantly, so I guess it's taking water at a greater rate than it's returning it. Is this normal, or is there a problem? Doesn't seem right to me. Thanks in advance for your help.