Best way to heat attached spa

[CHParker]

I have a 30,000 gallon in-ground pool with an attached spa. Typically the spa drain is closed, the main drain, shallow skimmer and deep-end skimmer are open. I have a heater with dual mode (pool or spa) capability. If I want to just heat the spa, what is the best setup for the drains? If I close everything and open the spa drain (and set the heater to spa) the water level in the spa drops by about 3-4 inches. Is this normal? Is there a better way to just heat the spa?

Thanks in advance.

[waste]

You should have valves to kill both the pool suction and the returns to the pool, so that you can run just the spa. It sounds like a return to the pool is still open when you are trying to run just the spa.