CONSIDERATIONS - There are three main issues to consider when draining the water in your pool's spa. First, closing off the return lines to your spa. Second, opening the spa drain. Third, deciding where to dump the Spa's water. Each of these issues is discussed in the following steps.
CLOSING RETURN LINES - Assuming that the water level in your pool is normal, you will be draining your spa by just moving the spa water over into your pool. This picture shows how the return valves in your pool system would be set to drain your pool. The Spa jets and Spa fountain are shut off blocking water into the spa. The pool spray heads [lions] are shown shut off but that is optional since they are going to the pool. The water flows from the pump/filter back out through the pool returns.
OPENING THE SPA DRAIN - This picture shows the suction valve settings when draining the spa. The spa drain is open to drain the spa. The pool settings shown are for a normal pool operation. The pool main drain and skimmer are open and the pool vac is closed (optional). Water flows from spa and the pool's main drain and skimmer to the pump to be filtered and returned to the pool Since no water is returning to the spa, it will eventually empty. Make sure that the diverter valve to the Spa drain is opened fully and that the drain is fully operational and not clogged. Also, make sure you shut off the pump before the Spa is completely empty so that you don't get air into the pool system and run the pump dry.
CHECKING WATER DISCHARGE REGULATIONS - If your pool is too full to handle the additional spa water, you may have to drain it out the waste line. Many municipalities prohibit discharging pool water into the streets. The approved process is to dump the water down your home's sewer clean-out usually located next to the house or on the wall. Check with your local authorities for what's acceptable. If you are dumping into the sewer line, you run the risk of too much water flowing and potentially backing up into the house. It is recommended that you reduce the GPM into the clean-out to a maximum rate of 12 gallons per minute. If you are already operating off a waste line with a ball valve, just close the valve down to half. If you are using the "Waste" setting on a multiport valve you might try adding a ball valve at the end of your waste port.