Quote Originally Posted by waterbear
Sometimes in a shared system the spa jets use a booster pump to supercharge them. Sometimes a blower is used for this also. In either case the main pump needs to be running and the valves need to be in the spa position. Usually when there is a second pump for the spa only it also has separate filtration, heat, and sanitation and works independantly of the pool system.
This gets to the central point of my question.

If there is a "booster pump", is it located in series with the main circulating pump (after the pool/spa diverter valve on the pump discharge)? That I could understand, although it would seem strange to me.

Or is it in parallel? If it is in parallel, I don't see how the main circulating pump and the booster pump could pump through the same returns/inlets/jets.

What makes sense to me is this:

Spa jet pump and spa blower are plumbed altogether separately from main circulating system (separate set of spa main drains - no diverter valves anywhere in this circuit).

While using the spa...

...pool/spa diverter valves in the main circulating loop - both suction and discharge - are in spa position.
...main circulating pump is running.
...heater is operating as needed to meet temperature set point.
...main circulating water is coming into the spa through a non-jet return.
...spa jet pump and spa blower are pumping/blowing into the spa through the jets.