I can expand on that: in just about all other spas, there is one pump which is plumbed to a T fitting. That T fitting goes to a filter on one side, and a suction fitting on the other. Start the pump with a nice, clean filter, and the suction is distributed evenly between the filter and the suction fitting. Very basic.
BUT
As the filter loads up with dirt, it becomes harder for water to pass through it. So, the suction at the suction fitting begins to climb. Now, remember this is a non-filtered water inlet near your feet in the bottom of the area in which you sit, and it could have a VERY powerful amount of suction if the filter gets really plugged. If the filter is ignored and becomes clogged, you could have almost the full suction the pump can generate drawing through the suction fitting. Most people do not let the filter go that far, and ETL, UL and the ANSI have all approved this design, so please don't take this as knocking the competition. I am just explaining one of many reasons that HS chose not to go this way.
HS designs their tubs with NO suction fittings. Instead, they put twin filters, one on each side of the T fitting, so that ALL the water must enter through a filter. They even build in a simple temp sensor device which will flash the red power light on the outside of the spa if the filter for the circ pump gets too dirty. This filter gets dirty far faster than the others, so if it's clean, chances are the others are too.
Robert's Hot Tubs, and a few others, use a similar approach with a twist: they put twin suction fittings, but then they run the filter downstream of the pump. So the suction fittings will ALWAYS be balanced, even as the filter loads. And if they plumb their tubs the way I think they do, those suctions are usually mounted on the floor, beneath the benches, out of the way.
