you all are great, one speed for each mode seems like how it should work. props to JasonLion for providing a number. now i can see about how strong the pump should be!
jblizzle, i like the continual idea because it is the simples way to achieve rapid response time for temperature drops at any time, which really matters in night visits to a hot tub. my dad's tub pump is on a timer with manual override buttons (they are funny air switches), but using that is too complicated. the alternative is adding a low-voltage circuit linking a sensor right in the tub with a switching relay to turn on the pump, which is even more complicated, but as you say, it sure would save some power.
say i did want to continually turn over 500G three times a day, which means a rate just over 1 GPM, and had a round 14" filter of just over 1 ft^2 area. regular circulation would see 1 GPM/ft^2, and in order to get 15 GPM/ft^2 for backwashing i'd need a second speed (or a second pump) 15 times as powerful. that's nuts, so i can see that i'd have to get a way larger pump or use a way smaller filter. exactly what i need to know before buying things. cheers!