How does a boiler pump circulate water?
does the spin on that impeller (at a high velocity) really create enough momentum to move water up seven or eight stories?


It doesn't have to create momentum as it isn't throwing water ballistically. It just has to create a small difference in pressure.

Note that you ask about circulating water, not pumping it up into a tank. The pump does not have to overcome the head of water because the water is in a sealed circuit that also returns downhill. The weight of the water in the down-pipe balances the weight of water in the up pipe.