If the impeller was changed but no water is coming out of the thermostat location, that indicates a blockage somewhere between the pump and the upper powerhead. Quite possibly the old impeller was broken and not all of the pieces were accounted for when it was replaced. A piece of impeller vane can be lodged in the copper tube leading to the powerhead or could be caught under the block or in the passages under the exhaust cover. Either way, the powerhead should come off to find the blockage.
