Im flushing my Rheeme Prestige series tankless water heater but the return flow coming back out of the heat unit is very low. It starts out ok but after a few minutes it slows down to a trickle. I tried running the submersable pump in my sink and it has tons of flow/pressure. it didnt do this the first time or two but now it does. Any ideas why its doing this? We have good water pressure in the house when hooked up normal. This just happens when trying to flush the system.