I have a variable speed pump (Ikeric) and a 400,000 BTU heater. When the new pump was initially installed, it was only programmed to go to high speed when water is flowing to solar. When I ran the system in spa mode the pump was on low. (Note that I have a separate pump for the spa jets. The spa shares the pool's filter pump.) The water coming into the spa was way too hot. I suspect that something in my almost new heater should have prevented this as a safety mechanism but I had the contractor that installed the pump set the pump to go into high speed whenever the water is flowing into the heater as well, when I close the new automated heater bypass valve. The apparently scalding water mixing into the spa concerned me. Be careful.