Which heats quicker High or Low speed pumping

[mas985]

I think your second point is probably why they set the lower limit on flow rate. I suspect that the heat exchanger could handle much lower flow rates but you run the risk of scalding water comming out of the returns. Although not quite the same, tankless water heaters work at very low flow rates, < 6 GPM. If the flow rate dropped to 10 GPM on a 400k BTU heater, the delta T would be close to 64 degrees! On top of 80 degree water would present a significant hazard.

[salinda]

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.