Agreed,
@ajw22 is surely correct that the flow basically won’t matter much at all. The heater will impart the BTUs and more water flow just means more water at a slightly lower temperature, versus less water flow exiting at a slightly higher temperature. Provided you stay within the GPM flow limits of the heater. Too low and the low pressure (ie low flo) limit of the heater will stop the heater with a flow message – presumably because the heater does not want is condenser coil (in the heat exchanger) to get too hot. I suppose the high flow (high water pressure) limit is due to the heater’s tested rating for maximum psi.
Also, I think it’s impossible to know the actual flow you’re getting – certainly not based on pump ratings of flow and/or HP nor based on psi readings. With all the variables of pump rating, speed, pump efficiency, head pressure from outlet nozzle types, bends and elbows in the piping, together with filter cleanliness, the only way to know the true flow is by installing a meter (if even most of those are accurate, LOL).
All that said, I have run some measurements on compressor load and the related condenser temperature based on how well the condenser is being cooled, and I found I could reduce condenser temp, and the resultant current drawn by the compressor, if the condenser runs cooler. The difference is small, but I think that is basic heat cycle / compressor physics. So in theory the heater will use a bit less power if the cooling water flow through the exchanger is greater. On the other hand if you have to run the pump on a higher speed to increase flow, you’ll pay way more for the power to run the pump faster than you would ever save from reducing compressor current drawn.
As a semi-related aside – two stories: I always run my two-speed pump on low which produces 40GPM. But even at 40 GPM my 20 year old heater complained about low flow, so I bypassed the internal flow switch in favor of an external adjustable pressure switch. I figure the internal pressure switch probably wasn’t that accurate anyway. Then my neighbor comes along, who bought a new heater but had always run his pump on low, with resultant 30-40 GPM (measured by by a flow meter). The new heater wouldn’t run (low flo) with the pump on low so the pool guys have him running the pump full speed, wasting about $30-$50 per month on pump electricity. Ugh. I net out that I don’t trust the heaters for flow indication. He could bypass his switch, as I did, but that would void the heater warranty. More Ugh. One day I’ll have to get a new heater and I’ll have to hope the internal pressure switch is accurate!
