I replaced a 1 hp full speed pool pump with a dual speed pump with the goal of hopefully substantially reducing the electric power used. I was concerned if the pump would startup and work with the solar heat on low speed. Testing has shown that it will always startup and work when water is going to the pool directly, ie non-solar. It will also startup and work OK if the water is going through the solar heater, but only with a clean DE filter, or one that is maybe halfway to needing to be cleaned. Without a clean filter the pressure required is too great. So, I'm going to put in the time delay relay system that we discussed in the other recent thread to have it start out at high, then switch to low. Once it gets going on high, I can switch it to low in a minute or so and it then runs solar on low fine.
I also did some power measurements by counting the revolutions of the power meter for a few mins with the pump off, on high, and on low. The pump is using 1/4 the power at half speed as it does at full speed. This means if I run it twice as long at 1/2 speed to move about the same amount of water, it will cost about 1/2 as much. That's great news. Even better is that at
1/2 speed the pump is now perfectly matched to the amount of time it should run for solar. Before for solar heating, it was running about 9 hours a day at full speed, which is more than was needed to keep the water clean. Now it will run 9 hours at half speed, which is enough to keep the water filtered. So, instead of just cutting the power usage by 1/2, I'm actually cutting it to just 1/4. Should be saving about $55 a month in electric cost, making for a quick payback.Also, found out something else interesting. Before measuring via the power meter, I used a clamp-on ammeter. Looking at amps going from full speed to half only cut the amps by just a little over half. I new this wasn't accurate, but thought that if the power factor was about the same at high and low speed, then it would be a proxy for true power. But obviously the PF must change substantially, because as reported above as measured at the electric meter, the true power used drops to 1/4 at low speed, not just by 1/2 as would be indicated by measuring amps. I guess as the speed drops, more of the power is reactive and less is real.
Bottom line, what I've measured is consistent with what I've read about the substantial savings you can get by going to a dual speed pump. In this case, all I had to change was the motor ($180) and an additonal box, some switches, etc. Still have to add that relay stuff. All in, probably cost $230 total.