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I've got a little experience in this area. We currently have two of the Grundfos Constant Pressure systems, and two of the standard single speed submersibles (spread out over different places, but I pay for repairs on all). There are advantages and disadvantages to both. I have a constant pressure system here at my house/main farm. It was originally a single speed submersible, but the tank went bad and I upgraded the whole system. The VFD really does allow the pump to cycle less, which is good. I can't speak to energy use, but overall I've been satisfied. I've got the pressure set at 60 psi, which is awesome for cleaning up equipment and filling tanks - and the showers in the house are like going to a car wash. The pressure stays between 59-62psi no matter what the demand is (within reason). The main single speed submersible that we have is at a farmstead where we mix most of our spray loads. The pump was put in in the early 90s, and has never missed a beat. I'm sure it's on borrowed time, but I'm not messing with something that works. That particular pump replaced a jet pump that was put installed in the 60s, which replaced a pump jack put in when the REA came through in 1939.

Someone mentioned lightning. I suppose the CP system might be more susceptible, but my experience has been that when there's a lightning strike, we're going to need a new pump. And new TVs and appliances. The second single speed has been replaced twice in my memory - both times due to lightning. It has a surge protector, which may or may not have saved it a few times - hard to say.

The constant pressure system is very particular about the placement of the pressure sensor. Much like anything, it is important to have a professional that understands and is trained on the system install the thing. But honestly, since my kids are drinking this water, I'd say let's go ahead and use a professional for anything well related.

The tank on my house CP system (15 gpm) is a 1 gallon tank. It looks tiny but has performed beautifully. That being said, if I didn't have an automatic generator, I'd rethink the installation. The time between when the power goes out and when the water stops is pretty quick if you're in the shower somewhere between lather, rinse and repeat. 30 seconds waiting for the generator to kick in isn't too terrible.

Finally, we had a problem a few years back at the well where the other CP system is located. The pump was at 65 feet in a well that was 108 feet deep. During the summer, the static level fell below the pump. The CP system shut itself down (until the pump was lowered). My experience has been that a single speed in the same situation will run until it burns up.

Sorry for the long post.