I've seen a lot of discussion lately about flow rates, pumps etc. It seems no-one uses a positive displacement pump for loop water which is somewhat understandable. Pros to PD: --With a VFD, I could easily control my flow rate and keep my pumping efficiency much higher than a centrifugal circulator pump. --Simple linear relation between frequency and flow will make control easy. --efficiency is almost always better than a centrif....best centrif I saw was about 60% Cons: --Marginally more expensive to purchase depending on the pump. --Maintenance will be more expensive, but again depends on the pump and the size. Few of the curves you can download will indicate the power consumption for the circulator pumps so it's hard to tell how efficient they are. and YES> it's kind of splitting hairs but I'm just curious.