I am in the midst of draining and refilling my hydronic system. That discussion is in another thread here. Initially, my system design was for a variable speed circulator pump that held constant pressure in the distribution system. At the time, the only pump I could find was a Grundfos that had external speed control. I got a pressure sensor to control it, but the pump was way too noisy. I believe the noise problem was a result of using a Triac on a normal AC motor. Not what I wanted, but all I could find. Today there are alternatives. PM motors with variable speed controls. While looking at alternatives, the Taco Viridian series is the most impressive. Taco does not recommend this series of pumps for my application. Looking at the pump curves it is clear that they are correct. My system has such a small requirement that it would be operating way below any reasonable speed/flow on the curves for any of the VS pumps. That is my problem. The calculated numbers for the system are 13' head at 12 gpm. This is so far off the curves for pumps designed for residential hydronic systems that I have to question the load calculations. This is a 6,000sq ft house. Plus heated greenhouse and garages. I believe that it is all piped with 1/2" PEX. I have 6 manifolds and a total of 32 solenoid controlled loops. Theoretically the least load could be a single bathroom, and the maximum all of the circuits. The max is unlikely; the minimum, not so much. Does the 13' and 12 gpm make sense to you guys for 6,000 sq ft, some outdoors (sort of)? It just feels wrong to me. The house is very heavily insulated with good glass. When I had no boiler, a 1.5 KW heater in the great room (plus 2 people, fridge/freezer, cooking, TV, computer) kept the central part of the main floor of the house above 60 degrees with outside temps going into the upper 20s. I need to decide if there is a fundamental error in the sizing and, if so, redoing it. If the numbers are way low, I have a much better set of pump options.