understanding 4-way mixing valve performance

RonV_2

When sizing a 4 way mixing valve, I note  that the manufacturer defines a "Normal Operating Range" on the head loss vs flow rate performance curve.  For example, the Tekmar 720 series valve data sheet (D720)  includes such a curve, as does Heatlink on their Tech 1-635xx sheet.  Heatlink calls it the region of "optimum performance" on their curve.  In both cases, the recommended operating range is shaded and is defined by upper and lower values of head loss.   I am having trouble understanding exactly what determines the lower limit value for these recommended operating ranges.  What exactly happens when you operate these valves at flow rates that result in head losses below the recommended operating range? 



I spoke to both Heatlink and Tekmar, and both were helpful but could not quite completely satisfy me on this question.  Both suggested that the lower limit has to do with the difficulty that the control system, of which this valve is a major part,  has in smoothly regulating the mix temperature at very low flow rates.   I don't quite get it and am wondering if anyone here can explain it precisely?

Paul Pollets

Sizing

the mixing valve should be correctly sized according to load. If you're output required is 15K BTu's or less, a 1/2" 4 way would be used; if less than 45K BTu's, a 3/4" valve will be optimum. If the valve is oversized, it won't open enough at lower load requirements to properly allow flow.  Oversized 4ways (and 3ways) are a common problem.  Calculate your GPM requirements and size the valve accordingly.