If you are using a "closed loop", if you trim (control flow) to a load, you need to bypass the remainder available back to the main return line. As far as flow meters go, they are pricey and usually are used to secure a fixed flow rate. Flow regulators (either electronic or pressure actuated) come in two or three way varieties, the two way being less expensive, but you still need to add the bypass line, like you automatically have for the three way valve. With small diameter valves, the three way is a space saver as well. If you are running with city water, you obviously will have a wider swing, summer to winter- closed loop you can reset your reservoir tank or chiller op stat to a higher tempp if it is too cold for ya.