I've been running for years with the Poor Man's Variable Speed Circulator and decided that it's time to bite the bullet and use a real variable speed circulator, managed by my NFCS. The control objective is to vary the boiler circulator speed to obtain the desired boiler outlet temp. Lower speed means more dwell time for the water in the boiler means higher outlet temperature - should work like a charm. With hotter fire or higher inlet temps, higher circ speeds would be possible while maintaining the desired outlet temp. It works wonderfully, except..... With slower flow through the boiler, you also have slower flow through the zones, which means colder inlet temps, which means you need slower flow through the boiler because you now need more rise. Same thing happens the other way around when everything is cranking. High flow though the boiler leads to high flow through the zones and a smaller drop, meaning higher inlet temp at the boiler. Bottom line - variable speed boiler circ doesn't solve all the world's problems.