I've had flow problems in my hydronic heating since getting my expansion tank replaced a couple of years ago. Last year it sounded like the Flo Control valve was rattling, looking over a diagram of how they're put together, might well be that the weight and washer had become detached. It was broadly working then though. Now this year, there's no circulation at all with the pump working and running, heat doesn't get further than about a foot above the boiler. I'm suspecting the flow control valve (maybe 3 feet up above the boiler) may have just jammed shut and gummed up over the summer.

It's one of these Bell and Gossett valves, top centre of picture:
http://mywebpages.comcast.net/swcooper/BoilerRear.jpg

How likely are these to fail like this - is it just wholly more likely that there's a physical blockage in the boiler pipes somewhere?

How hard will it be to unsolder one connection and then unscrew from the other one?