Yes, it was me that had to move my coal boiler circulator to the return side of the boiler.
Here's a in depth discussion of why. First, I fully agree with the "pumping away" principles as described by John Siegenthaler in the link posted by Sting. But, many boiler installations are not as simple as a single boiler and a single circulator connected to a radiation system that the "pumping away" examples use. That's true in my application. I have a oil boiler and a coal boiler piped into a primary secondary loop. From this loop water flows to my home via insulated underground pipe. In my home there are multiple zones, including an indirect hot water heater, all with individual circulators.
Coal boilers are unique, because heat continues to be produced even when there is no demand. How much heat is produced depends on the boiler design and the "keep fire alive" method. Ideally, this excess heat is not distributed, unless needed. One way to stop this flow is with one or more check valves. Often the check valve is an integral part of the circulator. I have both in my system. A further complication is if the unwanted heat distribution is blocked, it still must go somewhere. This could be a dump zone or an over-sized expansion tank.
All boilers have safety pressure relief valves. Indirectly this is what caused my problem. I had an over-fire condition which cause the safety pressure relief valve to release. The release also discharged a considerable volume of water. When the boiler cooled, the automatic fill value re-filled the boiler. But ... My circulator mounted on the top of the coal boiler has an integral flow check. When the pressure relief blew all water was removed from the circulator; both sides including the impeller side. Now when the automatic relief valve returned water to the boiler, it could not get to the impeller side of the circulator. There was a air pocket cause by the check valve. So, what's the solution? Move the circulator to the return side of the boiler. This way if the pressure relief valve blows there will still be water on both sides of the now low mounted circulator. When the automatic valve refills the boiler operation will resume.
After this problem. I did two things. I moved my coal boiler's circulator (with integral flow check) to the return side and increased the size of my expansion tank. The increased expansion tank size captures the expanding water volume due to any over-fire condition. This assures the pressure relief does not blow. I recommend this method over a dump zone. During very low demand and warm temperatures I routinely have a boiler water temperatures of 240 deg F. This hot water is stored in the boiler and is ready for use when demand occurs. If you design a system like this make sure all your boiler piping is rated for high temperatures.