You might read this for Dump Zone information. Dump Zone hook up.
Can you post a picture of your expansion tank, modern tanks have bladders in them and older units require different care.
You might have some success filling in your location in the user control panel, upper left, assistance may be in your area.
Relief valves somtimes need servie also, often replacment.
Water expands when heated, this expansion goes into the expansion tank, if it still has air in it, older systems fill and drain from the bottom and hold air in the top. Newer units have a rubber diaphram that separates the air for the water and require less attention. If your expansion tank is full of water with no air the expansion squirts out the preasure relief. If your boiler is getting hot and making steam, no expansion tank will hold that, and you need to prevent your boiler form getting to hot, this is where having a zone valve or two open when the boiler gets to say 205 or whatever you want will help to prevent overheat situations, the house may get warmer but that is better than popping the relief. Lastly you may want to repalce the relief valve, if it has been active scale can build up on the seat and cause it to weep, or stick shut.
Check the expansion system, a dump zone on hand fired is probably a very good idea, and think about the relief valve. We save money with coal but you still need to service your heating system.