Hi Freddy,, let me expand on what Richard has described. Lets say your house has not called for heat for the last 20 hours, all the water in the pipes and the baseboard units is cool,, the water has reached room temperature, say 65*. The boiler's aquastat is set for 150*, and the water in the boiler IS at 150*.. Then, at 7AM, the whole house decides it needs heat, so all the baseboard zones start to circulate the water,, the 150* water in the boiler is quickly replaced by the large volume of cool water in the pipes and baseboards..
In the meantime,, for the last 20 hours, the stoker in the boiler has been in it's idle or pilot mode,with a keep-fire timer' The timer has been adding a little coal to the fire every hour or half-hour, just enough to keep the fire going. This small fire has been enough to keep the small volume of water in the boiler up to temperature, 150*. When the 7AM demand for heat comes,, the pilot or idle fire is VERY small, probably a strip of burning coal about 1"x1"x8", or about the volume of a fat hot dog.. The stoker starts adding coal to this fire,, it takes about 15-20 minutes to increase the burning coal volume to a carpet of coal that is 2" deep by 8" wide x the length of the grate,, lets say 8".. You have an 8x8x2" or 128 cubic inches of BURNING coal.. Where before you had 1x1x8" or 8 cubic inches of burning coal, just a pilot fire. A huge increase in volume of burning coal,, and remember, for the last 20 hours, the 8 cubic inches has been enough to maintain water temp in the boiler.
Once the large volume of coal gets burning, it [finally] can make lots of heat, and the temperature demands of the house and the water in the boiler are satisfied... BUT.. the fire is still going strong... your stoker will back off the combustion fan, but you still have 128 cubic inches of coal burning, so even though the stoker has stopped adding more coal to the fire,, the fire is still creating a lot of heat,, so the water temperature rises in the boiler.. and it can get hot enough to reach 220* +, and the pressure relief valve open. Even though the combustion fan has shut off, the chimney draft still pulls air through the fire.
Add to this scenario the keep-fire timer, lets say it cycles on for it's one minute run, right when the big fire was finally cooling down,, the combustion fan fires up the whole bed of coal again, and the fire and water temperatures are given a big push... here comes an overheat..
If you have a heat dump circuit, it would be set to open and circulate at say 210*, and dump the excess heat somewhere.. Either into a garage heater, a hot water resevoir, or a 'waste-it' circuit.. hopefully without A/C trying to compensate for the excess heat.
In my situation,, I have a shop that I can dump overheated water into,, but so far I've never seen an overheat, and I haven't hooked up the valves, controls and pump to use the heat dump. For the spring, I have my aquastat at 160*, and ocassionally will see overshoots up to 185*,, but well short of an overheat.
Every instalation and lifestyle is different,, you may not need a dump zone.. If you keep your target water temp low during the summer and fall, I doubt that you will need a dump zone.. but it IS a good idea..
Hope this makes sense... Greg L