You need to think through the controls on the boiler. When the aquastat calls for heat, does it turn on a combustion fan? Or do you only have natural draft for combustion air?
You must have a barometric damper.. without one, you never know when the chimney draft can increase because of wind or temperatures, and the added draft pulls more air through the fire, increasing heat and overheating your water.
An aquastat needs to be able to shut off most of the air to the fire, otherwise it is NOT in control of the water temp.
You need to keep track of the wind, temperature, the chimney draft [a manometer gauge] and then adjust your manual combustion air control. Unless you have some automatic features using the aquastat, YOU are the controler and it can be complicated, I'd write down all the parameters I listed and the air setting you used,, so you can refer back and see what worked or didn't work.
Most boilers that are hand fed, but have aquastat controls have a combustion fan, or a motorized air flap that controls the air through the coal bed.. without this type of control, the fire is OUT of control..
AHS uses a combustion fan with a motorized flap over the intake of the fan,, the aquastat first opens the flap, then starts the fan, reaching target water temp, it reverses the process..
Hope this helps.