Harold Kestenholz
If you have only one pipe attached to the bottom of the radiators, then your system is a one-pipe system. Partially closing the hand vavle will partially block the return of condensed steam water back to the boiler. As stream tries to force its way past the resisiting water, the water will sizzle and perhaps bang. Leave the valve wide open.
The rate and quantity of steam is regulated by the size of the hole in the air vent. A smaller hole will let air out slowly, letting steam in slowly. When steam hits a good vent, the hole shuts to prevent more steam entering. So if you want less steam, use a vent with a smaller hole. The vent valves are rated A,B,C,D,E or 1,2,3,4,5; use the one that lets the air out at the best rate for room comfort. Adjustable automatic vents are available at more cost that let you set the control vent to a satisfactory setting, then the valve takes care of the temperature sensing in the room.
