Hot water radiator systems heat your home by boiling water in a central location and then pumping it out through the area to be heated. Along the way, it passes through radiators, which warm the air and cause the temperature to rise. In most installations radiators have bleed screws, which allow air trapped inside to be expelled. Some radiators don't have these screws, but it's still possible to bleed air in hot water radiators without radiator valves by bleeding the boiler itself.
Turn off the power for your heating system. This can either be done by switching the power off on the heater or at the central electricity breaker. Identify the circuit where the heater is installed and flip the switch to the "Off" position.
Wait until the water inside the system has cooled slightly -- for about 20 minutes. This will help prevent scalding.
Locate the bleed valve on the water heater or boiler. These are usually located near the top of the heater. Unlike most radiators, they don't require a special key and can be opened with an adjustable wrench.
Slip the wrench around the bleed valve. Place a towel over the valve to catch any water that may trickle out. Slowly open the valve. You will hear steam escaping. When water begins to bubble through the valve, tighten it shut.
Top up the radiator system with water, if needed, by opening up the fill valve.
The steps listed here are a basic guideline -- always follow instructions for your specific radiator system.