In addition, after you have let the air out of all the radiators, you need to refill the boiler to its working pressure. For a 2 story house, the pressure is 12 psi. You will find a temperature and pressure gauge on the side of the boiler. Do not exceed 30 psi or the water will pour out of the relief valve. There should be a water supply valve coming from the cold water line into the boiler. Open it slowly and watch the gauge. Then recheck the radiators for air. Make sure you check the top floor. Just because they are warm does not mean there is no air inside them. Bleeding should be done once per season.