In most cases fixing your boiler drain valve will require you to open up the valve in order to bleed the air out and any water in the valve before attempting the repair. If the leak persists after trying the steps below, you may want to consider calling a professional to accomplish this task.
Step 1 - Bleed the Radiator Valves
Turn on the heating system to your home. The boiler feeds steam to the radiators, which need to be drained and bled before you attack the leaking boiler valve. With the heating system engaged and working, go around to each of the radiators throughout the house and use the valve key to open up the valves. The valves should make a hissing sound when opened and water may spit out during this process. Use towels and place them underneath the bleeding valve. Make sure to not place your hand over the hissing steam because if it is hot enough it will cause burns.
Step 2 - Drain the Boiler
Open up the boiler valve and allow all of the water inside to drain out. Again, place towels underneath the valve in order to contain any of the spillage that will seep out of the boiler. With the boiler drained, look at the discharge to determine whether there is rust present in your boiler system. You can tell if this is the case because the color of the discharge will be brown.
Step 3 - Add a Rust Inhibitor
Take a rust inhibitor and spray the boiler valve. This is more than likely the cause of your leak and spraying the rust inhibitor will help to seal the valve and make it work properly again. Apply the rust inhibitor as prescribed by the instructions on the sprayer.
Step 4 - Refill the Boiler
Use a garden hose to refill the boiler with water. Only fill the boiler to its recommended water level. With the boiler refilled and the valve sprayed with the rust inhibitor, close the boiler valve and watch it for several minutes. Check to see if any water is leaking out. If it appears that the leak has stopped you can proceed to the final step in this process.
Step 5 - Close the Radiator Valves
With the boiler valve leak properly sealed and the boiler working properly again, go back to the radiators in your home and close up the valves. Use the valve key again to turn the valve position back to the off position that it was in when you started this project.