The boiler relief valve or the blow off valve is a safety valve that protects your heating system from building up to much pressure and possibly blowing up. It's a very important part of your heating system. Some times the relief valve or blow off valve will leak, and naturally people assume that it is because the relief valve is defective.
The First reason:
Nothing lasts for ever, and a relief valves or blow off valve will go bad. A boiler relief valve needs to be tested every three months to be sure it is operating properly. if you have not checked yours out for a long time, you should have it checked out ASAP.
A boiler relief or blow off valve, that has not been tested in a long time may become rusted closed, causing it to be as dangerous as not having one at all. You can test it yourself but it can be dangerous you can get scalded by the 180 degree to 210 degree water, Water this hot will cause a very serious burn. and in most cases once you pop the lever, the valve will not seat again, and the valve will need to be replaced. if you want to do it your self turn the power to the boiler off and leave it for about 2 hours before pulling the lever. Check the temperature first.