When the pressure-release valve on the top of a boiler or a hot water tank is leaking it could mean several things.
The first thing to try is to simply lift the little lever on the top of the tank to let out some hot water. Before you do this, make sure that the valve has a discharge pipe attached to it and that it is directed to the ground, away from your feet. It will be very hot water coming out. If there was simply a bit of grit stuck in the valve, this will solve the problem.
If the valve still leaks, either the valve is defective, or that there is a pressure problem in the tank. In either case, don't mess with it. Get a plumber in to check things out.
If you want to change the valve yourself, OK, but if the new one leaks, that really means that you have dangerous pressures in the tank and you need the help of a professional.
Did you know that the valve manufacturers recommend that you trip the valve handle once a year just to check if it is working? Has anyone ever tripped one of these things? Something worth thinking about.
And while I am at it -- if you have a hot water tank higher up in the house with finished ceilings below, you might want to consider buying a retaining basin to put under the tank. Then if it leaks, you have a chance of catching the problem before it ruins the ceiling below.
And if it is your in-laws below you, you may even want to put a 'water alarm' into the basin, to let you know when it leaks. Most alarm companies now sell water sensors to add to their alarm systems that can detect any spill on the floor near near or under an appliance. For a unique flood alarm that you might find useful for more than just detecting water spills - like knowing when the bathtub or swimming pool is full, follow this link to "Rialco Flood Alarm"