An annual test of the relief valve is a must. This is done to ensure a safe passage for water, in the event that the system was ever to over pressurize. The relief valve's job is to release the pressure of the water. The system is usually designed to never exceed 30 PSI. Raising the pressure on water raises the boiling point of the water. If there was ever a leak, allowing the atmospheric pressure into the system and the water temperature was higher than it's boiling point, it could cause the water to immediately vaporize to steam.  This could cause an explosion. The relief valve is deigned to prevent this from happening by opening (thus reducing both pressure and temperature) before this unsafe condition can occur. There should be a tail pipe on the outlet of the relief valve to direct the water to the floor, and not allow it to blow all over the room.

To test the relief valve, lift up on the handle and then let it go. Water should flow out the tail pipe, then stop. If the water pressure falls below 12 PSI, open the water line and reset the pressure. Then turn off the water valve.