Water heaters should have a temperature and pressure relief valve for safety purposes.
When the temperature gets too high, the valve opens up and water runs out through it; this relieves the high water pressure and prevents the water heater from rupturing or exploding.
If there is no discharge line connected to the T&P valve and it opens up, those near the water heater may suffer 1st, 2nd or 3rd degree burns.
Children and hot water
Should a child or adult be standing near the water heater when the valves opens to let out a little of the hot water, they could get burned by the hot water. Therefore, having a pipe hooked to the T&P valve and ran to a safe discharge location is needed.
Where to run the discharge line
In general the discharge line should be ran to a safe and proper discharge location. On new homes, it is often ran to the exterior of the home. On older homes it is not uncommon to run it down to the garage floor if the water heater is in the garage. It should not terminate in the crawl space under the house, have a trap or be subject to freezing.
Copper or galvanized pipe are the most common materials for this discharge line. PVC generally is not approved for use as a discharge material. Another requirement is the end of the line doesn’t have threads on it; that it terminates 6 to 24 inches above the garage floor or ground and that it does not discharge horizontally, but have a 90 on the end of it, directing it towards the ground
Who to consult? Plumbers and qualified handymen.
Home owners can generally install a discharge line themselves. Plumbers and qualified handymen can do this type of work quickly and inexpensively.