As water gets hotter, it expands. If it gets too hot and expands too much in a water heater, then the pressure needs to be relieved. T&P valves do this. They open up and this reduces the pressure. Most water heater manufactures, as well as codes, require this or some safe method of reducing the pressure.
Like a rocket
Water expands when it get hot and if there is no T&P valve on the water heater that opens up when the temperature reaches a certain temperature, then the tank may rupture or even explode like a rocket and go through the roof. Therefore it is wise to have one.
They’re inexpensive and easy to install.
Getting the proper size of valve and properly installing it is key
T&P valves come in several sizes and settings. It important that the correct valve be chosen for the water heater that it is being installed on. The water heater manufactures installation instructions will provide this information with the water heater and it is usually also available on line as well.
The valves are usually located on the top of the water heater or on the side of the water heater near the top. The T&P valve must be installed so that the temperature-sensing probe is immersed in the hottest water area of the water heater. It must be submerged in the tank a specific distance.
Occasionally, a plumber or home owner will first install a short nipple to the water heater inlet for the the T&P valve, then install the T&P valve with part of the temperature probe running through the nipple. This is wrong because the temperature probe is not deep enough in the proper section of the tank. The concern is that the water in the nipple itself will be a little cooler than that in the tank, thus the valve does not open when it should, due to the temperature in the nipple being a little cooler.
Who to consult? Plumbers and qualified handymen.
DIY home owners can generally install a T&P valve themselves. Plumbers and qualified handymen can do this type of work quickly and inexpensively.