Welcome to Just Answer, I will do my best to Answer your question to the best of my professional ability. Water heaters have a life span of only 6-9 years depending on water quality. Now to replace a pressure relief valve you must first turn the heater off and wait a hour for it too cool down or drain the hot water out of it. Then you are ready to use a pipe wrench , you need a pipe wrench along with a pipe that you can slide over the handle of the wrench to extend the wrench and give you more leverage on the wrench, this acts as a breaker bar. A 3 foot piece of pipe about 1-2 inch in diameter works best , this the extra leverage will help you loosen the pressure relief valve for removal. Some times it is best to tighten the part first and apply some steady pressure clock wise , this will break the seal , then you can go counter clock wise to loosen and remove. Keep in mind the pressure relief valve is installed from the manufacture and is machine tight and will not come easy and metal expands when hot so it would be best to let it cool off first so the metal can go back to its original size and make this as easy as it can, also when installing the new one, make sure its ratted for the exact same psi range as the old one you are taking out. Also use pipe thread compound on the treads of the new one so you don't have a leak.
Water heater after drained can take up to one hour to heat back up or longer , as long as you started it back up and the burners are on and burning then the water will be start to heat up in the next hour or so. Also If this is a electric water heater then if you turned the power on before the tank wall filled all the way then you burnt out the heating elements and will now need to change them. Also the pressure relief valve must be solid ridged pipe metal iron or copper most towns do not allow plastic , I know they sell them at home depot but most all towns do not allow them and must be metal and the pipe should be cut 4-6 inches above the floor to allow for a air gap .
Yes i was telling you that the pipe must me metal and not plastic , I don't even know why they sell them at the store , they are a code violation and must be copper or iron pipe. The heater will take up to a hour to heat back up and thats why you don't have hot water yet. Every hot water tank has a step by step instructions on how to light the tank , these will be written on the side of the tank and once you get the tank lite it will take several hours for the water to heat back up before you have hot water again.
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