In order to install a thermocouple, it's important to first determine whether or not the whole burner needs to be taken out. Find out how to tighten a thermocouple after it's been installed with help from a master plumber and heating specialist in this free video on water heaters.
We're going to go over how to change a thermocouple. First things first, what you're going to want to do is shut the gas off to the unit. You want to make sure that's done. If your pilot light has been on and you're doing this as just a maintenance thing because you feel it's necessary you're going to want to let that cool off because if you reach your hand in there right after you shut it off you're going to get burned, it's hot, just so you're aware. But typically you do this after it's cold and you haven't been able to light it but you've tried so it may be a little bit warm so let it cool off for a minute. Gas off, next you open the door, both the outer door and then the inner door by letting it roll up and pulling it upside down makes it simple. So we know our pilot light doesn't work, so we've determined that our thermocouple's probably going to have to be replaced. It's not a difficult task, sometimes it requires pulling the whole burner out, if it gets to that point I recommend that you do call a certified person to come out and take care of this for you. It's a relatively simple process, what you do is you put your wrench on, it's a hex nut up there, and you spin it off. Remember you're looking for the copper, that's the one that you need to take off. It takes just one little turn, it's not in there super tight, and then it just simply pulls out and down. Now what you do is basically you put your finger on this end of it, and I give it a twist. Some of them are just this simple, others, like I said, you do have to remove the burner, and pull it out not really aggressively because you don't want to bend anything, but you pull it out and there's your thermocouple and there's the piece. Now the new one that you put in is exactly like what you've got, and they make universal ones that have a bunch of extra little fittings inside of them and you simply reverse the process. You take the end of this and slide it into that little hole that's left of the pilot tube and you give it a little bit of a twist while you push it in. Now after you do that it's pushed up in and you can see that it's pushed past where the pilot end is, you know you've got it, you simply push that end back up into the gas valve and you may have to do a little bit of wiggling as you thread it back in. Be careful to not cross-thread because if you do that you can mess up both the gas valve and your thermocouple. And you twist it, if it doesn't twist easy you're probably cross-threading it. Sometimes it takes a little bit of wiggle to start it but that's just how it works. Then you take and you tighten it about another ninety degree turn and that's in there. Make sure that you don't have much shaft wiggle, when I say "shaft wiggle", the copper line there wiggling inside of the fitting itself. If you do, which it looks like this one's wiggling just a little bit, you give it just a little more, and if you'll notice it tightened it up, just like that. Now we go through the process of lighting the pilot light, once again turning it to "On", depressing the button, getting your flame out, lighting your pilot. You hold your button down for about a minute or so, and then release. Make sure that your thermostat's in the vacation or pilot mode, go ahead and turn it to "On", now turn this up past the click, you'll notice the fire fire up and it may come out. Stand back, it may come out just a little bit, you'll feel it, but then you know it's working. Now go ahead and shut it back off and then we reverse the process on the doors. You take the inner door upside down, that's the easiest way to get these back on, stick it down in. The outer door, replace it and then set your thermostat to your setting that you had it at before. Always pay attention to where that is just so you know where you had it because if you were comfortable with it there's no need to change it, it doesn't change. And that's how you change out a thermocouple.