Modern water heater tanks have a “temperature pressure relief” safety valve called a TPR valve which is located either in the top or side of the tank. This valve is piped such that it vents hot water to the atmosphere when one of the following two conditions Exist:
1) High hot water tank pressure greater than the design set-point of the relief device, this is typically 150 psi on home hot water tanks.
2) High hot water tank temperature greater than a reasonable setting for your typical controls, most are set for 200 degrees Fahrenheit.
This valve is designed to be the failsafe which prevents the hot water tank from rupturing due to one or both of the conditions previously mentioned. These valves can work reliably for the life of your water heater and only rarely do they fail. When these do fail it is fortunate they tend to fail in an open position allowing water to vent even when you don’t need it to. This is caused by mechanical wear and tear and by corrosion byproducts getting stuck inside them and underneath the sealing surface which keeps the water in the tank.
Many people have no idea that this device even exists until it begins to leak slowly onto their tile flooring. Since most are tucked away in the basement or a closet most people barely even notice the small drips that can come from these valves on occasion. I have had really good success with these valves over the years by doing a semi-annual burp test on them. This test is where I intentionally open the valve using the activation lever that is built into the top stem of the valve and I vent into a bucket at least two gallons of water. For this burp test I will usually do one long blast for the first gallon followed by two shorter half gallon blasts. Once this is done I make sure the valve seats by pressing down on the stem that the activation lever is attached to and then I watch to make sure the flow has truly stopped. If it has not then I will repeat the short blasts followed by pushing down on the stem when I let it go closed.
This test can help clear away debris which can collect on the internal sealing surfaces of the valve. I have witnessed more problems with this valve on well water than on city water. In my experience tanks with higher water temperature settings above 130 degrees Fahrenheit tend to drip and leak more often than those with settings lower than 130 degrees Fahrenheit.
One smart thing to consider is to route this vent pipe to bucket so that if it begins to weep and drip then you do not have a wet floor before you notice it is leaking. You must never block or restrict this valve opening! It is very dangerous to do anything which will limit its ability to vent off pressure automatically. I was once asked by a homeowner why I could not just solder a valve onto the end so that they could shut it off if it began to leak. This would void all warranties as it could turn the tank into a bomb if the heating elements were to stick in the on position or if somehow the water pressure went too high. So obviously this request was politely refused with a short and firm explanation of the possible consequences of such a plumbing modification.
NOW getting down to the actual repair!
First thing you need to do is get a new replacement valve. There is a metal data tag on top of these valves under the release lever and all you need to do is take this data to the plumbing supply or home improvement center that you use. Once you are back with the part turn off the heating source whether it is electric or gas you need to shut that off. Then you need to isolate the hot water tank by closing the inlet and outlet valves.
It is wise to shut off the well water pump to prevent any possible problems due to the pump cycling while you are doing this repair. Next you want to drain out some water down to the level of the fitting you are about to remove. This level is easy to guess if your valve is in the top of the tank as all you need to do is open this vent valve and allow the water to drip into a bucket. Once you have done that you can leave the relief valve and open the bottom drain on the tank to allow more water out so that you do not have a flood of water coming out of the tank once this relief valve is unscrewed.
In order to get the valve to turn the four to six turns that will be required for it to be removed from the tank you will need to remove the extension pipes that are connected to this valve. Your plumber should have installed this pipe with a union or some sort of threaded joint such that you can unscrew the long copper extension pipe. If there is not way to undo this line then you may have to cut it off leaving enough room to make a coupling or union joint in the new installation.
Take your long pipe wrench and with a long bar laid across the base of the tank inlet and outlet fittings on top of the tank for leverage you then draw the pipe wrench counter clockwise to loosen this vent fitting. This vent valve will probably be very tight and will resist your efforts to unscrew it. This is a job where patience will win out and you do not want to hammer on anything as this can case you to break things that are not part of your original repair plans. If you can not get this fitting to budge then add an extender pipe to the pipe wrench to give you more leverage. If the tank begins to tilt or rotate along with you then you will need to secure the tank so that you do not break the inlet and outlet pipes.
I have used braided ratchet straps around the tank with a board caught underneath which gives me a push point to wedge my shoulder against when I am pulling on the pipe wrench. There are times when an extra helper is a good idea providing there is room for the second person to get in there and hold onto something. As you unscrew the fitting watch for water that is higher than the top of the fitting you are unscrewing the relief valve from and open the bottom drain valve to let out more water. Make sure you close this valve when the water level is down below the top edge of the tank fitting.
Preparation of the new fitting calls for a cleaning of any residual sealant left on the threads in the tank fitting, and putting pipe dope on the threads of your new device. For the record the Rector Seal brand sealant is the only pipe dope that I will ever use. Once you have applied a thin even band of pipe dope onto the first three or four threads of your new TPR valve it is time to install it into your tank. Screwing the part into the tank by hand as far as you can go then looking at the point where you want the drip pipe to go consider that you will be able to get at least one full turn on the valve from where the hand tightening stopped. You are better off rerouting the vent pipe if the new valve stalls at a different location than the spot from before. It is okay to try for a quarter turn more if you absolutely have to but I would not suggest going backwards as this can cause the valve to leak at the threads. Do not use a cheater bar to tighten the new fitting into the tank, only use a 10 or 12 inch pipe wrench.
Once the fitting is tight then you must reconnect the vent line and route it into a bucket so that the tip is above the top edge of the bucket. This prevents accidentally causing any back pressure on the valve which can spray hot water all over the place if the vent valve does leak in the future. Now turn the water back only slowly and check the new fitting for leaks at the tank threads. Next you need to turn on the well pump if you have it disabled, then relight the pilot flame on a gas burner or turn the breaker back on if it is an electric water heater.
I always check back in on the job in a day or two to ask how things look in case a drip has started. I feel that it is better to know now than it would be three months later. For the timid the new device has installation instruction which come with it, and you can always get advice from someone in the know. The Internet and Associated Content are great places to get more detailed information if you need. Good luck and never fear!