Water Heater Safety
When you are done reading this, you need to check your water heater. Your water heater has a safety device called a Temperature & Relief Valve (T&P Valve). This valve is connected to a drain pipe to take hot water and pressure out of the tank. It is a vital safety device that opens if the temperature of the tank exceeds 210 degrees or if the pressure in the tank exceeds 150 psi. The drain line runs from the T&P valve downward to the exterior of the building, a floor drain, etc.
This drain line is critical for your safety. It carries away potentially scalding hot water, or highly pressurized hot water, or both, to allow cold incoming water into the tank which should prevent boiling. The diameter of this discharge line cannot be less than the T&P valve's discharge outlet and has to be made of heat-resistant material (no garden hose, PVC, etc.) The most common violation I see is that the drain line is "trapped".
This drain line must be gravity fed, i.e. it can't have any uphill rise in the pipe. This would "trap" water in the pipe and act as an obstruction for the hot, pressurized water. This water could also freeze which would be the equivalent of not having a T&P valve installed at all. A T&P valve doesn't open often, but when it does, the water and pressure inside the tank needs to make a quick exit, or the tank could explode.
In many cases, fixing the drain line is easy. Many drain lines are connected with a flexible copper line and you can bend them in such a way as to remove any uphill rise. In some cases though, you may have to replace or re-route the drain line altogether.