When you look at your hot water tank you may just see another appliance, but the engineering term for such a component is a pressure vessel. The definition of a pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure (normal atmospheric pressure). There are many codes and standards written about pressure vessels as they can be very dangerous if they don’t have the proper safety features installed. If the vessel doesn’t have the right pressure safety valve to relieve pressure when it starts to rise past the set limits, then the thing has the potential to explode like a bomb. You can see an example of this in a Mythbuster’s episode where they purposefully take a water tank to its limits (click here for the video clip).
Don’t worry, just because you have a hot water tank in your house, does not mean it is likely to explode. Most water heaters have several controls and safety features to prevent your tank from ever over-pressurizing, but I do want to stress the importance of proper temperature/pressure relief (TPR) valve installation on your hot water tank, boiler, etc. The TPR valve is a safety relief valve responsible for opening and relieving pressure when the temperature and pressure get too high in the tank.
How does the TPR Valve Work?
TPR valves open to relieve pressure or temperature from the tank when the water in the tank exceeds the preset pressure or temperature limit. Normal tanks operate between 120 to 140 degrees Fahrenheit and are regulated by a temperature sensor. If the temperature sensor and other fail-safes go bad, and the burners are left on too long, the pressure will rise in the tank as the temperature increased due to the water wanting to expand. Most TPR valves are set to open at 150 psi or 210 degrees Fahrenheit to relieve pressure before the tank gets to its limit. Note that different tanks and components may have different requirements, so it is important you check the nameplate rating on the water heater and make sure you select a TPR valve that will actuate at or below the maximum operating pressure/temperature limits of the tank.
The TPR valve itself is spring-loaded in the shut position. The springs are set to start to give and open the valve when the pressure or temperature limit is reached, and then close again when the pressure has been properly relieved. Most likely when it is properly operating the valve will cycle on and off, versus if it is defective it will probably be leaking constantly.
Proper Installation of a TPR Discharge Line
As you know by now, TPR valves are vital components of keeping your water heater safe from overheating/over pressurizing. That is why it is very important that they are properly installed to ensure the safety of you and your family. The part of the TPR system that is often not properly installed is the TPR discharge piping which is responsible for guiding any water released by the valve to a safe location. Here are some important installation guidelines to check when it comes to the TPR discharge piping (Please note to also follow the manufacturer’s guidelines for your specific unit as well).
The main job of the TPR discharge line is to direct the flow of any water that is released by the TPR Valve. Therefore, the material of the line must be able to withstand the pressure and temperature of the water exiting through the line. The line should be constructed of a material that is rated to handle these conditions. It is preferred that the line is constructed from metals like copper, galvanized steel, or stainless steel that are rigid and can easily handle the scalding hot water coming out. There are some special plastics that are also allowed like CPVC, polyethylene, and polypropylene that can handle those high temperatures as well. PVC and other non-approved plastics should not be used since they can easily melt.
Ensure Proper Drain Flow Without Restrictions
It is important that the flow of water out of the discharge line is never restricted as this can cause back pressure in the line and cause the valve to not release at the right pressure. The line itself should not be smaller than the outlet diameter of the valve. For instance, if the valve outlet is 3/4” diameter, then the piping should be 3/4”. The line should never be reduced downstream before it discharges as well and should be a straight shot to the floor, not using any unnecessary elbows or teeing off at any point.
In certain circumstances, the TPR valve is on top of the unit and will require an elbow for the line to go vertical. For the horizontal run, the piping should be sloped slightly away from the valve so water doesn’t backflow and puts back pressure on the valve seat. Lastly, this may seem like common sense, but do not cap, plug, or install an isolation valve on the line. This will completely block the line and make it as the valve doesn’t even really exist.
Air Gap and Safe Discharge
The last thing that we need to be concerned with for the TPR Valve and its piping is where it discharges. There is an air gap needed between where the line ends and the drain. This means the line should not be directly connected to a drain because there is a chance for back pressure affecting the valve set pressure as also some chance of back siphoning. If your water heater is not located near a floor drain or is located near a finished area, a drain pan underneath the water heater is always recommended. However, the discharge piping should point into the drain pan and not be directly connected.
Lastly, the discharge piping should terminate within 6 inches of the floor. This is to prevent scalding hot water from burning anyone in the vicinity of the tank if the valve releases. Also, you don’t want water splattering all over the place getting onto electronics, or getting walls wet.