A common defect we find when a water heater has been replaced by someone other than a professional plumber is that the TPR valve discharge pipe has been trapped, which is not allowed by the building code. Recently we had a homeowner ask us why we were calling out a trapped TPR discharge pipe when they couldn’t find any clogs, crimping, or obstructions in the line—because they misunderstood the meaning of a “trap.”

    A plumbing trap is a section of pipe that allows a volume of water to stay in place after the pipe has drained by gravity. A P-trap under a sink is one kind of trap that is both useful and required, because the pool of water keeps sewer gas from rising up into a home from the sink drain. 

    But a trap in the discharge piping from a TPR valve—which allows standing water to sit in a the pipe after the valve has closed, likely grow bacteria in it over time, and then drain backwards into the water heater if the TPR valve is opened when there is no pressure in the tank—is a safety hazard. An example is shown at the top of the page, with an arrow  indicating the height of a column of water that will remain in the pipe, and another one below.

    The International Residential Code (IRC) and the Residential Edition of the Florida Building Code (FBC 2803.6.3) both have a list of eleven requirements for TPR valve discharge piping, but the two that relate to this defect state that the piping must “be installed to flow by gravity” and “not be trapped.”

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Here’s links to a collection of more blog posts about WATER HEATERS:

• Are water heaters required to be raised off the floor? 

• Can a Temperature and Pressure Relief (TPR or T&P) valve be mounted to discharge horizontally?

• What does ECO INSTALLED mean on a water heater data sticker?

• Where are gas water heaters not allowed to be installed?  

• Is the minimum size water heater inlet pipe 1/2" or 3/4" according to the building code?

• Can you use a light switch for a water heater disconnect? 

• What is the minimum clearance to doors and windows for an outdoor tankless gas water heater? 

• What is required clearance for access and working space in front of an electric water heater? 

• Why is the water heater older than the house? 

• Does a water heater need a shut-off valve? 

• Why should a tankless water heater have an isolator/service valve kit installed?

• When was a gas water heater first required to be elevated 18 inches above a garage floor?  

• Can the temperature and pressure relief (TPR) valve discharge pipe for a mobile/manufactured home water heater terminate under the home? 

• Does a tankless gas water heater still work with no electricity during a power outage?  

•  Why do water heaters have a sacrificial anode? 

• What is the difference between a manufactured/mobile home water heater and a regular water heater? 

• Does a tankless water heater require a pressure relief valve?

• When is a water heater drain pan required?

• Why is there water in my water heater drain pan? 

• What is the difference between a single element and dual element electric water heater? 

• What is an FVIR water heater?

• What is a heat pump water heater?

• What is a dielectric union?

• What's that powdery crust on the pipe connections at the water heater? 

• What are the most common installation mistakes with water heater replacement?

• Why is my water heater making strange (rumbling, gurgling, knocking or banging) noises? 

• What can I do to make my water heater last longer?

• How can I determine the age of a water heater if the serial number is missing or decoding it is impossible? 

• How does a hydronic heating system work?

• What is backdrafting at a gas water heater? 

   Visit our WATER HEATERS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.