What are the code requirements for the discharge piping from a Temperature-Pressure Relief (TPR) valve at a water heater?

Thursday, March 7, 2019

Temperature-Pressure Relief (TPR) valves rarely go off; but, if one does, it’s a big deal. Super-hot and high-pressure water gets released to keep the tank from exploding like a bomb. So, both the International Residential Code (IRC) and the Residential Edition of the Florida Building Code (FBC P2804.6.1) have the following requirements to make sure that there are no obstructions or the possiblity of injury from the scalding water if the valve opens. Here they are outlined below. And, unfortunately, we have an abundance of examples of how not to do it.

1) The discharge pipe cannot be directly connected to the home’s plumbing drainage system, but an air gap located in the same room as the water heater can used to connect to the drainage system. An air gap eliminates the possibility of backflow from the drain into the discharge piping. The photo below shows TPR and catch pan drains that dump into a laundry sink without necessary minimum 1-inch air gap above rim of sink. To read more about it, see our blog post What is an indirect waste receptor?

2) The pipe should not be smaller than the outlet of the valve and must run full-size (no reductions) to the termination. 

3) Each TPR valve must have its own discharge piping, not shared with any other devices or equipment. In the example below, an upper pressure relief valve drain (not visible behind vent connector) has been tied into the drain for the lower one.

4) Termination of the discharge piping can be to any of the following:

5) The piping must discharge in a way that does not cause injury or damage. When there is no TPR discharge piping, or it terminates horizontally, anyone unlucky enough to standing in front of the water heater when the valve opens will be severely burned.

6) Must terminate in a location that is readily observable by the occupants of the home. If the water heater thermostat fails to turn off at its temperature setting and continues to heat the water indefinitely, the TPR valve will open and let the overheated water out; but, if the termination of the piping is not readily visible—such as if it is located under the house—the water will continue to drain until the homeowner gets an amazing electric or gas bill and goes searching for the reason. Also, sometimes the valve fails by opening partially and leaking a continuous small stream of hot water, like the one shown below, under an older mobile home..

7) The piping must drain by gravity and not be trapped.  A plumbing “trap" is a part of a pipe where water remains in place after gravity has drained the rest of the water. Because this stagnant water could sit for months and possibly drain backwards into the tank if the TPR valve is opened manually and there is no water pressure at the time, introducing bacteria into the tank, the pipe must completely drain by gravity. A trapped TPR dischage pipe is a common defect when a replacement water heater’s TPR valve is lower than the original one, and the installer connects to the exisitng discharge piping at the wall. 

8) Termination of the TPR drain pipe cannot be more than 6-inches, or less than two pipe diameters, above the floor, ground, or waster receptor flood rim level. Again, for safety of anyone nearby when it goes off. The example shown below, with the pipe almost touching the floor, will impede the water flow and cause it to splash upwards.

9) Must not have a threaded connection at the end of the piping. Multiple instances of water heater explosions have been caused by a handyman capping the end of a nuisance drip from TPR piping. Threading at the end appears to make it acceptable.

10) No valves or tee-fittings in the piping. Nothing should impede or split the direction of the flow. For some reason, the installer of the water heater shown below decided that the TPR drain piping needed a vent pipe.

11) Pipe should be of approved materials. Copper, glavanized steel, CPVC, and PEX or PE-RT are all approved. PVC is not approved, because it is not rated for transporting hot water.

12) If the discharge piping is PEX or PE-RT tubing, then it must be one nominal size larger than the relief valve outlet. Also, the flexible tubing should be fastened in place. 

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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?

 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? 

 How can I tell if a water heater is HUD-approved for mobile/manufactured homes?

 Can you wire a 240-volt water heater with 120 volts? 

 Is it alright to have a shut-off valve on both the hot and cold water pipes at a water heater?  

 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? 

 What is the purpose of a thermostatic mixing valve above a water heater? 

 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 does it mean when a water heater Temperature/Pressure Relief (TPR) discharge pipe is "trapped"? 

 Can I leave a gas water heater in place when remodeling a garage into a family room or bedroom? 

 Where do I find the water heater in a mobile home?

 Does a tankless water heater in an attic require a drain pan? 

 Does an electric water heater require a disconnect? 

 Is a catch pan and drain piping required for a replacement water heater?

 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 the difference between a regular water heater and a direct vent water heater? 

 What is the difference berween a regular water heater and a power vent water heater?

 What is backdrafting at a gas water heater? 

 How do I determine if a water heater is gas or electric?

 What does it mean when a gas appliance (water heater, furnace, or range) has been "red tagged"?  

• What's the valve with the flip-up handle on the water heater for?

• Why is an older water heater an insurance problem? 

   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. 

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