Cody Casey
The Temperature-Pressure Relief (TPR) discharge pipe is an important component of a water heating system. It is designed to safely direct hot water away from the heater in the event of a temperature or pressure surge, preventing injury or damage. While some installations may not include a pipe, it is improper and unsafe. The TPR discharge pipe should be made of approved materials such as copper, galvanized steel, CPVC, or PEX and should terminate within a certain distance from the floor to ensure safety and comply with local codes. In some cases, a catch pan with a water alert device or drainage is also recommended. However, directly connecting the TPR discharge pipe to a drainage system should be avoided to prevent potential contamination of the potable water system.
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What You'll Learn
T&P discharge pipe and drain pan line can't be directly connected
The T&P discharge pipe and drain pan line cannot be directly connected because it would be against code to directly connect the two pipes together due to the resulting cross-connection between a potable system and a drainage system. The pipe that drains the pan is considered part of a drainage system (either sanitary or storm, depending on what it connects to or where it terminates). The T&P relief valve's discharge pipe is connected directly to the potable system and is therefore considered part of the potable system.
In a typical installation, the relief valve's discharge pipe terminates indirectly above the drain pan's drain opening with an air break that prevents the two systems from being cross-connected. This configuration ensures that water flowing down from the relief valve does not flow back into the pan and cause an overflow. The requirement for a drain pan's drain pipe is that it be twice the diameter of the relief valve's discharge pipe.
To install a drain pan under the water heater, one must follow the code requirements. This includes ensuring that the relief discharge pipe is no longer than 2m (6' 6") and that no more than two 90° elbows are used. The T&P discharge pipe should also be installed at a height of approximately 6" above the floor, directing hot water away from the face in case of a release.
It is important to note that while it is common for the T&P valve not to have a pipe attached, it is improper and can pose safety risks. The T&P valve is a safety device meant to release hot water or steam in a safe direction away from people, and piping it to the floor or into a catch pan with a water alert device is a recommended practice.
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T&P discharge pipe must drain to an approved location
The T&P discharge pipe must drain to an approved location, which is usually just a pipe that is attached to the T&P and sits around 6 inches above the floor. This is because the T&P discharge pipe is connected directly to the potable system and is considered part of it. As such, it is against code to directly connect the T&P discharge pipe to the drainage system, as this could contaminate the potable system.
The approved location for the T&P discharge pipe is typically just above the floor, where it can direct hot water and steam away from people's faces and towards the floor, reducing the risk of burns in the event of a release. The pipe should be made of approved materials such as copper, galvanised steel, CPVC, or PEX or PE-RT tubing. PVC is not approved as it is not rated for transporting hot water.
The T&P discharge pipe should terminate in a readily observable location to prevent accidental scalding or property damage. If the piping is not visible, such as if it is located under the house, it could go unnoticed, resulting in costly water damage. Additionally, the piping must drain by gravity and not be trapped to prevent stagnant water from sitting for months and possibly draining backward into the tank, introducing bacteria.
In some cases, the T&P discharge pipe may be required to drain into a catch pan or drain pan. This is necessary when a hot water tank is located in a ceiling or roof space, or over a floor of wood construction. The catch pan should have a water alert device and be piped to drain. However, it is important to note that the drain pan's drain pipe must be two times the diameter of the relief valve's discharge pipe to prevent overflow.
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Drain pan requirements
Drain pans are required when a hot water tank is located in a ceiling or roof space, or over a floor of wood construction. The pan must be made of an approved material, such as galvanized steel or aluminum of a minimum thickness of 0.0236 inches (0.6010 mm), plastic of a minimum thickness of 0.036 inches (0.9 mm), or other approved materials. A plastic pan beneath a gas-fired water heater must be constructed of material with a flame spread index of 25 or less and a smoke-developed index of 450 or less when tested in accordance with ASTM E84 or UL 723.
The pan shall be no less than 1.5 inches (38 mm) deep and shall be large enough to catch all dripping or condensate from the tank or water heater. The pan shall be drained by an indirect waste pipe of no less than 0.75 inches (19 mm) in diameter. The pipe that drains the pan is considered part of a drainage system (either sanitary or storm depending on what it connects to or where it terminates). The drain pan's drain pipe should be two times that of the relief valve's discharge pipe diameter.
The T&P relief valve's discharge pipe is connected directly to the potable system and is considered part of that system. It is against code to directly connect these two systems as it could result in the contamination of the potable system. The relief discharge pipe should be no longer than 2m (6' 6') and should not use more than two 90-degree elbows. If the piping exceeds these specifications, the valve manufacturer's installation instructions must be followed to ensure that the piping does not restrict the valve's discharge capacity.
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Pipe diameter requirements
The pipe diameter requirements for drain pans vary depending on the specific application and local code regulations. Here are some general guidelines and considerations for pipe diameter requirements:
For a typical installation, the T&P relief valve's discharge pipe should terminate indirectly above the drain pan's opening with an air break. This prevents the potable system and drainage system from being cross-connected, which could potentially contaminate the potable system. The drain pipe for the pan should be at least twice the diameter of the relief valve's discharge pipe.
In the case of water heaters, the General Services Administration Residential Code specifies that the drain pipe for the pan should be no less than 3/4 inch (19 mm) in diameter. The pan itself should be at least 1 1/2 inches (38 mm) deep and large enough to collect dripping or condensate from the water heater.
For air conditioning units, local codes dictate the size of the condensate pipe. The slope of the pipe should be at least 1/8" per foot or 1%, meaning for every 12" horizontally, there should be a drop of at least 1/8" vertically. If the air conditioner is in an inaccessible location, such as an attic or above a ceiling, a secondary drain pan and piping system may be required to protect the building from water damage and to alert occupants of any leaks.
In some cases, it may be necessary to repipe the drain line to a larger diameter to accommodate the installation of a drain pan and T&P discharge pipe. This is often dictated by code requirements.
It is important to note that the pipe diameter requirements may vary based on local codes and specific applications, so it is always recommended to refer to the local code regulations and seek professional guidance when designing or modifying plumbing systems.
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TPR piping must drain by gravity and not be trapped
Temperature-Pressure Relief (TPR) valves are safety devices installed on water heating appliances, such as boilers and domestic water supply heaters. TPRs are designed to automatically release water if the pressure or temperature in the water tank exceeds safe levels. If temperature sensors and safety devices such as TPRs malfunction, water in the system may become superheated (exceed the boiling point). Once the tank ruptures and the water is exposed to the atmosphere, it will expand into steam almost instantly and occupy approximately 1,600 times its original volume. This process can propel a heating tank like a rocket through multiple floors, causing personal injury and extensive property damage.
Water-heating appliance explosions are rare due to the fact that they require a simultaneous combination of unusual conditions and the failure of redundant safety components. However, if a TPR valve does activate, it will release a powerful jet of hot water from the discharge pipe. This is why it is critical that discharge pipes meet specific requirements.
The TPR 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, it could drain backward into the tank if the TPR valve is opened manually and there is no water pressure at the time. This is why it is against code to directly connect the potable system and drainage system pipes, as it would result in a cross-connection that could contaminate the potable system.
The discharge pipe should be piped independently of other equipment drains, water heater pans, or relief valve discharge piping to the point of discharge. It should also be constructed of an approved material, such as CPVC, copper, polyethylene, galvanized steel, polypropylene, or stainless steel. PVC and other non-approved plastics should not be used since they can easily melt.
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Frequently asked questions
Yes, a TP discharge pipe can drain to a pan. However, the pan should have a water alert device or be piped to drain.
The pan should be not less than 50mm (2") larger than the tank and have a drain located directly under the TP discharge pipe.
The piping must drain by gravity and not be trapped. It must terminate in a readily observable location and discharge in a way that does not cause injury or damage.
Approved materials for the pipe include copper, galvanised steel, CPVC, and PEX or PE-RT. PVC is not approved as it is not rated for transporting hot water.
