The REAC Guru Bulletin Board

What is an example of a water heater drip leg pipe with restrictions and what does it look like when
properly installed?

I think you’re asking, “What would cause a restriction in the pressure relief valve drip leg?” Correct me if I misunderstand.

First of all, the pressure relief drip leg or extension tube/pipe is going to look different in Florida, Texas, and California than it does in states where we deal with freezing outdoor temperatures. You will find pressure relief lines that run out through a wall to the outdoors in these warmer regions. You will also NOT find basements. In much of the country, you’ll find the water heater in a basement, where it is below ground level, so the water in the pipe cannot run outdoors via gravity.
So, in most areas, a pressure relief valve has a pipe that points down toward the floor, maybe into a drain pan, maybe into a floor drain.

IF the diameter of the pipe is reduced at some point, that is a restriction. If the pipe is bent, causing a kink, that is a restriction. If the pipe terminates too close to an immovable object, that is a restriction. In other words, what we are trying to avoid is any reduction in the capacity of the pipe to carry a sudden burst of fast moving water under high pressure. Anything that might restrict the flow so the pipe explodes or the water sprays everywhere is a serious problem.

According to the NACHI web page at https://www.nachi.org/tpr-valves-discharge-piping.htm

It is critical that discharge pipes meet the following requirements, which can be found in InterNACHI’s Water Heater Discharge Piping mini-course, at www.nachi.org/education. A discharge pipe should:
1. 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.
2. not be smaller than the diameter of the outlet of the valve it serves (usually no smaller than 3/4").
3. not reduce in size from the valve to the air gap (point of discharge).
4. be as short and as straight as possible so as to avoid undue stress on the valve.
5. be installed so as to drain by flow of gravity.
6. not be trapped, since standing water may become contaminated and backflow into the potable water.
7. discharge to a floor drain, to an indirect waste receptor, or to the outdoors.
8. not be directly connected to the drainage system to prevent backflow of potentially contaminating the potable water.
9. discharge through a visible air gap in the same room as the water-heating appliance.
10. be first piped to an indirect waste receptor such as a bucket through an air gap located in a heated area when discharging to the outdoors in areas subject to freezing, since freezing water could block the pipe.
11. not terminate more than 6 inches (152 mm) above the floor or waste receptor.
12. discharge in a manner that could not cause scalding.
13. discharge in a manner that could not cause structural or property damage.
14. discharge to a termination point that is readily observable by occupants, because discharge indicates that something is wrong, and to prevent unobserved termination capping.
15. be piped independently of other equipment drains, water heater pans, or relief valve discharge piping to the point of discharge.
16. not have valves anywhere.
17. not have tee fittings.
18. not have a threaded connection at the end of the pipe so as to avoid capping.