We all know that a water heater discharge pipe cannot be less than a 3/4 inch pipe without a reduction in size but city/county inspectors have a different opinion…until now! I gave the building inspectors (Arizona: Flagstaff, Williams and Coconino county) the inside diameter measurements of a 3/4 inch copper pipe, a 1/2 inch copper pipe and a PEX B fitting. We all know that a 1/2 pipe is not acceptable for a water heater discharge pipe. I demonstrated that the inside diameter of a PEX B fitting is smaller than the inside diameter of a 1/2 copper pipe. Changes are in the works! I hope this is helpful!
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Everyone and their brother knows that it will likely never be an issue in the real world, but, yeah, that type of elbow would not be allowed on the discharge piping.
Curious, why would it never be an issue? I have never seen a triggered TPR valve with PEX B fitting, so I cannot possible say with any experience. What data do you have to support your statement?
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Mainly because of this…
Nobody has ever really seen one, because it is so rare. Try and find a video of a release into a slightly constricted discharge pipe. You will likely not find one.
The valve is going to release small amounts of water and/or steam at a time if it should pop off. It will not be like a steam whistle.
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What am I missing here? Is this possible? Can the outside diameter be smaller than the inside diameter? (the pex fitting is likely accurate)
Then, I found this chart online. Nominal sizes are not what we think they are.
Where ***M is thin walled, above ground residential and light commercial uses.
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I’ll be honest. TPRV discharge piping is not a hill I plan to die on. My stance is to just use common sense. If it happens to discharge, can it do it in a safe manner? I will call out the reductions in size if significant, or the occasional plugged-off discharge, and if the piping has a spot that can retain water. Also if PVC or another improper material is being used. But, I’m not willing to get into the decimal places of the size of the piping.
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I completely agree. I don’t spend much time on gotcha moments unless it has a reasonable negative impact on the home. I have seen inspectors write up TPRV pipes that are 1 inch too high from the ground. They put the tape measurer in the pic and everything 
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You’re a couple of years behind. There is already a comment in the current IRC about upsizing the discharge fitting size when using an insert fitting such as PEX.
I don’t know if this is referenced in the 2018 IRC. It may have been, this is about the time this change was made. If this is what your jurisdiction follows then a 3/4” PEX fitting would not be an issue. In the next IRC adoption, it will be an issue.
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To answer your question: You are missing calipers. I have provided a picture of the two differnt ones I have. Measure twice cut once. Please refer to the actual sizes of the OD and ID of the pipes. Have a wonderful day.
Yes, you are correct. My point here is that, like Ryan and the city/county I live in, so many have accepted the status quo. I often find city/county new construction that does not pay attention to item number #14 of the 2018 IRC code. I cite it and yet they still say its OK with the smaller PEX B fitting. So, I had to demostrate in front of them at their offices by showing them with emperical evidence. Which, is also why I created the picture for my results. New construction should be built to code not by status quo
P2804.6.1 Requirements for discharge pipe.
The discharge piping serving a pressure relief valve, temperature relief valve or combination valve shall:
14. Be one nominal size larger than the size of the relief-valve outlet, where the relief-valve discharge piping is installed with insert fittings. The outlet end of such tubing shall be fastened in place.
“A relentless barrage of ‘why’s’ is the best way to prepare your mind to pierce the clouded veil of thinking caused by the status quo. Use it often.”
– Shigeo Shingo
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Your response is good though, at least for the picture I add to my reports. I will change it to the listed data in the table with the emperical data. Because someone is likely to challange the measurements, like you did. I will also add the calipers too. In any case, a TPR valve and discharge pipe are considered in the realm of device safety. Not willing to let safey take a 2nd chairs to “Let common sense rule the day”.
Geeze guys. All this fuss about the diameter of a short piece of piping that for 99.999% of homes will never be utilized to blowoff any significant amount of super-heated water. If there was no TPR valve discharge piping at all, none whatsoever, the exit of the TPR valve is in most cases is already facing downward.
That’s a definite defect in my book worth mentioning! Many of the installations I encounter are in the laundry room. Sudden discharge with no pipe is a serious safety problem for the laundress (female-XX) or launderer (male-XY).
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I do as well, but put it in the “maintenance items” category. Just being pragmatic. Most water heaters are in the garage in our neck of the woods. Water heater overheating is exceedingly rare and the chance that someone just happens to be sitting underneath the TPR valve when that happens is inconceivably small.
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Something to chew on: What happens when a TPR valve fails? They explode I suppose, never seen it it myself but there are plenty of videos and pictures on the internet when you google ‘water heater explode’. So, some those might be related to gas issues, so who knows…right? But it happens enough to be on the interwebs 
If some of those water heater are exploding due to failed TPR valves, then the question becomes how many TPR valves are failing versus succeeding? You should see the myth busters video; that will blow your mind!
If no one has observed a triggered TPR valve then how do you know: It will not be like a steam whistle
Anyhow, good discussion here all around. I thank you for your input.
Hey Martin, I thought you might enjoy reading this little nugget from the City of Flagstaff. While the city does post the codes that they are referencing, it does not mean they know the codes.
@rkenney @bcawhern1 Thought you might like this too
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Or the reality that likely, no one has lived to relate what they saw when it blew!!
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Nice job Bill!
And that Bill is what I called job security!
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