Pex & Tpr

I inspected some new construction the other day where they had a PEX line running from the TPR on the water heater.

I’m not real familiar with PEX and was wondering how flexible it gets when hot water is running through it. Is this discharge going to turn into a broken fire-hose during a major discharge, or will it remain mostly rigid?


I’d write it up, siting that its use is restricted to cold water supply, only.

Are you sure about that? They had both Red and Blue PEX (hot and cold) throughout the house. I’ve seen this before, just not on a TPR.


Depends on who you talk to.

The vendors say it can be used for both systems, but several plumbing resources (i.e. "Plumber’s Pocket Manual, 10th Edition, Miller, Miller and Almond, page 183) advise using it only with cold water systems.

In your case though, since it is used for hot water throughout the house, why not the TPR - unless the copper pipe serves to provide a rigid channel to force the scalding discharge to the ground where the PEX could have it shooting in all directions?


This was discussed not to long ago, and the answer is some locals allow it on the TPR and others do not.

No one gave an instance where it has failed on a TPR, so if/when I see it my write up is going to basically say that various areas allow PEX, others do not but I know of no known instances where it has failed.

Hope that helps!

– bz

I called the folks at (, and asked the guy who answered the phone if it was a proper usage of the material. His response was “absolutely”. He said the material is pretty rigid, and there should be no problems with a discharge.

I found their catalog to also be fairly informative as well (pexconnection catalog) and helpful in helping educate me a little more.

Thank you all for your comments. That’s very helpful.


I would right it up as not a appropriate application. Think about 150 PSI and 210 degrees…you get a fire hose flying around.


Hey Curt,

I clearly see your point, and agree with with what you are saying. What would you say in the report?

Because I could see this as becoming a huge “p…ing” match between me an the builder. Especially if the manufacturers say it’s appropriate, and the AHJ doesn’t have a problem with it.


Ah, but the pressure is not 150 PSI on a open unterminated line.
PEX is just fine for this application though I would recommend it be secured to prevent whipping in the event of TPR discharge.
My opinion. Take it or leave it.:smiley:


Agree but how do you PROPERLY secure it? no matter what if the TPRV has to be used it will cause a nightmare even if for just a second as a hot water waving wildcat! Way easier to make it copper or gold guard. IMHO.


For the record everyone… This PEX wasn’t particularly flexible. It’s pretty firm. I could move it a few inches, but that’s about all. It was just slightly more flexible can a copper tube. My concern was how it would react after it got hot and under pressure.

I wrote up another TPR valve that same day that had a rubber hose connected to it. That one was totally flexible.

I just don’t know with this one. That’s why I posted, and it’s also why I’m so appreciative of this ongoing discussion.

The best course of action whuld be to check with your local AHJ as I know at least some states do not permit the use of PEX for the TPR discharge piping.

If you want a definitive answer for what is permitted in the areas you inspect, contact your AHJs.

I have never heard of pex becoming more flexible with hot water in it. If it was, the spacing of horizontal supports would be different. I seriously doubt it is a problem on the TPRV----some jurisdictions require that it be strapped to the side of the tank at the bottom, which I think is a good idea any way. It just looks kinda funky because it usually curls a little:D

I am not an expert on PEX but have worked with it in my last two homes. Even when hot water is run through it, it stays fairly rigid. I’d have a hard time believing it would actually whip around upon discharge of the TPR.

My opinion, if the local AHJ allows it, then it’s not wrong so why write it up???


As an inspector, PEX is no problem but we require it to be secured at the bottom. Usually installers will use a hose clamp with 2 very short screws into the metal jacket. Code doesn’t spell out the method or materials of the discharge line, just per mfr’s recommendations.

Uponor bought out Wirsbo a couple years ao and they are the industry leader in PEX >>>

PEX as the TPR drain line is allowed in all the jurisdictions where I have checked. Whenever I have a question, I usually check the city of San Diego and then the cities where all the rich people live (La Jolla, Heritage Golf Estates in Poway, Coronado, Solana Beach, Cardiff by the Sea). If the rich people accept it, then Mr and Mrs Average Joe Blow, the poor people in Logan Heights, and the rednecks in Lakeside and Santee tend to follow along.

That may be true in CA but not all areas of the country.


**This regulation does not allow Cross-linked Polyethylene PEX pipe or fittings to be installed for T & P discharge on water heaters.
**This regulation does not allow Crosslinked Polyethylene PEX pipe or fittings to be installed directly into a water heater opening.

That doesn’t surprise me, which is why I said “in all the jurisdictions where I have checked.”

PEX wasn’t even allowed in California at all until, I believe, February 2002, when a Superior Court Judge up in, I believe, Ventura County basically told the Plumbers’ Union to go back to their caves because they didn’t have the public’s best interest in mind by continuing to disallow the use of PEX by their plumbers.

PEX is not allowed for use as TPR drain piping in CA. It must be specifically listed for this use, and PEX is not.

CPC 608.5 Relief valves located inside a building shall be provided with a drain, not smaller than the relief valve outlet, of galvanized steel, hard drawn copper piping and fittings, CPVC, or listed relief valve drain tube with fittings. . .

The CPC does not even allow PEX to be used for the plumbing distribution system, but it allows local jurisdictions to use it at their discretion.

Local AHJ’s here allow it. I guess they are using their discretion.