I'm installing a steam generator and a tankless water heater. I want to know if I can use ProPEX for the P&T relief and for the steam generator's auto drain valve.

The googling I did on the P&T valve was that the plumbing needed to be secured and that it was hard to secure PEX. for the tankless I was going to use a PEX to Brass threaded fitting with ears and secure that to the wall and exit the side of the house.

The steam generator P&T and drain is going to be plumbed into the grate of a 10x10 floor drain and secured to the drain cover. Of course with the 1" air gap to where the actual water drains.


  • Talking to the Manufacturer of the Steam unit, the concern for the not using PEX was that out of the steam unit the water can get up to 212 degrees F, and the PEX is only rated between 180 and 200 degrees. He said that typically what they do is install the first 18-24" of the line in Copper and then switch to PEX. Though this does not answer the Connection for the water heater. Though I would think it would be similar as long as the drain exists to the outside and is securely mounted. – scooter133 Jan 12 '15 at 4:40

The code requirement in most areas for the P&T relief plumbing is that it must be a material suitable for potable water plumbing - which in turn typically means it must allow a working pressure of 100psi at 180f. This might seem like an inadequate set of requirements for something intended to drain boiling water or steam, well over 180f, but the key term there is working - that is, it needs to remain durable when exposed to that pressure and temperature over long terms. The relief drain plumbing does not need to support pressures that high, and will only briefly be exposed to high temperatures so long term reliability is not required.

You may notice that PEX tubing is marked as complying with ASTM F877. ASTM F877 standards require that the tubing is able to endure 150psi at 210F for 30 days straight.

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The acronym for that pipe is PEX and they do have a hot water version but I wouldn’t do it. The drain is important, and the pressure relief is important and I would stay with a rigid material, something could happen to a plastic material causing it to collapse and then your pressure relief in your drainer isn’t going to be working properly.

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  • Thank you for your reply, There are a Few types of 'PEX', I mentioned ProPex which is Pex-A by Uponor/Wirsbo to reference the type of Fittings, ProPex Expander, that I'm using. – scooter133 Jan 15 '15 at 17:21

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