PEX Hot Water Line Temperature Rating

I looked at a house yesterday where a number of the hot water heating pipes for the boiler were replaced with the orange PEX tubes. The stamped temperature rating on the tubing is 180 degrees so I ran the boiler for a while to see how high the boiler temperature would go. The temperature reading on the pressure gauge and the digital readout on the limit control both showed a high temperature reading a bit over 180 (182 degrees to be exact).

How significant is this in the fact that the boiler heated the water to a temperature in excess of the stamped rating on the PEX tubing? If the piping goes through multiple heating cycles where the water in the piping reaches this temperature, how likely is failure to occur? The easiest fix would probably be to have the limit control adjusted or changed to ensure the water in the boiler does not limit out that high again. The more costly fix would be change out the PEX tubing to copper but I don't know if that's necessary.

Thoughts?

The 180f rating on the PEX has a built-in tolerance that would exceed that ratting but it will not be recommended or even published for the public. Kind of like the Sell By Date on food products. We all know that the food will be good for a few days or even months after the date, but do you want to take the risk?

From what I can tell after doing some research for a litigation case involving PEX, once you get over the max temp ratting the PEX is subject to deformity with higher temps and pressure. This is when the fittings and couplings will start to fail. In other words if you go over the 180f degree ratting for the material you do so at your own risk....

Good info Scott. Thanks. I'm going to state in the report that the operating temperature of the boiler water appeared to exceed the maximum temperature rating stamped on the PEX tubing and could make the tubing susceptible to leaks/failure.

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Originally Posted by Scott Patterson

From what I can tell after doing some research for a litigation case involving PEX, once you get over the max temp ratting the PEX is subject to deformity with higher temps and pressure. This is when the fittings and couplings will start to fail. In other words if you go over the 180f degree ratting for the material you do so at your own risk....

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The 180°F temperature is only one of two critical factors you need to know when considering what effects the temperature will have on the PEX piping.

First, the temperature itself, if high enough and for long enough, can cause the material to start to deteriorate simply due to the high temperature, although (as Scott said) 182°F is not going to do that as that has a conservative 180°F rating on it. As I recall, the safety factor is 2x+.

The other critical thing you need to know, and is sometimes even more important that the temperature, is the pressure. The minimum rating for water-distribution piping (which includes PEX) is:
- P2904.5 Water-distribution pipe. Water-distribution piping within dwelling units shall conform to NSF 61 and shall conform to one of the standards listed in Table P2904.5. All hot-water-distribution pipe and tubing shall have a minimum pressure rating of 100 psi at 180°F (689 kPa at 82°C).

However, if you have 182°F and the pressure is 150 psi ... you got problems.

Conversely, if you have 190°F and the pressure is only 60 psi, you will likely not have deformation, which would lead to the problems Scott referred to. The maximum pressure allowed for the water-distribution system is:
- P2903.3.1 Maximum pressure. Maximum static pressure shall be 80 psi (551 kPa). When main pressure exceeds 80 psi (551 kPa), an approved pressure-reducing valve conforming to ASSE 1003 shall be installed on the domestic water branch main or riser at the connection to the water-service pipe.

If the maximum pressure is kept under that 80 psi pressure, there should be no problems with the PEX at a slightly elevated temperature such as your 182°F.

Yes, at some point in elevated temperature with the pressure at the maximum 80 psi pressure, there will be deformation. I'm not sure if Scott has been able to obtain what that deformation temperature is at the maximum allowed 80 psi pressure.

This is from the ASTM standard for PEX:
- 3.2.6 standard thermoplastic tubing materials designation code—The tubing material designation code shall consist of the abbreviation for the type of plastic (PEX) followed by four Arabic digits that describe short-term properties in accordance with applicable ASTM standards.
- - (1) The first digit is for chlorine resistance tested in accordance with Test Method F 2023. A digit “1” indicates the PEX tubing has been tested and meets the F 876 requirement for minimum chlorine resistance at the end use condition of 25% at 140°F (60°C) and 75% at 73°F (23°C). A digit “0” indicates it does not meet this requirement or it has not been tested.
- - (2)The second digit is a “0”. This digit is reserved for another PEX tubing property.
- - (3) The last two digits are the hydrostatic design stress for water at 73°F (23°C) in units of 100 psi with any decimal figures dropped. Where the hydrostatic design stress code contains less than two figures, a zero is used before the number.
- - Thus, a complete material designation code for PEX tubing shall consist of the three letters “PEX” and four digits. For example, PEX 1006 is PEX tubing with a 630-psi design stress (1250-psi HDB) for water at 73°F (23°C) that meets the minimum chlorine resistance requirement. PEX material designation codes are listed in PPI TR-4.

In the attached table, note that the design pressure for 180°F is shown as 400 psi, with the water pressure rating being 100 psi.

Note that at 73.4°F the design rating is 630 psi and the pressure rating is 160 psi.

At 200°F, the design rating is 315 psi and the pressure rating has dropped to 80 psi.

This piping was for the boiler which has a pressure relief valve designed to open at 30 psi so as long as that valve is functioning properly, there should never be a pressure issue.

Be careful here. The referenced standards and codes are correct; but not applicable in this case. The codes cited are for "distribution" water. There are separate PEX ratings and codes for heating purposes. This covers both hydronic heating and simpler boiler applications. You will find the PEX (color coded for material) with a 180°F stamping, will be rated for about 200°F for heating purposes. This is generally consistent with the thread here, as the pressure is much less in a heating system. BTW the "orange" tubing is the correct tubing (vs red or blue or white) for a heating system. This is because it has an oxygen barrier.

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Originally Posted by Mike Grant

Be careful here. The referenced standards and codes are correct; but not applicable in this case. The codes cited are for "distribution" water. There are separate PEX ratings and codes for heating purposes. This covers both hydronic heating and simpler boiler applications. You will find the PEX (color coded for material) with a 180°F stamping, will be rated for about 200°F for heating purposes. This is generally consistent with the thread here, as the pressure is much less in a heating system. BTW the "orange" tubing is the correct tubing (vs red or blue or white) for a heating system. This is because it has an oxygen barrier.

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From the IRC:

Table M2101.1 Hydroponic Piping Materials is the table for hydronic piping, this table allows PEX meeting ASTM F 876 and F877, the table says "install in accordance with manufacturer's instructions"

Table P2906.5 Water Distribution Pipe, this table allows PEX meeting ASTM F 876, F 877, and CSA B137.5.

From the IMC:

Table 1202.4 Hydronic Pipe is the table for hydronic piping, this table allows PEX meeting ASTM F 876 and F877

Notice that all of the tables allow PEX, and each of the tables states the same ASTM standards, as shown above.