I was just replacing an electrical hot water tank.

Had to call a plumber because I didn't fancy my soldering skills on something that could flood the house. The plumber simply cut off the old copper lines (after the shutoff valve) and used pex to connect the tank and as a vent pipe.

Apart from if I'd known you could do that I would have done it myself - is this OK?
Seems to have been tightened with proper synch connectors and it certainly works.

  • 1
    FYI, if you don't want to deal with soldering copper connections in the future, use SharkBite connectors. They're not cheap (about $10 a connector) But that's still about 1/20th the costs of having a plumber do it.
    – DA01
    Commented Dec 12, 2013 at 8:08

5 Answers 5


PEX is usually rated at 180° F (82° C) -- it should say on the side of the pipe what the actual rating is. As long as your water heater temperature is below that, you're fine.

From wikipedia:

There are two seemingly conflicting safety issues around water heater temperature—the risk of scalding from excessively hot water greater than 55 °C (131 °F), and the risk of incubating bacteria colonies, particularly Legionella, in water that is not hot enough to kill them. Both risks are potentially life threatening and are balanced by setting the water heater's thermostat to at least 54.4 °C (130 °F). The European Guidelines for Control and Prevention of Travel Associated Legionnaires’ Disease recommend that hot water should be stored at 60 °C (140 °F) and distributed such that a temperature of at least 50 °C and preferably 55 °C is achieved within one minute at outlets.

As @Michael points out, a T&P safety valve is rated to go off at 210°F (100°C), so if something does go wrong and your water heater overheats, there is a possibility the PEX will fail. If the PEX does fail, it almost certainly will happen right above the tank (which will be the hottest place, obviously). It will also serve somewhat the same job as the T&P valve, except that a T&P valve will close once the temperature/pressure has lowered, while a burst pipe will just keep gushing water until someone shuts off the supply.

The chances of water heater overheating like this are quite low: electrical heaters have thermal fuses in them that serve as an electronic safety (very hard to find the temperature rating of these: seems like it's anywhere from 167 to 210°F). In addition, the electronics are pretty reliable, and just very unlikely to fail "on", and without drawing too much current that would cause the breaker to trip.


There's no need to use Pex on your water heater, and you probably shouldn't, seeing as how your T&P valve won't go off until after you've already exceeded Pex's maximum operating temperature. Instead, you can use a flex line like this:

enter image description here

Also, domestic hot water should not be 100-120F, but rather around 130F. First off, the average person showers in roughly 105 deg water, and you're probably going to get a 3-5 degree drop from the water heater to the shower, so anything under 110 would be rather out of the ordinary. Second, 100-120 is the typical and preferred growth range for legionnaire's disease. Unless you want to die on account of your water heater's temperature, you should turn it to at least 125F... or, if you don't want to do that, you should at least blast it to 140F once a month.

  • Has there ever been a case of legionaires from a domestic water system? Only ever heard of it in hospitals etc, which is why you scald yourself every time you turn on a tap there. Thanks for the flex-copper link - never seen that.
    – mgb
    Commented May 4, 2011 at 4:28
  • 2
    Yes, there have been cases from domestic hot water systems. See, for example, cdc.gov/legionella/patient_facts.htm . The most recent famous outbreak was at the Playboy Mansion where many people fell extremely ill in a matter of minutes, presumably because the hot tub's bubble jets made it extremely airborne, causing a rapid onset of symptoms.
    – Michael
    Commented May 4, 2011 at 4:40
  • 1
    I shall be careful to stay away from the aerosoled bodily fluids of Mr Heffner and his small rodent friends !
    – mgb
    Commented May 4, 2011 at 12:50
  • Keep in mind that raising the temperature of your water heater >120F increases the risk for burns, especially with young children. Almost any new water heater is set to 120F by default for this reason. Commented May 4, 2011 at 12:55
  • @msemack - I turned the water heater down to 55C (120F) because I would rather have a shower of hot water than boiling water + cold. And I don't want to pay hydro bills to heat my basement all summer!
    – mgb
    Commented May 4, 2011 at 15:49

The Pex piping is rated by code, to withstand the 210 °F temperatures for up to 48 hours. However, national code does not allow for PEX to be used within 18 inches of the water heater. If the plumber ran the pipe all the way to the water heater, then it is not up to code, and he needs to come back out and fix it (at no cost). If he will not, then contact the board from which he would have gotten his license in your area/state (usually as a last resort) they can act as an intermediary to make sure he gets the job done correctly.

Here is a very useful link for PEX piping http://www.huduser.org/portal/publications/pex_design_guide.pdf


PEX is suitable for all domestic freshwater, but it's not perfect.

  • It may leach, as it's a plastic. However, every freshwater plumbing material seems to leach something - copper, ABS, whatever.

  • It's sensitive to UV light. As long as it's in a lightless area, it'll be OK.

  • PEX fittings can be expensive. However, long runs of PEX can usually be done as one piece, while copper needs a fitting at each bend.

Because PEX is flexible, it's less likely to fail due to freezing or earthquake, compared to rigid pipes. It's also way cheaper than copper, which is attractive.

  • How about using PEX for the popoff valve?
    – BrianK
    Commented May 4, 2011 at 1:53
  • Water here is pretty acid so copper pinholes, it's in the dark. It's done in a straight run with copper right angles which seems to defeat the point - but it was certainly quick and easy.
    – mgb
    Commented May 4, 2011 at 4:29

I'm staying at a hotel that has pen running in the ceiling for hot water line. Their temp is set at 140 degrees and there per is showing stress factor on the per and leaking which makes me believe that pex and hot water doesn't work at 140 degrees

  • Correlation is not causation. You don't (and can't) know that the leaking and stress factor (whatever that is) is due to the PEX and its proximity to hot water. Could be improper installation...
    – mmathis
    Commented Sep 14, 2017 at 16:59

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