Thermal Expansion of Underground Pex

Rick Stanley · Mar 24, 2009

Here are a couple shots of where the pex enters my cellar. I've anchored the 2x6 shown here to a 26" long 2X4 sunk into concrete. You can't see it here because it's under the 2x6.

Anyway, I'm confident that the 2x6 won't move, but am wondering if I anchor the pex to the 2x6 near the ball valves, if that will be ok. Anyone had an installation similar to this? Experiences?? According to the specs on the manufacturers website, this stuff is gonna try to grow about 7 inches with a force of 560 lbs if the pipe extends into the room 10 inches. I'm at like 4 feet into the room. Should I shorten the pex as much as possible or will anchoring it down as is be ok? Thoughts?

Rick Stanley · Mar 24, 2009

Oh yeah, I had a helper down there the other day.

He was kinda grouchy.................. :bug:

Robby · Mar 24, 2009

I like your helper, reminds me of my first wife !!!

!

I think you should take a look at the method (straight to exchanger) on the end of Pex. I had to release mine. I had fastened pex down with the plastic clamps w/nail. As soon as I heated it it expanded so much that I has esses between each clamp and excessive side pressures. That was 1.5" pex.

I also used pex inside the house between zone valves and radiators. Same problem. Moved tubing around to allow it to expand in a "S", then everything OK.

I am old fashioned and ran everything in straight lines, all neat etc. Pex didn,t like that. As soon as I allowed it to move about it worked better. Maybe a loop at end of line?

Robby

Jim K in PA · Mar 24, 2009

Milk snake - we have them too (but no usually in the basement!).

Rick - I would not anchor the pex. It will grow/move as the temp rises. You probably don't want to hear this, but I also advise against mounting the HX so close to the floor. I would use 90 degree fittings (pex/pex) and run a length of pex up the wall, and let the free end of the short leg act as the strain relief for the main pex run. I would also mount the HX up higher on the wall or at the joists.

I brought my PEX into the house much higher on my foundation wall (mine is stone also) in a sweeping turn in the trench, then 90s on the outside for a short run into the house. That has worked well so far.

Rick Stanley · Mar 24, 2009

Thanks to you both, Jim and Robby, what you are saying makes sense to me. I come from a construction background, with some auto repair, propane, logger, equipment operator, farmer and all around cheap skate do-it-yourself-er experience thrown in and that's the first time I've ever heard of trying to STOP thermal expansion. Whether a large concrete pour or applying vinyl siding it's common practice to ALLOW or at times CONTROL expansion but never to fight it. That's why I asked the question, it didn't look right to me, despite Garn's and Rehau's, um well; recommendations.

And as usual, I came to the right place.

Wow Robby, What a coincidence!! That snake looks like my first ex-wife, too.

SteveGH · Mar 25, 2009

Hopefully the wives looked better during the engagement.

I know this thread is about Pex which I've worked with and loved it. I see installs with this and with black pipe. I'm sure this group had thoughts on both sides and being that I'm in the very early leaning/planning phase I'm curious.

Rick Stanley · Mar 25, 2009

Hi Biker,

What we're talking about here is the thermal expansion of the underground part of the system. Insulated pex, in one form or another, is the weapon of choice for that. You're right, expansion can be eliminated, once you're into the building, by changing to copper or black steel or some combination of the two, eventually.

In this case, we have a 170 ft pex run that will, according to manufacturer, grow about 5 inches when I put 200 degree water through it. Once I get a flexible connection at the hx done it's gonna be 1-1/4" copper from the hx the last 30 ft to the oil boiler.

On the Garn end of things, the pex actually enters the building through the wall above ground, so I should have the same big sweep effect that will allow movement as Jim has. I'll post pics of that when I get to it. Anyway, I think I can just anchor that end to the wall inside and let it move outside. It'll have to be boxed-in and insulated of course. Then again, yes, once I'm inside, it'll be a short run of copper to the pump then a black steel manifold off the Garn.

steam man · Mar 25, 2009

Looking at where your heat exchanger is in relation to the pex I would be concerned about the stress from expansion on the HX. Piping systems typically have to take into account the amount of expansion and deal with it such as using U-bends every so often, floating hangers, flexible connections, and whatever. Even allowing the HX to float could work depending on the rest of the piping. If you could install the HX at a 90 deg bend with a decent amount of pipe on it that would seem OK. Another idea would be to use a properly installed flexible SS hose connection.

Mike

Rick Stanley · Apr 10, 2009

Here are a couple shots of changes I've made as far as thermal expansion and a plumbing scheme.

I've tried to address issues with hx plumbing discussed here in the past. I have unions, purge valves, ball valves on all four lines so I can isolate, flush out and/or remove the hx. There are wye strainers on the two incoming lines with temp gauges on the incoming hot line and the outgoing heat line to see heat exchange temps and it's piped counter-flow. Any thoughts?

Thank You

Rick Stanley · Apr 10, 2009

Another shot

Rick Stanley · Apr 10, 2009

one more.........

steam man · Apr 10, 2009

It doesn't look soldered yet so I would consider changing the outlet line with the temp gauge to maybe match what the inlet line looks like, 3 elbows instead of 4. Looks like thermal expansion shouldn't be a problem. Get some pipe hangers in there and I'd say go with it.

Mike

Rick Stanley · Apr 11, 2009

Great!! I'll eliminate that one elbow and do it up solid. Thank you!!

SteveGH · Apr 11, 2009

Just curious...is the expansion mostly linear or does the diameter increase also. Just wondering about the underground pex in a foamed trench which is what JimK did. Does the Pex expand and shrink causing air gaps? I haven't read about any issues with that so it must not be a big problem.

Rick Stanley · Apr 12, 2009

Here's a link to a tech manual for the Rehau stuff that I have. I didn't see anything other than linear but you can check it out closer. Lots of info there...............Now that you mention it, I guess if air gaps from expansion were an issue, it would be an issue with the pre-fabbed stuff as well as "sprayed in the ditch" stuff.................wouldn't it? Now that I mention it, thinking out load here, I guess the problem with gaps would be water infiltration which would be a non-issue because of the outer sleeve on the pre-insulated products.

Rick Stanley · Apr 12, 2009

Ooops, forgot the link http://na.rehau.com/files/INSULPEX_Tech._Manual_10.03.pdf

Gooserider · Apr 12, 2009

I don't think it would be a factor to worry about... I've never seen any mention of thermal expansion of the diameter on PEX tubes, and if it were a factor I'd expect to see mention of it since it could be a problem with fittings...

As a thought experiment, I tried assuming that there was a similar diameter expansion as there is to length. Per their appendix, with a 150* temperature change, there would be a 5" expansion in 100' - rounding to 6" that makes the expansion rate about 0.5%. Half a percent expansion in a 2" diameter pipe is going to be about 0.001" - hardly enough to be a significant problem.

Gooserider

Gooserider · Apr 12, 2009

Rick Stanley said:
Here are a couple shots of where the pex enters my cellar. I've anchored the 2x6 shown here to a 26" long 2X4 sunk into concrete. You can't see it here because it's under the 2x6.

Anyway, I'm confident that the 2x6 won't move, but am wondering if I anchor the pex to the 2x6 near the ball valves, if that will be ok. Anyone had an installation similar to this? Experiences?? According to the specs on the manufacturers website, this stuff is gonna try to grow about 7 inches with a force of 560 lbs if the pipe extends into the room 10 inches. I'm at like 4 feet into the room. Should I shorten the pex as much as possible or will anchoring it down as is be ok? Thoughts?

Actually Rick, I just read through the install manual you linked to, and it seemed to say that the expansion of the buried part of the line was "self limited" by the friction between the jacket and the covering material, the distance numbers were for exposed / non-burried lines. I may well be missing something, but the way I read it, the only part of the lines that will actually be trying to grow is the foot or so that is sticking out of the wall. Per the manual, that small length should only grow about a couple hundredths of an inch...

They did say to anchor the pipe down where it terminated, but they also said NOT to attach to the pex itself, or to the jacket, but rather to the body of the fitting - i.e. bolt down the ball valves, not the PEX.

From the manual - Page 9

3.7 Thermal Expansion
A unique property of INSULPEX is that it is self-compensating when buried in accordance with the instructions in the installation guide. The frictional force between the fill sand around the INSULPEX and the outer casing is sufficient to limit thermal expansion of the pipe under typical operating conditions. However, when INSULPEX is installed in a non-buried application, the system design must account for the natural tendency of the pipe to expand due to temperature changes. The total thermal expansion, ∆LT, is expressed as:

∆LT = a x ∆T x L

Where a is the INSULPEX coefficient of thermal expansion, being approximately 3.29 x 10-4in/ft °F (4.94 x 10-2mm/m °C) ∆T in this case is the difference between the highest and lowest temperatures the INSULPEX will experience. Appendix 5, Thermal Expansion, may also be used to determine an approximate length of expan- sion based on known temperature change.

Example, given:
∆T = 100°F Lowest ground temperature of 50°F and highest system temper ature of 150°F
L= 100 ft of INSULPEX
∆L T = (3.29 x 10-4 in/ft °F) x 100 °F x 100 ft= 3.29 in, approximate change in length

And from Page 11 (bold added)

4.4 Termination
At the end points of the INSULPEX system, the INSULPEX pipe will transition to non-INSULPEX pipe. This is called termination. The INSULPEX system must be anchored on the fitting, at all termination points. Anchor only to the body of the fitting, not the INSULPEX outer casing, the EVERLOC sleeve, or the nut of a compression nut fitting. Anchor the pipe that the INSULPEX fitting terminates to, if it is rigid pipe, see Fig. 4.5. Use an anchor designed for the forces listed in Table 4.5. To limit thermal expansion, use this table to set the distance, A, that the INSULPEX projects into the room.

Sounds to me like with your current setup, you need to anchor the elbows where you transition from the Insulpex to the house plumbing, but otherwise you probably don't have much to worry about.

Gooserider

2.beans · Apr 12, 2009

they're saying pex will grow? mine has shrunk two or more inches.[the pex] i had to add more copper to the supply line to straighten it out.

Rick Stanley · Apr 12, 2009

Goose,

I did read that about anchoring at the first fitting. I also read the part about how far it extended into the building being a factor. What I didn't understand, I guess, is that the 5'' of growth/100ft was for fully exposed un-buried pipe. That's in line with what the Garn guy told me. He said to anchor both ends and that the "movement will happen in the ground". That agrees with what you're saying, I guess.

So, anyway, I was concerned about what it would do, posted the question here and you can see the replies I got warning not to anchor it. I also talked with my local supply house (they handle Wirsbo pex) and they advised strongly to allow for expansion especially if it's a long straight run. (geez, I guess they understood it was underground) So, if you look at the last pics I posted in this thread you'll see that I've made allowance for linear movement. So I think whatever it does when I heat it up, I'm covered. WE'll see...............

Beans,

What brand of pex did you use?

Gooserider · Apr 12, 2009

Well the way you have it set up now looks like it should cover you, no matter what the line does, so that should be OK.

On my own setup, I haven't decided on a brand yet, as I'm still in the "severe fantasy" stage :cheese: I'm less than thrilled with the way our Encore is working, and would love to do a solar / boiler / storage / setup w/ in-floor radiant, pool heat, and DHW; but I need to convince the GF that it's a good idea, and figure out how to work the money....

However, I won't be needing to worry about underground lines unless my dreams have a really drastic change in direction - I'm thinking more in terms of doing a basement install, so all my PEX will be inside.

That said, if I were to do underground lines, I'd probably go for one of the pre-insulated lines, but not sure which one.

Gooserider

2.beans · Apr 12, 2009

the first time i did this i did it wrong. i went with some really bad advice, i dont want to point blame at fred seton but, that pex was wirsbo, and that shrunk a foot at least. im sure it was from overheating the pex during a power outage without a back up power source which ive since corrected. the boiler unsweated the joints. my new stuff has seen one hot supper but didnt shrink any wheres near the wirsbo pex. im not sure now what brand it is though. it seems like after its been hot and cold a number of times (one year) its stopped moving that you can see. im sure it still moves a little but nothing like it did when it was new. your set up looks like youd be all set if it wanted to move. i wouldnt worry if i were you. i probably run mine a little too hot.

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