CPVC Pipe for Geothermal

sniffdogDecember 16, 2012

This is a cross-post from HVAC.

I have a 5 year old geothermal system. Slinkly loops outside are made with HDPE but interior plumbing is CPVC. Anti-freeze is a water-methanol mix (about 10% methanol). The static pressure is about 20 psi and when running about 35 psi.

I just repaired a small leak at the manifold where there was a 2" to 1 1/4 reducer bushing - looked like a bad gluing job.

I decided to look on line and was surprised to see an article that said to never use CPVC pipe for any geothermal application. I also read a few articles about schools that had used PVC (Schedule 80) for their inside plumbing and they had leaks - and replaced all their pipes.

Am I sitting on a time bomb? What would cause the CPVC to fail?

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I don't see any reason why you can't use CPVC for the supply & return lines from the condenser to the ground loop.

CPVC is rated as approved for handling pure Methanol so your 10% methanol mix should not present any problems.

CPVC is pressure rated at 73degF and if the working temperature is increased you have to derate the working pressure considerably...

By example, 1/2" CPVC is rated for a max 400psi @ 73degF, but at 120degF the pressure max is reduced to 260psi and at 180degF it is down to 80psi, however, the highest temp you incure in a ground loop is the actual natural geothermal earth temp which is approximately 55degF so your geothermal loop temps should run about 55degF from the loop to the condenser and about 35-40degF return to the loop, so I can't see any reason you can't use it....

One the other hand, you could not use it on the heat distribution piping from your geothermal unit, and many times the codes will prohibit it so that installers don't mistakenly use CPVC on the heat distribution lines by mistake.

Personally, I would not use anything but copper or PEX from the condenser to the ground loop.

    Bookmark   December 16, 2012 at 7:49PM
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Thanks for the reply. I was wondering if the temperature variation of the anti-freeze from winter to summer could cause the glued joints to fail? I thought the range would be from 32 to about 60 degrees and that didn't seem like a lot to me but I am not a materials expert.

Or could it be that in the cases cited on line, they were just badly glued joints?

In the mean time, I am in the process of hooking up a low pressure sensor on my loop so that if there is a failure and the pressure drops below an adjustable threshold, the loop pump and heat pumps will shut off. I have always feared the possibility of having 300 gallons of methanol-water dumped into my house.

    Bookmark   December 17, 2012 at 9:26AM
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Actually in winter during the heating cycle the temp differential would be in the order of 35-40 to 50degF, however, in summer during the cooling cycle the differential could easily be 170-190 to 50-55degF.

I doubt very seriously if you have anything near 300gal of water in the system. The ground loops are typically made with 1" pipe and 300gal would require a loop of 1" pipe nearly 1.5miles long.

    Bookmark   December 17, 2012 at 12:49PM
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I have 12 slinky loops that i thought were made with 1.25" pipe - but t could be 1", I am not certain. The HVAC company that did the install told me that it took 290 gallons of fluid. I know that the pit that was dug was 120' long x 50 ft wide. I think the slinky loops were 100' long each - but I don't know how long that would be if you unstreched the loop. I think the width of each slinky loop was 3' and they put a 1' space between each loop. I am curious on just how long that would be end to end.

I didn't realize the temp delta could be that high in the summer. if I had a 35 to 190 degree swing between winter and summer, do you think that might put any stress on the glued joints that could cause a leak?


    Bookmark   December 18, 2012 at 8:42AM
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Although the chemical used to make the joints is called glue and we refer to them as glued joints, technically speaking, that is incorrect.

Technically speaking a glue joint is when a chemical adhesive is applied between two mating surfaces and the glue holds the pieces together by clinging to microscopic surface depressions in the materials.

One the other hand, when a PVC glue joint is made up, we begin by applying the solvent/cleaner. The solvent cleaner removes all dirt, dust & debris from the surface of the pipe & fitting, and it also removes the smooth plastic glaze on the surface.

The glue then penetrates the PVC on both the pipe & fitting and softens it so that when the two pieces are fitted together the plastic from both the pipe and fitting fuse together in what is technically defined as a "Chemical Weld". When the joint has sufficient time to cure the joint is just as strong as the pipe & fittings that it is applied to. In fact, if you were to check the "burst tables" you would see that in some cases the joints have a higher burst strength than the pipe.

My concern would be that PVC would be fine for a geotehrmal heating system, but if they are also using the geothermal ground loop as a condenser for the AC system the high side of the AC system could easily produce temperatures that would approach the red line for PVC, and under some circumstances could exceed the listed limits.

    Bookmark   December 19, 2012 at 7:39AM
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thanks for that explanation on the solvent and "glued" joints - i never knew that. I feel better about my plumbing now.

I decided to do come calculations to see if I could come up with the 290 gallon number that my HVAC company gave me. I checked my notes from 5 years ago - Does this seem reasonable:

Each slinky loop segment was 3' in diameter and was 115' long. That would mean that there were ~38 individial circles each with a length of 2*pi*r = 4.7 feet = 178 feet. Then I had to add another 115' for the return back to the manifold outside (they used parallel loops) so that would give me a total streched length of 293 feet.

I have 12 slinkly loop segments so that woul dmean there would be 3516 feet of HDPE pipe in the ground. I believe the pipe was 1 1/4, and that has an ID of 1.157 inches. That would hold 192 gallons of fluid.

Then I have 2" HDPE that runs between the house and the loop field. I guessed that it is a round trip of 200' which gives me another 22 gallons.

And then I have the inside piping which is 1 1/4 CPVC. I estimated about 300' total which would hold another 23 gallons.

Added all up, I come up with 237 gallons. I know that isn't 290 gallons but it is in the ballpark. My loop lengths are estimates - the people who put the system in knew exactly how much pipe was used. So I think the 290 gallon number that my HVAC company gave me is probably right.

I remember vividly that there was 30 gallons of anit-freeze put in becasue I had an issue with the gylcol they first used - it was gooing up due to extremely cold temps that year (0 degrees for about 10 days). They replaced the gycol with methanol. So a 30 gallon methanol + 260 gallon water mix would give me an 11% anti-freeze solution and I think that translates into a freeze point of ~20 degree F. That would give me ~15 deg of freeze protection which I think is OK.

Hopefully these numbers add up.

    Bookmark   December 19, 2012 at 9:10AM
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