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The short answer is that, while ProPress is well suited for underground use, the decision on whether it is allowed is up to your local inspector. Often they will allow it but some will not. Contact me or your local Viega rep if you need assistance in working with your local inspector.
The long answer is:
Many contractors are using ProPress underground instead of flared connections or silver soldering.
ProPress connections are well suited for underground use. We base our knowledge on the performance testing completed by NSF International (an independent lab). In addition to confirming that ProPress fittings comply with NSF-61 and the standards in the code, NSF confirmed the ability of ProPress fittings to withstand pressure, temperature, water hammer, bending forces, torsion, thermal expansion and contraction, vibration and vacuum.
When it comes to underground, some people are concerned about whether, if you get back pressure in the pipe, there will be infiltration into the water supply from the soil. The fact that ProPress can hold a vacuum demonstrates our ability to withstand those conditions.
Others are concerned about an undeground connection experiencing stress as the ground shifts. The torsion, bending and vibration tests demonstrate our ability to withstand those conditions.
Finally, some are concerned about how an underground connection will hold up when subjected to temperature change. NSF subjected the ProPress connection to 5,000 cycles of temperature change from 68 degrees F to 200 degrees F every 15 minutes to prove that a ProPress connection can handle temperature change over time.
If you would like a copy of this test I would be happy to send it to you. The bottom line is that no other method for connecting copper has stood up to the same performance criteria.
Director of Marketing