In various posts in the forums here, it has been mentioned that the depth of burial for the underground PEX piping is of the order of 30" (these lines are either the direct burial lines with insulation and encapsulation already incorporated into them, or the standard PEX piping with the use of closed cell spray insulation).

In contrast, the GARN installation manual on underground piping states that, "Trench depth in cold climates should be 4 feet (grade to top of pipe) if possible."  My mechanical consultant is also of the opinion that it is prudent to bury the lines below the frost line (in my area, this is 4'), hence I would have to dig a trench in sandy soil almost 5' deep.  With wetter fall weather on the way, and collapsing sandy soil, I may have to go the direct burial route - fortunately, the trench is only 45' long.

I could see if there was an extended power failure and the pipes were buried only the 30" that there would be a potential for the lines to freeze, but has anyone calculated (or experienced) how many hours/days would be required for this to happen?

Comments on the trench depth issue from seasoned veterans "in the trenches" would be greatly appreciated by this HVAC newbie installing a GARN 2000. 

Doesn't seem like a 45' trench 4-5 ft deep in sandy soil should take more than a day to dig.  If you already have the encapsulated pipe on hand and the building/mechanical inspector notified you could even be backfilling the same day.  I vote for burying the pipe as deep as you can get it.

Ralph,

Thanks for your comments.  I got the encapsulated PEX piping in yesterday and even got it backfilled - it was a long day, but took advantage of some nice weather.  I went 5' deep with a local 6' deep spot entering the garage to enable the bending of the PEX under the existing garage footings that are founded 4' below grade.

Kari

That's great. Now you don't have to worry about the underground.  Hope the rest goes smoothly.