Install Underground Insulated Pex

April 2, 2023

Installing underground PEX (cross-linked polyethylene) like Thermopex, Rhinoflex, Rehau, or other high quality underground PEX that contains an insulation jacket with closed cell foam is an excellent choice due to its flexibility, durability, and high insulation value. Installing PEX tubing underground requires proper planning, excavation, and installation techniques. In this article, we will guide you through the steps to install underground PEX tubing.

Find out more on the process and tips on installing a wood boiler.

Step 1: Plan for the underground PEX

Before you start digging, it is essential to plan the installation carefully. Start by identifying the location of the boiler and the entry location into any buildings in the system.  Identify any obstructions that may be underground and will need to be avoided or dug around including tree roots, foundations, wells, septic, and underground utilities. Identifying utilities can be done by calling diggers hotline and any other public or private utility marking services. It is a good idea to stay outside the drip edge of your trees to avoid any tree kill. Your outdoor boiler will need power, adding direct bury 12-2 wire and this is a good time to plan where the cable will terminate on both end of the trench.  Generally speaking to estimate the amount of underground PEX, measure the distance of the planned route of underground pipe plus add on an additional 10' for the amount needed for going down 2' and leaving enough pipe at both ends to work with.

Step 2: Plan the building entrance

PEX entrance into the building has many components to consider. There are two general options to enter into a building with a basement or crawlspace. First option is to core drill through the basement foundation. This will require a special core drill and drill bit between 5" to 6" in diameter in which the underground PEX will enter the building through this hole. Second option is to go through the rim joist, which may be the only option for buildings without a basement. WIth this, the underground PEX will come up from the ground and with a drill through the side of the building will allow for PEX to enter. Remember that your underground wire feed will also go through this hole.

Step 3: Excavate the trench

Using a backhoe, excavator, trencher, or shovel, dig a trench that is deep and wide enough to accommodate the PEX insulated tubing. A trench of 6" wide should be sufficient but if it is a wide trench (more than 12"), you can lay the wire and the underground pex side-by-side with at least 6" of separation. The depth of the trench will depend on the codes and soild conditions in your area, but a general rule of thumb is to bury the tubing at least the electrical wire direct bury depth. Consult with local building codes but generally a 24" depth is required for direct bury electrical wire. Where the PEX will protrude from the ground, a depth of 4' will be required to have the PEX have enough bend radius to protrude being perpendicular to the ground. If the underground PEX pipe will be in an area with heavy machinery or under a driveway, then going down atleast 4' or more may be required.  Check with the manufacturer of the product for depths for heavy traffic areas.

Step 4: Install Underground Pex and electrical wire.

Before installing the underground PEX, cap or tape off the PEX lines to avoid any debris from entering the pipe. Start by laying the electrical wire in the trench, if you are requiring electrical at the boiler. Then sprinkle 4" of loose fill with no large or sharp rocks. Then lay the PEX in the trench leaving atleast 2' at the boiler end protruding from the ground above the boiler foundation. If the PEX entrance into your building is though the hole in your basement, then push the PEX through the foundation with 1' of pex left on the inside. Taking hydronic cement and a waterproofing coating, fill in any gap between the foundation and the pipe.

Step 5: Connect the fittings

Once the tubing is in place, it is time to connect the fittings. First trim the back the outside PEX insulation jacket and clean the PEX tubes getting the PEX to the correct length. For PEX-A or PEX-B, fittings should be connected using a crimping tool or a clamp ring tool. PEX-A may use the appropriate expansion tool and expansion fittings. Make sure the fittings are properly seated on the tubing and the installation routing is appropriate.

Step 6: Pressure test the system

Before covering the tubing with soil, it is important to pressure test the system to ensure there are no leaks. Use a pressure gauge to check the pressure of the system and make sure it is within the recommended range. If there are any leaks, repair them before proceeding.

Step 7: Backfill to cover the tubing

Once the system has passed the pressure test, it is time to cover the tubing with soil. Use a layer of sand or clean fill to protect the tubing from any sharp rocks or debris that may be in the soil. The soil should be compacted around the tubing to prevent settling and to ensure that the tubing remains in place. Making sure that any sharp or large rocks are not layed near the underground PEX. Generally, any rocks larger than a baseball should not be near the PEX. Backfill and remove any remaining dirt leaving a smooth and level surface.