Laying the Pre-tempering Ground Loop of PEX Tubing

Posted from: MD, USA

The exterior foundation walls have now been waterproofed and the insulation and drainage board installation is nearly complete. The steel for basement columns and beams is at the site, and we await the arrival of PanelWrights on Thursday to begin framing of basement walls and first floor framing. The first floor framing must be in place before final backfilling is completed so that there is resistance at the top of the foundation walls to the pressure of the earth.
Yesterday Foley’s crew (Foley Mechanical, Inc.) was at the site to lay the PEX tubing for the pre-tempering ground loop. What is a pre-tempering ground loop? Basically we will temper the air coming into the household ventilation system with what you might call a “poor man’s” ground source heat pump. We are running a loop of about 300 feet of PEX around two sides of the building deep underground. Water will circulate through this tubing to a heat exchange coil in the ventilation air intake duct. The water, which will be at constant year-round temperature of 56 degrees Farenheit, will cool the incoming summer air and warm the incoming winter air. Foley will install monitors to measure the exact amount of btu’s that are transferred annually. A ground source heat pump operates according to the same principles as this, but runs refrigerants like freon through the underground lines and uses a compressor to “pump” the energy— considerably more complicated and expensive.

We have integrated laying the tubing into the process of backfilling. The first step of backfilling involved putting down perforated footing drains in a gravel bed around the permiter of the house. Any water that gets down to that level will enter those drains and be taken away.

Over that gravel is placed a geotextile fabric — basically a filter that keeps dirt from getting below to clog up the drains, but allows the free passage of water. It is the dark cloth at the bottom of the picture showing the exterior of one of the foundation walls. In this picture you can also see foundation insulation and drainage board, and above that the dark waterproofing.

Over the geotextile fabric is about 8” of dirt, which is what you see at the bottom of the trench in the video. The men have laid down the first loop of PEX, keeping piping as far apart in the trench as possible. They are covering it with dirt by hand to hold it in place.

Then a backhoe will dump 24” more of soil, compacted in 8” lifts (layers). The men will then repeat the process of laying another loop of PEX. The goal is to keep the individual tubes as far apart in the soil as possible so that the ambiant soil temperature will not be affected. If they were all laid in one big bunch of 4 tubes, the surrounding soil temperature would be changed, reducing the delta T for energy transfer. The top layer will be 5’ below grade.

The PEX we used here is a Rehau product, proven in underground use in Europe over many years. Foley only has confidence in Wirsbo and Rehau as makers of this tubing. It is completely inert and will not deteriorate over time.