Using PEX for thermal storage
If you’re familiar with hydronics, you know crosslinked polyethylene, or PEX, tubing has a history of use as distribution piping to transport heating fluids to and from various types of terminal units, including radiators, fan coils, and baseboards. And application options are continuing to evolve with PEX, especially as the industry incorporates the larger diameters of PEX pipe, now available up to 3 inches.
But a less common application that is gaining more attention is the use of PEX tubing for thermal storage. In such applications, the medium for maintaining the heat or cold is often a sand bed, typically located under the building.
The key advantage of this type of application is the minimal amount of disruption it has on the rest of the building space. The thermal storage bed is literally out of sight and out of mind.
Ensuring top performance
When installing a system where PEX will provide thermal storage, there are some key points to consider:
Will the radiant loops contain a water/glycol mix if needed for freeze resistance?
Are the loops installed in a manner that will not cause any kinks to develop in the lines?
Is the system free of any airlocks?
Are the loop lengths properly sized and balanced to ensure even flow within the system for optimal performance?
Thermal storage in a typical mechanical heating and cooling system has historically taken the form of a large water storage tank. Large tanks are great for storing energy, but they also take up a lot of space. That can be challenging for engineers and architects trying to maximize usable square footage in a commercial project.
Some applications can incorporate large tanks into the property landscape or on the roof of the building, rather than in a mechanical room (which is often already at maximum capacity).
But another option is to go underground with the thermal storage, using a PEX piping system to connect it with a building’s heating and cooling systems.
Building a bank
Thermal storage is like a battery for a heating and cooling system and is most often used when trying to optimize the overall efficiency of the mechanical system.
For example, ice storage tanks can be used with chillers. These banks of frozen water get charged at night when energy costs are typically lower, due to off-peak rates and when there are lower demands for cooling from the chiller.
Throughout the day, when demands are higher, the ice bank in the tank helps cool the space until the ice is depleted, and the chiller repeats the process again the following night.
If you think about it, a domestic hot-water tank is, in essence, a thermal storage tank as well. The water is heated periodically and then held for use. Solar hot-water systems are another great example of a thermal “battery” storage application.
Positioning the bank
Here are a few examples of PEX-based thermal storage applications:
- A commercial renovation project: In this instance, a thermal storage area was located where contaminated soil was initially discovered beneath a building. Since the project required removal of the contaminated soil and backfill of clean soil, the project team decided to cover the bottom and perimeter of the area with rigid insulation. That way, the space could become a thermal storage bed and the piping could connect to the geothermal heat pump system.
The project incorporated coils of PEX tubing in long loops, much like a radiant-floor installation, in multiple layers of sand (which was then compacted). Adding this thermal storage area allowed the heat pumps to transfer any surplus heating or cooling energy into the sand bed for later use when needed.
This type of application is ideal for minimizing run times on heat pumps and may even reduce the number of total heat pumps or geothermal wells required on a project, thus creating cost savings for the overall mechanical system.
- A new commercial project: PEX-based thermal storage was to be incorporated into the design at the onset of this new project. As a result, the entire area under the building’s slab-on-grade floor was covered in a single layer of PEX tubing within an insulated sand bed.
The system was also connected to a set of geothermal heat pumps. The size and quantity of those heat pumps were optimized to capitalize on the benefits of the thermal storage bed.
- A residential project: PEX-based thermal storage is not only for commercial projects — the concept has been used in residential applications, as well. One residential application I have seen was an area beneath a two-car garage.
During construction of the home, the builder covered the foundation walls below the garage and the floor of the opening down at the footing level with two inches of rigid insulation. He then backfilled the space with sand and installed layers of PEX piping before insulating the top of the sand bed and then pouring the garage floor. The PEX loops were then connected back to manifolds on the supply-and-return headers to control the flow of water within the loops in the thermal storage beds.
It’s easy to see that PEX-based thermal storage is a smart solution that can work for both commercial and residential applications. Check it out if you’re looking for a way to offer system efficiencies, cost savings and a practical way to free up valuable square footage in the design of a structure.