A geothermal valve vault serves as a central point where the manifold collects flow from the entire loopfield with a single pair of supply-return lines running back to the building.
A well-designed vault has incorporated bypass and butterfly circuit isolation valves, which are used during system fill, flush & purge and pressure testing. These features are important and necessary, but you don't need to install a vault to include them in your design.
Here are the top four reasons to avoid using a geothermal valve vault in a commercial loopfield according to Howard Newton, Director of Geothermal Design at Image Engineering Group.
1 First Cost
As with any mechanical component, cost-benefit must always be considered. Whether pre-manufactured, or built on-site, The up-front cost for a geothermal valve vault can be significant.
If mechanical room space or distance from the building are non-factors, a detailed cost comparison like the one performed by Dr. Kavanaugh in ASHRAE's commercial geothermal design guide (Example 9.2 on page 353 - "To Vault or Not to Vault") to look at the loopfield investment with and without a vault.
2 Large Pipe Must Be Used
In order to accommodate the combined flow for the entire system, large diameter piping must be used.
- Large diameter piping is expensive, and has much higher internal volume requirements, which results in the need for a large amount of antifreeze (when used)
- Fusion equipment is larger, more expensive and unwieldy for the installer
- Larger fusion fittings are less plentiful and accessible
- Large diameter piping is heavy, less flexible and harder to work with
- Large diameter pipe can only be purchased in straight lengths, which increases the amount of labor to fuse sections of pipe for long runs from the vault to the building
3 Site Work Complexity
The site work associated with a vault on site is generally more complicated and labor intensive:
- A large pit must be dug to completely bury the structure.
- Vaults require the use of concrete which leads to the need for a properly constructed and leveled gravel base, concrete forms, a concrete truck, etc.
- If the water table is high on the jobsite, the contractor will have to contend with the buoyancy of an HDPE vault or the permeable walls of a concrete vault.
- The height of finish grade must always be considered. Vault load ratings depend on the amount of backfill cover. Also, if the vault isn't installed at the proper depth, the manway may need to be extended or cut down.
- If a sump pump or ventilation fan are required, electrical service will need to be provided.
4 Increased Exposure
While buried underground, a vault is still installed outside. With that being the case, the risks for possible damage due to work done by other trades, leaks or floods, etc. will be higher compared to a system with a manifold located in the mechanical room.
A steel manifold may corrode due to leaks, floods, or high humidity inside the vault structure, which is why their use is not recommended. This was a big issue on the project discussed in the "Burying Mistakes" webinar by Lisa Meline.
An interior manifold with circuit isolation valves, loopfield bypass, P/T ports and flush/fill ports can be used to provide the same level of functionality of a vault, at a fraction of the cost without the increased site work complexity and risk for damage.
About the Author
Howard Newton
Howard Newton is the Director of Geothermal Design with Image Engineering Group (IEG), a MEP firm in Westlake Texas that specializes in Geothermal, Net Zero, and High Performance Schools.
Howard’s experience and expertise is based in the design and construction of ground heat exchangers for commercial GSHP systems. He also has experience with industrial ammonia refrigeration and has worked in a number of positions, including commercial service, sales, marketing, and as VP of a geothermal design/build contracting company.
Howard is a graduate of Texas State Technical College with an Associate's Science Degree in Air Conditioning and a Bachelor of Science degree from Oklahoma State University’s School of Engineering Technology.