By, Ryan Carda 
Simply put, a vault is a buried structure that houses an external manifold for a geothermal loopfield. Think of it as a buried mechanical room where your manifold will go. Simply climb down the ladder through the built-in manway to gain access.
For large systems, it usually does not make sense to connect all of the ground loops with a single pair of supply-return lines without first breaking the field into smaller parallel circuits. For example if you had a 100-bore system, you may decide to to break the field into 10 circuits with 10 bores connected to each circuit (or 5 circuits of 20 bores each for that matter).
By doing so, you can more easily:
- Flush and purge the ground loop to remove air and particulates prior to system startup
- Balance flow to each circuit
- Measure the performance of each circuit (flow, temperature, pressure drop, etc.)
- Isolate circuits in the event that a problem arises rather than being forced to shut down the entire field for repair
When breaking the field into multiple parallel circuits, proper SDSU-RR header design becomes critical to ensure that adequate flushing velocities and head loss characteristics can be achieved. The Auto-Header tool in LoopLink™ PRO simplifies this (formerly cumbersome) task.
Manifold Options
With the field broken into multiple circuits, you will need to incorporate a manifold somewhere in the piping system to combine those circuits to a single supply return line before connecting to the circulating pumps (whenever a centralized pumping system is used - a topic for another day).
There are two basic options when it comes to manifold location - placement inside or outside of the building. An inside manifold will typically be located in the mechanical room. An outside manifold is buried and typically located in a vault or valve pit.
Inside Manifold (No Vault)
Outside Manifold (Buried Vault)
Why Use a Vault in the First Place?
Per IGSHPA’s RLC Design & Installation Guide:
In any installation, the exterior portion of the ground loop piping should always remain buried 4-6 feet below the ground surface (which will be the depth of the header trench).
Additionally, per Paragraph 1D.3 in IGSHPA’s Design & Installation Standards:
All mechanical connections must be accessible.
With an inside manifold, this problem takes care of itself. With an outside manifold, all mechanical fittings (such as butterfly valves and P/T ports) being used need to be accessible in some way for maintenance, performance checking, flushing/purging, etc., which is the primary purpose of a vault.
Factors to Weigh
The decision whether or not to use a vault is project-specific. Generally speaking, these are the things you should think about before making your decision:
- Available space in the mechanical room
- Distance from the loopfield to the building
- Cost for labor and materials
- System operating costs
- Plans for future addition/expansion
Safety Considerations
There are two primary OSHA Standards that must be considered in every vault design.
- OSHA Standard 1910.146 Permit-required confined spaces which includes discussion of entry/exit ways, permitting, air quality, ventilation, etc.
- OSHA Standard 1926.1053 Stairways and ladders which provides the ladder requirements for use in entry/exit of the structure.
If you plan to incorporate a vault into your next geothermal loopfield design, read 5 Features of Good Vault Design to ensure that it is well-suited for the application.