In Joe Pejsa's experience there are common elements in every bad vault design. Here are his top five things to avoid when specifying the requirements for a geothermal valve vault.
1 Circuit Balancing Valves
The primary issue with circuit balancing valves is the fact that they only control flow under peak flow conditions. Typically, a loopfield only operates at peak capacity for a few hours in a year. For most of the year, the system will operate at part load which means the expensive circuit balancing valves serve no purpose for most, if not all of their installed life.
Circuit balancing valves add considerable cost to a vault, create additional points of failure and increase system head loss and associated pumping power requirements. A better alternative to circuit balancing valves is to simply use butterfly isolation valves to balance flow (when necessary). The butterfly valves will already be incorporated into a good manifold design, and they are about 20% of the cost of a circuit balancing valve.
The occasional spec calls for a space heater to be installed inside of the vault. Although the circulating water temperatures in a GSHP system can fall as low as 25-30F, the inside temperature of the vault will not fall below 40F. The vault is buried and the surrounding soil will moderate the inside temperatures relative to outside air temperature extremes. Also remember that in cold climates, the entire system will be protected with antifreeze (such as propylene glycol) to freeze point temperatures well below the lowest temperature seen inside of the vault. The inclusion of a heater adds unnecessary cost and increases the complexity of the installation due to the need for electricity.
3 Steel Manifolds
Vault manifolds should be constructed from HDPE rather than steel for several reasons.
- During the installation process, moisture is constantly present and depending on location, the air will also be full of humidity. The excess moisture can cause a steel manifold to rust and corrode quickly.
- The expansion/contraction of grooved fittings causes leaks. Routine maintenance is needed to make sure the steel is holding up to the environment and the fittings are tight to the grooved end.
- Labor costs can be very high in certain areas. Steel manifolds are very heavy and for the most part, need to be constructed on site. The use of HDPE allows for the manifold to be fabricated and installed by the vault manufacturer so that when it arrives at the site, it will be completely assembled and ready for installation.
4 Individual Loops to the Vault
Geothermal vaults are typically used when there isn’t adequate space in the mechanical room to accommodate the number of circuits. The size and cost of the vault is directly impacted by the number of connected circuits.
To use a vault as a manifold that provides access to each individual loop in the field is expensive and generally unnecessary. When a vault is needed, the most common and cost effective solution is to join the loops in groups of multiple parallel circuits, and bring each circuit to the manifold inside of the vault.
There are instances where individually connecting the loops to the vault is the best answer (for example: an installation under a building or a parking garage), but most loopfields are installed in green spaces or playing fields and can be repaired easily if a leak occurs. Butterfly valves allow for repairs on a portion of the field without requiring shutdown of the entire system if/when that time comes.
5 Tiny Housings
Geothermal vaults are confined spaces that need to meet OSHA ventilation, entry and electrical standards. Forced air ventilation and ladders are just some of the OSHA regulations that need to be followed. When design features such as bypasses, isolation valves, purge ports, gauges and accessories are included, the space becomes very congested.
Installation workers and maintenance personnel need space to work comfortably and safely. If a situation arises, they need to be able to easily and safely escape from the hazard. Being forced to kneel or crawl into a vault that is too small is at best inconvenient and at worst dangerous. Always remember to consider the ability of a real person to maneuver and work when sizing your vault.
Now that you know what to avoid, read 5 Features of Good Vault Design for pointers on how to improve your next vault design.