Geothermal Loops: 5 Reasons for using HDPE & PEXa

Friday, March 10, 2017


With so many available options, why has the geothermal heat pump industry gravitated toward exclusively using polyethylene (PE) - specifically HDPE and PEXa - for ground loop construction, especially considering it is one of the most insulative piping materials available?

Aside from the fact that PE piping accounts for a tiny fraction of the overall thermal resistance in a loopfield1, it offers a lot more benefits than deficits.

Photo courtesy of ISCO Industries.

Industry Standards

HDPE and PEXa are the only materials that IGSHPA formally approves for use in the buried portion of a closed-loop GSHP system. Per Section 1C of IGSHPA's Design and Installation Standards:

The acceptable pipe and fitting materials for the underground portion of the ground heat exchanger are high-density polyethylene (HDPE), as specified in Section 1C.2 and cross-linked polyethylene (PEXa), as specified in Section 1C.32.

These recommendations were born out of a combination of past experience along with the acknowledgement of the many advantages that polyethylene (PE) has to offer. Aside from being the industry standard, here are the top 5 reasons for using PE over the alternatives:

1 Affordable & Available

Polyethylene is used in a wide range of applications such as food packaging, plastic bottles and bags, pool liners, and of course, geothermal piping. It is a commodity plastic and is among the least expensive types to make. Geothermal grade polyethylene pipe is mass produced and readily available in the marketplace at commodity prices.

2 Durability

Geothermal heat pump systems operate under a wide range of temperatures and pressures. It is normal for ground loop temperatures to swing from 25-30F in heating mode to 90-100F in cooling mode. Thermal expansion and contraction of the piping due to temperature swings will cause system pressures to follow suit.

Polyethylene is highly resistant to damage due to fatigue (as well as damage due to abrasion, weathering, corrosion, etc.). It can withstand the abuse of pressure fluctuation due to temperature changes, as well as the abuse of being transported and handled on the jobsite. According to the Plastic Pipe Institute, it can even withstand damage due to an earthquake:

The toughness, ductility and flexibility of PE pipe combined with its other special properties, such as its leak-free fully restrained heat fused joints, make it well suited for installation in dynamic soil environments and in areas prone to earthquakes.

The durability of PE pipe is tough to beat (pun intended).

3 Installation Ease

Mechanical fittings are not necessary when PE pipe is used. Simple heat fusion techniques are used to join pipe and fittings together in a leak-free, virtually fail-proof manner. Even if leaks or other errors occur, they are extremely easy to fix.

PE pipe is also relatively flexible, lightweight and very easy to manage on the jobsite. Pipe coils are generally available for purchase in any 100-ft increment, leaving it to the installer to pick the length that best suits the project without the hassle of a special order.

4 Service Life

The life expectancy of polyethylene is greater than any mechanical component inside of the building, and even the building itself. According to Chapter 7 in the Handbook of PE Pipe:

The service life of HDPE pipe manufactured from today’s materials is expected to exceed 100 years.

In fact, most pipe manufacturers offer a 50-year warranty to guarantee that the pipe will perform according to specifications without failure of the material itself.

5 Maintenance Free

The long service life coupled with the use of heat fusion in lieu of mechanical fittings virtually eliminates the need for maintenance on the pipe itself. Once installed, the buried ground loop will be a permanent fixture on the property for as long as there is a building to heat and cool.

Polyethylene is also corrosion resistant and inert to most chemicals. It does not promote biological growth and helps to minimize the amount of water quality-related issues typically associated with a water-source HVAC system. Alternative piping materials such as steel, copper and galvanized iron are much more demanding from a maintenance point of view.

All things considered, HDPE and PEXa are far and away the most practical choice for geothermal loopfield construction.

Notes:

1GeoPro’s Importance of Grout TC illustrates the fact that pipe is a very small portion of the overall thermal resistance in a loopfield. In fact, LoopLink PRO can be used to show that the thermal resistance of a basic HDPE or PEXa u-bend accounts for only 10%-12% of the overall total.

2Refer to IGSHPA's Design and Installation Standards for further information on pipe manufacturing methods and materials, pressure ratings, dimensions, tolerances, etc.

Understanding Auto-Header

Tuesday, March 7, 2017

In LoopLink PRO we spent a lot of time on header design. We spent so much time on it in fact, most users will never need to spend more than a few seconds on the page. But the speed with which the auto-header function does its job, belies the complexity and importance of what it is doing.

This article describes generally how the LoopLink PRO auto-header system works so it feels less magical and becomes more useful.

Fixed Assumptions

In order to automate the design of your loopfield headers, the LoopLink PRO auto-header tool was built using three fundamental assumptions.

  • Headers are broken into evenly-sized groups of bores we call circuits.
  • Circuits are designed using step-down, step-up reverse-return principles (SDSU-RR).
  • Regardless of u-bend material being used, the header pipe is constructed using HDPE.

We arrived at these assumptions through our own experience and by reviewing hundreds of header designs from other engineers and system designers. From our research, we have found that this method of piping design is the most common and least error prone.

Basic Logic

With those assumptions set, the auto-header function follows a pretty simple set of rules based on IGSHPA's Design & Installation Standards and supplemented by best practices. The scope of the problem is bounded first by the pipe sizes you select to include. After that, we simply analyze the theoretical velocity and pressure drop through a section of pipe and make a choice as to which pipe diameter makes the most sense to minimize head loss while maintaining the minimum velocity required for the flushing process.

You Choose the Available Pipes

The smallest pipe size available is always the diameter and dimension ratio of your U-Bend selection. When your header is initially designed, LoopLink PRO will by default use the same dimension ratio as your U-Bend selection for all sizes of pipe. After that, you can select the nominal diameter and dimension ratio pipe you would like to include in your design.

So, if for example you wanted to design headers to skip 1.25” pipe, all you would do is turn off 1.25”. If you want everything larger than 2” to be DR 13.5… turn those pipes on.

LoopLink PRO will only design your headers using the pipe sizes you specify.

When to Step

LoopLink PRO is designed to first minimize head loss then maintain minimum flushing flow velocities. The minimum flushing flow velocity is defaulted to 2ft/s based on IGSHPA's Design & Installation Standards. That said, you can easily override this value if specified by local code or as a matter of professional preference.

LoopLink PRO will first determine the smallest available pipe size that will result in a head loss value that is less than 3 ft. H2O per 100 ft. of pipe length but is still capable of maintaining the minimum flushing velocity specified in that section.

At every intersection where the header will ‘feed’ a loop, the loop’s design flow is subtracted from the total flow required to provide design flow to the last bore in a circuit. The auto-header then decides if the next section of pipe should be the same size as the last or if a diameter change is needed to minimize head loss and maintain flushing velocity.

The process is exactly what you would do if you were solving the header on paper… just faster and with a lot less eraser debris on your desk.

You Can Step In

LoopLink PRO does its best to behave like we do during header design but it isn’t always practical or reasonable to program common sense choices that fall outside of design standards. Sometimes an engineer needs to step in and apply their experience. Which is why we included manual edit mode.


This allows you to start with an excellent base header configuration and quickly apply changes to make the design your own. The system will even warn you if you fall outside of the standards and provide you with feedback that will help you choose if the decision is appropriate.

The auto-header isn’t meant to take the designer out of the equation. It is meant to take the tedium out of your design process so that you can iterate and optimize quickly to find the best solution for your application.

Top 4 Reasons Not to Use a Vault

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.

Photo courtesy of 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
Image Engineering Group

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.

Video: Burying Mistakes

Wednesday, March 1, 2017

Duration:

31:45

Description

Industry leader, Lisa Meline, P.E. shares a cautionary tale about what happens when geothermal designers trust but don’t verify. A failure analysis of a system points to a design that was sound in principle but not properly overseen during installation. The result was of course an expensive analysis and repair that could have been avoided.

About the Presenter

Lisa Meline, P.E.
Meline Engineering

Ms. Meline is principal engineer and owner of Meline Engineering. She has 25 years of mechanical engineering experience and over 30 years of experience in the construction industry.

Ms. Meline is a licensed professional engineer in four western states and a Certified Geoexchange Designer. She is Standards Sub-committee Chair for ASHRAE TC6.8, Geothermal Heat Pump and Energy Recovery Applications, Chair of IGSHPA’s Standards Committee, and recently served as Special Expert on the IAPMO Uniform Solar Energy and Hydronics Technical Committee.

Ms. Meline is an ASME Fellow and one of the founders of the California Geothermal Heat Pump Association.

Top 6 Reasons to Install a Geothermal Vault

Monday, February 27, 2017

Although some may argue against the upfront cost, there are many advantages to the use of geothermal vaults in large commercial projects. It can save both the owner and installer time and money over the long haul.

Here are the top six reasons for using a geothermal valve vault in a commercial loopfield according to Stuart Lyle, Director of Geothermal Sales at ISCO Industries.

Photo courtesy of ISCO Industries.

1 Create More Space

A vault can free up valuable area inside of a mechanical room by greatly reducing the number of pipes entering the building.

By utilizing a vault to create a manifold space outside of the building, the system designer can:

  • Maintain the integrity of the building by reducing the number of cored holes that must be drilled through the wall or foundation
  • Create more space for other system components such as pumps and controls
  • Decrease the loopfield contractor’s time inside of the mechanical room, which helps keep the project on schedule by reducing the need to coordinate work times with other trades

2 Separation of Scopes

Vaults not only serve as a central location for circuit piping, they also provide a clean stopping point for the loopfield contractor until system commissioning takes place. In new construction projects, the geothermal scope is often one of the first to get underway and also one of the first to finish. With a vault, geothermal contractors don’t have to wait on the mechanical contractor to finish their scope.

With a vault acting as a stopping point between the loopfield and building:

  • The loopfield contractor can properly pressure test, flush and purge the system without first having to connect to the interior mechanical piping.
  • With an incorporated bypass valve in the vault, the loopfield can remain separated from the supply-return lines that run back to the mechanical room. This allows the mechanical contractor to proceed with the interior installation without compromising the loopfield after it has already been tested.

3 Facilitate Repair

Another advantage of the separation between loopfield and building can be realized when making repairs to a damaged circuit or while searching for leaks. With the loopfield manifold located in the vault (and not in the building), the loopfield contractor won’t need to access the building’s interior to make repairs.

By having butterfly isolation valves on each circuit along with a building bypass, the loopfield contractor can identify the damage or leak, make the repair and complete the pressure test/flush/purge sequence without stepping foot in the building.

In the event that a leak occurs, the vault also prevents spillage or mess inside of the mechanical room, minimizes disruption of activities within the building, and allows the contractor to conserve antifreeze and rust inhibitors by isolating the problem during repair.

4Convenience

Prefabricated vaults offer the contractor a “plug and play” option. Rather than allocating valuable time and labor to building a manifold on site (often while paying prevailing wages), the loopfield contractor can focus on drilling operations and pipe installation while the vault is being made off-site.

Once the vault is on site and in place, the geothermal installer can easily connect the circuits, then sequentially pressure test/flush/purge the circuits, subfields and entire loopfield to prepare the system for operation.

5 Accommodate Larger Distances from the Building

In some cases, the loopfield can be separated from the building by a large distance (500 ft or more). A vault acts as the central point where the combined flow from the field is collected, and a single pair of (larger) supply-return lines are run from that point back to the building. A relatively narrow trench can be excavated to accommodate those lines. Also, supply-returns can be sized for minimal head loss such that distance from the building to the loopfield is a non-factor.

6 Plan for Future Expansion

In some cases, a loopfield may be installed in multiple phases. A vault can be designed with extra circuits to allow for future expansion. In such cases, loopfield expansion can be completed with minimal disturbance inside of the building.

With the many advantages of using a vault as part of a commercial geothermal loopfield design, the first cost can typically be justified.

About the Author

Stuart Lyle
ISCO Industries

Stuart Lyle serves as geothermal sales director and provides technical support for ISCO’s customer base in the United States and Canada. A 10 year veteran of the geothermal industry, Stuart has served in both operations and sales roles. Before joining ISCO Industries, Stuart served as a project manager for a nationwide geothermal installation company. Prior to entering the civilian workforce, Stuart had a prestigious military career in the 3rd Marine Division where he earned a meritorious promotion to the rank of Sergeant and the Navy and Marines Corps Achievement medal for outstanding performance while serving abroad. After completion of his tour in Afghanistan, Stuart continued his military service in the Georgia Army National Guard while simultaneously earning a BS in Physiology and BA in Criminal Justice from the University of Georgia in 2006.

LoopLink RLC Update: Pond Loop GHEX Design

Wednesday, February 22, 2017


Pond loops are closed-loop, surface-water heat exchangers. Regulation permitting, any nearby body of water, such as a lake, stream or pond, can serve as a lower cost heat source/sink for a geothermal heat pump system than conventionally bored or trenched systems. That is of course assuming the pond loop is properly designed and installed.

With our latest update to LoopLink RLC, you now have a simple to use tool to properly design a pond loop.

The RLC Approach

LoopLink RLC uses the calculation methods presented in ASHRAE's Design of Geothermal Systems for Commercial and Institutional Buildings (Kavanaugh and Rafferty, 2014) to perform pond loop design calculations. The calculations account for project location (weather conditions), peak heating and cooling loads, GSHP capacity and efficiency as well as the pond temperature, size and condition (i.e. clean water or muddy).

You have the option of designing pond loops in two configurations:

To make things simple, we assume ASHRAE’s minimum spacing recommmendation of 10 feet for both configurations.

As you work, LoopLink RLC will check that the maximum recommended heating and cooling rates are not exceeded (i.e. the pond is large enough and deep enough to accommodate the load) and will display a warning if necessary. Even with those warnings built in, there are some key things to things to think about when designing a pond loop

Size Matters In Cooling

Rejecting heat to a pond in the summer months even in the warmest of locations is fairly easy due to natural convection and evaporation. The most important thing you need to watch out for is the heat rejection rate.

If the pond (reservoir) is too small, you can change the natural temperature of your pond which is bad for the plants and fish. Plus, you can create excessive amounts of evaporation and run the risk of running your small pond out of water… also bad for the plants and fish.

Size Matters In Heating

Water is pretty amazing. One of the most amazing features of water is its behavior at and near freezing. We won’t get into the physics of water freezing but there are two things you need to know about frozen ponds.

  1. Ice floats
  2. Liquid water is densest at 39.2°F (4­°C)

So, if a lake is frozen at the surface, the temperature of liquid water at the bottom of the lake and it is 39.2°F (4°C). That is of course until we start extracting heat energy out of that water through a long winter.

Size matters in two ways in heating. First and foremost is the fact that if you have too small of a loopfield, you may locally freeze the water around your loops. This will make your loops more buoyant and may lift them off the bottom of the lake causing a host of problems not least of which is the possibility of catastrophic system damage.

The second size issue is the pond itself. If you don’t have a large enough volume of water you may suppress the temperature of the entire body which will again pose a risk to the plants and fish. It is possible to freeze a pond all the way through if the rate of heat extraction is higher than the rate of heat rejection from the soil below the body of water.

ASHRAE Says Size Matters

In any pond loop application, the body of water being used needs to be large enough so that the GSHP system does not alter the natural temperature of the reservoir by more than 1°F. According to ASHRAE, the maximum recommended load for a reservoir is 20 tons/acre in cooling mode and 10 tons/acre in heating mode.

Before designing a pond loop, a detailed study should be performed to ensure that the size of the pond is sufficient given the load and also to find the temperature of the pond in the summer and winter at the installation depth of the loop.

Top 5 Features of Bad Vault Design

Tuesday, February 14, 2017

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.

2 Heaters

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.

Corrosion
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.
Expansion/Contraction
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.
Cost
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.

About the Author

Joe Pejsa
Uponor

Joe is a Manufacturing Engineer at Uponor Infra. He has been involved in the ground source heat pump industry since July of 2003. His involvement in the industry has included technical support, field service work, estimating, installation, and troubleshooting of all types of geothermal systems. Joe has an extensive background in Geothermal Vault design, manufacturability, product development and confined space issues. Joe graduated with a BS in Manufacturing Engineering from South Dakota State University in 2005.