Water Quality in GSHP Applications

By, Ryan Carda

Water quality has a huge impact on GSHP service life and efficiency. Unfortunately, most systems are filled with whatever water is available on site with little or no attention to its suitability for use.

In fact, water quality is the root of many issues, including the premature failure of mechanical components. It goes without saying that if you aren’t already paying attention to water quality, it’s time to start.

Water treatment can be divided into two categories: physical and chemical. Physical water treatment consists of the removal of air, dirt and suspended solids from the circulating fluid. Chemical treatment consists of the modification or elimination of substances in the water to make it suitable for contact with the various components in the system.

1 Physical Water Treatment

Every geothermal system will start out with air in it. A new system will also have certain amounts of dirt, oil, pipe shavings, and other debris as a byproduct of manufacturing, transport, and installation.

The majority of air removal is done with power flushing prior to system startup. Dirt removal is done simultaneously with a filter during power flushing¹. Geo-Flo recommends filtration to 100 microns during the flushing and purging process. Once air is purged from the system, a finer 1-micron filter will remove sand, silt, and clay from the circulating fluid.

After system startup, additional air and dirt removal is possible with the right mechanical devices:

• Air removal: high-point vents or central air separators
• Dirt removal: filters, strainers and particle separators

Failure to remove air and debris from the piping system will cause service-related issues and even shorten the service life of the circulating pumps and other related components. Permanently-installed mechanical devices can provide ongoing levels of physical water treatment, but keep in mind that they will need to be cleaned periodically.

2 Chemical Water Treatment

Although air and dirt removal is essential, physical treatment doesn’t affect water chemistry.

Water is commonly referred to as the ‘universal solvent’. It is always full of dissolved minerals, sediment, dissolved gasses, etc., and the amounts vary from place to place. If left unchecked, poor water chemistry can lead to corrosion, scale, fouling and biological growth, which affects system efficiency and service life and becomes a human health and safety concern. Treatment may include the removal of chemical impurities, pH value adjustment, or the addition of antifreeze, corrosion inhibitors, microbial control products, etc.

There are many different ways to approach water chemistry. Here are a few that are common in GSHP applications:

Internal Cleaning

The process of cleaning an internal piping system is fairly straightforward. Simply circulate a cleaning solution (detergent) through the piping to remove dirt, oils and other impurities. After the process is complete, the system will need to be completely drained and washed with clean water prior to being filled with loop fluid.

By removing impurities from the system on the front end, it will be easier to achieve the desired level of water quality later on. Note that internal cleaning is uncommon in residential applications.

On-Site Water Treatment

One option to address water chemistry is to simply add inhibitors to prevent corrosion, scale, biological growth, etc. The inhibitor manufacturer will provide guidelines to determine proper concentration levels, handling and safety requirements, etc.

For heating dominant systems that use antifreeze, additional inhibitor is typically unnecessary as most antifreeze products are are pre-mixed with the necessary inhibition agents.

Transport

Rather than adding chemicals to available water on-site, another option is to purchase treated water (or a premixed water-antifreeze solution) from a water or chemical treatment company, haul it to the jobsite and completely replace the water in the system with the pre-treated fluid. This is the easiest way to guarantee that water quality will not be an issue, but also the most expensive.

Measures of Water Quality

In general, it is best to refer to the manufacturer of each component in the system to determine water quality requirements (pH level, dissolved solids, turbidity, chloride levels, etc.). If guidelines aren’t readily available, Section 3B in IGSHPAs Design and Installation Standards manual will serve as a great starting point.

The most relevant measure of water quality is dissolved mineral content.² In general, the use of de-mineralized fill water will alleviate the majority of issues related to scale and corrosion.

Remember to Check In

The circulating fluid should be tested periodically to check pH, turbidity, suspended solids, antifreeze and inhibitor levels, etc. IGSHPA recommends that water quality be checked at least once a year in a commercial system and once every five years in a residential system.

By comparison, GSHP systems are more forgiving than other HVAC system types when it comes to water quality, primarily because most of the piping in the system is the ground loop, which is constructed of inert plastic (polyethylene). But poor water quality will wreak havoc on pumps, heat exchangers, valves, and other mechanical devices in the system. As such, water chemistry always needs to be addressed.

Footnotes:

1. According to ASHRAE, 2 ft/sec is the recommended velocity to remove air and light debris from a piping system. However, this velocity is not enough to remove large or heavy particles and additional filtration is recommended.
2. Refer to Idronics #18 for additional information.

Pros and Cons of Centralized Pumping

By, Ryan Carda

With a ground source heat pump system (as with anything else), the designer must strike the right balance between installation and operating costs. This is true of all aspects of design, but is especially true of the interior piping design as well as the pump layout and selection. Even with the best design, the efficiency gains from a GSHP system can be completely erased by poor piping and pumping design (due to excessive head loss, oversized pumps, improper control, etc.).

While there are many options with respect to pumping system layout and design, they generally fall into one of two categories: centralized or distributed. While there are merits to both approaches, the process of finding the best choice for your application starts with an evaluation of the the project.

Centralized Pumping Basics

A centralized system will use one or more pumps at a central location to induce flow through the loopfield and then distribute it to the units scattered throughout the building, as shown in the illustration.

For large systems that fall in this category, variable speed control is common (and may even be required)¹.

In general terms, the use of a centralized pump will be well-suited for a building with a small footprint where the interior piping can easily be connected to the GSHP units that are scattered throughout². Centralized pumping may also be ideal for applications with significant load diversity where ‘load sharing’ principles can be used to reduce overall loopfield requirements³.

To determine whether this design approach is the best choice for your system, start by estimating the installation and operating costs and then comparing them to a distributed pumping approach. Next, take a full accounting of the pros and cons of centralized pumping, a few of which are as follows:

Pros:

• Large pumps generally have better overall efficiency values than the smaller pumps used in distributed arrangements.
• When required, maintenance is performed at the central pumping station (located in the mechanical room or dedicated pump house) which provides adequate access and minimal disturbance to the rest of the building.
• Maintenance costs and personnel requirements are generally the lowest with this approach.

Cons:

• A large interior piping loop must be used to connect all of the GSHP units to the central circulating pump, which requires proper design, increases complexity, requires the use of larger pipe sizes and drives up installation cost.
• VFD flow control can be complicated and central systems must be balanced for proper operation⁴.
• Failure or shutdown of the pumping system will cause the entire system to be down.

Best Suited For:

• Buildings with small footprints and/or significant load diversity².

Additional Notes:

• To avoid system shutdown due to failure or routine maintenance, redundancy with a standby pump (in parallel with the main pump) is recommended. With redundant pumps, duty cycling is important to help with pump longevity and to even out service life expectancy.
• When variable speed control is required, flow control measures (such as zone valves and pressure sensors) are necessary.
• With variable speed control, the pressure drop in the distribution piping should be kept low. Additionally, the system should be balanced during startup to ensure proper control can be achieved.
• Variable speed pumps should be designed to never operate below 25% of design flowrate to ensure that the motor and VFD efficiencies remain relatively high. For most GSHP systems, a large percentage of operating hours for the year will be at the lower end of the flowrate (idle or with 20% to 40% of GSHP units operating).

In a large GSHP system, a centralized pumping solution may be beneficial because of the low number of pumps required along with their central placement (which comes in handy when maintenance is required). But the system designer must select a pump that meets pressure and flow requirements while also providing economical operation, which can be tricky at part load conditions.

Because of the increased complexity of system design and control, as well as the lower limit on flow with variable speed equipment, it may be worth looking at the Pros and Cons of Distributed Pumping to see if it is a better fit for your application and more importantly, a better fit for your customer.

Footnotes:

1. Per ASHRAE 90.1 (2016), “Hydronic heat pumps and water-cooled unitary air conditioners having a total pump system power exceeding 5 hp shall have controls and/or devices (such as variable-speed control) that will result in pump motor demand of no more than 30% of design wattage at 50% of design water flow.”
2. Refer to Chapter 6 in Geothermal Heating and Cooling: Design of Ground-Source Heat Pump Systems (Kavanaugh and Rafferty, 2014).
3. LoopLink PRO can be used determine how much ground loop reduction is possible with load sharing principles for a given system.
4. A recent GSHP field study indicated that less than 10% of the ground-loop variable speed pumps with differential pressure transducer control were operating as intended due to faulty controls or had pumps large enough to provide near full-load flow rate at minimum motor speed (Kavanaugh 2012).

Video: Burying Mistakes

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.