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You already install furnaces, right? If you’re already installing furnaces, then you’re installing heat pumps too? What about minisplit inverter heat pumps? Well, why not GSHP (Ground Source Heat Pumps)? Retrofit or new construction: What’s the big deal?

GSHP systems have but one primary significant difference: the outdoor refrigerant coil and fan have gone missing. In addition to no outdoor coil/fan, the rest of the outdoor condenser’s components are now incorporated into the indoor unit, a hybrid condenser/evaporator unit. In place of the outdoor coil and fan, an earth-coupled heat exchanger nor performs giving up (air conditioning) or taking up (heating) energy from the planet earth or a body of water be it a pond closed loop or pump-and-dump from a stream or from one well to another or, in a few cases the municipal water main. While it might sound like a strange idea, geothermal heating and cooling systems are not some newfangled technology, although today’s high efficiency systems do include the same latest and greatest technologies the HVAC systems you’re already designing, selling, and installing incorporate.

A wide variety of configurations are available from standard air delivery heat pump to split condenser-to-air-handler to hydronic module and with open or closed earth-coupled loop configurations that can utilize water, water/glycol, or even refrigerant lines buried in earth. One thing to know in advance: weight can be an issue because of having now incorporated the outdoor condenser components into the same indoor evaporator/blower cabinet – something to consider when first installing a ducted geothermal HVAC system. If you are installing a unitary HVAC geothermal unit, there is no need to fiddle with refrigerants because it's a sealed system.

Retrofit applications

Replacing a gas- or oil-fired furnace with central air conditioning evaporator coil and outdoor condenser. It’s a system, so my first go-to when analyzing the possibility for retrofitting GSHP is the ductwork. What is the airflow capacity of the ductwork? I use a ductulator (https://www.acca.org/mobile) and for metal ductwork, friction factors of .08 for supply and .05 for the return because it’s easier to push than pull air and by using those friction factors, my air delivery system will be quieter.

As a basic rule of thumb, I use 400-CFM (cubic feet of air per minute) per ton (12,000-Btuh) of air conditioning. If my supply trunk duct is 8 x 26, then I can support 1,200-CFM or 3-tons of cooling. However, I may have an 80,000-Btuh furnace and here in the northeast, heating loads typically are greater than cooling loads and we’re looking at installing a heat pump in this retrofit application. The first question I’m asking myself is this: was the 80,000-Btuh furnace installed because its blower could provide the required 1,200-CFM or s that the actual heat loss for this home? For that matter, does this home actually need three tons of cooling?

There only one way to know and professional HVAC contractors will turn to a qualified heat loss/gain calculator that is accepted by the AHJ (Authority Having Jurisdiction) for the permit application. “Dave, Dave, Dave, you and I don’t need to do a heat loss/gain calculation. We have enough experience to look at a home and know what size heat pump will do the job.”

An actual quote from a fellow PHVAC contractor who had grossly undersized a heat pump and the owners could not warm their home above 55F during the colder days during winter. Don’t be that contractor. I use ACCA’s Manual J and it is universally accepted by AHJ’s throughout the USA. It’s also admissible in courts of law should you ever need to defend the sizing of any residential or light commercial installation and if you’re unlucky enough to have a builder shortcut a building envelope and compromise air/energy infiltration/exfiltration, this alone can save your hide. Been there, had that happen, and walked away victorious.

In addition to the duct trunk lines, you need to take each branch duct into account to ensure the trunk duct’s CFM capacity can be delivered. It’s a system! Beyond that, if you really want to perform your due diligence, check on placement of the supply registers throughout the home. Will any be blowing directly on occupants? Fossil fuel furnaces tend to deliver warmer air than do heat pumps, so this step can ward off potential complaints if you educate your customers in advance. Their skin surface temperature averages 84F and although a heat pump may be delivering air at 105F at the register face, delivered air is mixing with cooler room air and combined with velocity and moisture evaporating from skin, may feel cool.

What do you do when the heating load exceeds the airflow capacity of what’s required for sizing a heat pump to the heating load instead of the lower Btuh cooling load? The same thing you do with any heat pump: you add some type of auxiliary heat be it electric coils mounted in the air handler; or fossil fuel furnace to either supplement or replace the heat pump side.

New construction:

The sky is the limit! Get creative if your customer’s budget allows you to provide some awesome comfort.

As always, you can simply stick to air-based delivery systems or blend in hydronics as well as supplementing domestic hot water with the waste heat before rejecting that energy to the earth or target domestic hot water production directly.

If you do incorporate hydronics, lots of flexibility opens up to blend temperatures and mix/match various loads in the home. Check out the attached PDF (Link to PDF drawing) of one geothermal installation that incorporated a multitude of comfort delivery systems. Solar thermal was incorporated to assist when feasible. The property had added a remote garage large enough to house a pontoon boat, jet skis, and a motor home along with several cars. The second floor included sleeping quarters for up to 20-people and a bathroom. Hot water storage large enough to accommodate showering for the guests was included. This outbuilding was more than 100-feet away from the main home. PEX tubing in an insulated sleeve was installed between the main HydroNex panels and the garage HydroNex panel in the garage. A mix of radiant (low mixed temp) and hydroair units were easily incorporated.

One advantage geothermal has over solar is it is available 24/7/365 providing the earth-coupled heat exchanger loops are sized properly.

The heart of the matter: the earth-coupled heat exchanger

Three basic geothermal types that replace a traditional heat pump’s condenser coil/fan are: Vertical bore holes (wells); horizontal trench six to eight feet deep with either straight loops or a stretched out slinky coiled loops; and if a large enough body of water is available, pond loops weighted down so the sink to the bottom, a sled type flat-panel heat exchanger, or grid-type ballasted platform(s) that can be floated into position and sunk to the bottom.

The PPI (Plastics Piping Institute) has a wealth of great information here.

Aside from the training provided by manufacturers like WaterFurnace, the designer programs will provide the depth of bore holes needed per ton, HDPE plastic pipe size required, fluid volume and percentage of glycol required, and which flow station best matched the equipment being installed. At first, I was not comfortable allowing a well-driller to fusion-weld our distribution piping and thought our customers would be better served if we performed that portion of every installation.

After all, how hard could it be to heat up the HDPE fitting and pipe, push them together, and hold for a few seconds until the plastic solidified? Well my friends, it was not difficult, but I will freely admit our time was better invested in the more technical aspects of geothermal installations and, as I quickly realized, the better well drillers would gladly install the HDPW piping into the building with fusion-welded ends that could attach directly to our flow center. Work smarter, not harder!

What I learned early on was one competitor was deliberately under sizing the geo field so their installation price was lower than everyone else. There a fine art to determining sizing to handle the larger load between heating and air conditioning, which here in the northeast is almost always the heating load unless you’re dealing with commercial applications where the cooling requirements can sometimes outpace the heating load.

Our most excellent friend and one of my mentors, Lance MacNevin, director of Engineering, Plastics Pipe Institute building & construction Division, recently attended the IGSHPA International (Ground Source Heat Pump Association) where he gave a presentation on PPI MS-7 Model Specification for Plastic Piping Materials for Ground Source Geothermal Applications I asked Lance about the geothermal loops:

• The piping material is critical to the overall success of the closed-loop ground heat exchanger and must provide corrosion resistance, chemical resistance, flexibility, impact resistance, resistance to slow crack growth, long-term hydrostatic strength (i.e., pressure capability), and temperature resistance. In addition, the ground loop heat exchanger materials must provide suitable heat transfer capabilities. HDPE PE4710 is the dominant ground heat exchanger piping material and has proven reliable for these applications.
• In addition, PE-RT and PEX, which are both HDPE with enhanced temperature properties, are also approved for ground heat exchanger applications and are used when the extra capabilities are needed.
• Long-term pressure ratings for these piping materials are developed based on testing in accordance with ASTM Test Method D2837, and piping materials are listed according to PPI TR-3. While materials such as PE4710 have projected design life of 100 years or more in municipal water applications, ground source geothermal applications have more variables in system operation (e.g., temperature and pressure), so PPI conservatively reports that the life expectancy of these plastic piping materials, when specified correctly and installed according to industry and manufacturers’ guidelines, is typically well in excess of fifty (50) years.
• The PPI Plastic Pipe Design Calculator has been upgraded to include geo fluids to help designers estimate their pressure drop values accurately, and to size pipes so as to minimize pressure drop.
• Fortunately, the higher strength and toughness of PE4710 material allows thinner pipes to be safely installed in boreholes, reducing pressure drop and increasing heat transfer through the pipe wall.
  • PPI now recommends that SDR 13.5 PE 4710 pipes may be used, where in the past these pipes were SDR11 or SDR9 to have thicker walls for added protection against damage. This is a new trend that is beginning in 2025 with updates to codes.
• Grouts have improved, and today’s thermally enhanced grouts can provide better conductivity (i.e., less resistance) without big issues with mixing and pumping.
• Drilling equipment has improved, with more automation, efficiency and reliability, to allow contractors to drill faster and be more productive.

Educating your customer wins bids

Another advantage the design programs grants, is the ability to calculate when you will have reached capacity for exchanging Btus with the earth-coupled heat exchanger. During winter, you will be extracting heat from and rejecting cold into the surrounding earth, which is temporarily finite. I say temporarily because the rejected cold (think popsicle) earth surrounding the loop will slowly return to its normal 55F.

However, in most cases, that cannot keep pace with the active heating or cooling required for comfort conditioning. I found educating the potential customer worked very well because it drew them into the decision-making process regarding their personal pain-point for bringing auxiliary heat into the mix. The geo designer program provided a breakdown of costs between various systems like gas furnace with central AC at various operating efficiencies, air-to-air heat pumps with a range of SEER, COP, and HSPF values, and air-to-water heat pumps compared to the geothermal system I was proposing.

Where the geo field was concerned, I could show them what percentage of the heating system was to be provided by axillary heat and that cost was broken out as a line item. The customer bought into the design process by putting them in the driver’s seat. An under sized geothermal loop heat exchanger field will result in higher electric (if the axillary backup source is an electric resistance duct coil) that will likely result in an irate customer.

Under sized geothermal heat exchangers can easily be corrected! Adding additional bore holes, trenched loops, or submerged heat exchangers, if the body of water supports the addition are easily incorporated. I prefer reverse-return piping to automatically balance flow rates between loops, which works well if each loop is close to the same length.

Variable speed technology is rapidly replacing staged output in order to provide better operating economy as well as increased comfort for the customers.

IGSHPA has a YouTube education channel: www.youtube.com/@igshpaOrg/videos

IGSHPA YouTube video: Dig Deeper with Lance MacNevin

Tax credits and financial incentives are available to help you sell more geothermal systems. Given that market share has been steadily increasing by around 11% year after year, there has never been a better time for you to become involved. MacNevin provided the following information:

• The geo tax credits in the IRA will continue to foster growth in this industry, leading to improved processes and equipment.
  • https://geoexchange.org/ has been working hard in Washington to ensure that lawmakers understand all of the domestic benefits of GSHP systems.
  • This website https://www.dsireusa.org/ provides a great list of incentives all across the country.
• Finally, the new organization https://missiongeo.org/ has just launched to help tell these benefits to owners, designers, specifiers, etc. so they can take advantage of all of the benefits of GSHP systems.