Geothermal Heat Pumps and Radiant Systems
Meanwhile, Back at the Ranch…
A geothermal heat pump system intersects with radiant floor design to provide one of the most energy efficient means to provide heating for residential systems, both in new construction and retrofit applications. About a decade and half ago, geothermal heat pump manufacturers started providing products that could be applied to radiant floor heating systems. Initially, these water-to-water units (as the industry labeled them) were too small in capacity to service any significant heating load. In most cases, these units had 1, 2 or 3-ton (36,000 BTU/hr) nominal capacities. While the units were undoubtedly useful at these capacities, it wasn’t until larger-capacity units became available (today’s single-phase units are available in up to 10- to 12-ton capacities) that really opened the opportunities to merge what is probably one of the most logical applications for geothermal heat pumps.
There are numerous advantages to using a GSHP system combined with radiant floor heating. Primarily, as most radiant system designs typically require supply temperatures between 100 to 120°F, this makes a convenient merger with geothermal heat pumps since most GSHP models also operate with their highest capacity and best coefficient of performance for energy consumption between those same temperature ranges. While some applications may require temperatures outside of these ranges (for which there are several easy solutions), a GSHP system can almost always optimize radiant systems in residential retrofit and new construction projects.
Additionally, combining a GSHP and radiant system often equates to a reduction in the mechanical piping of these systems by eliminating the piping system’s mixing valves – something that can be required with a conventional (non-condensing) boiler. To even further optimize the piping system, it is often designed with a buffer tank between the heat pump and radiant floor system to 1. eliminate short cycling of the heat pump unit(s) by hydronically decoupling it/them from the radiant system such that the GSHP unit(s) is/are required to only maintain the buffer tank’s setpoint and 2. allow the radiant floor system, during lower heating load periods, to work against the stored volume of hot water. During these circumstances, only the radiant system’s circulation pump is required for operation, saving additional energy as it is only a fractional horse power (hp) pump.
There are several other design considerations to be aware of to facilitate an optimized combined GSHP and radiant system design. One of the key design factors is to only install the radiant pipe in concrete or thin slab (ex. PEXa pipe in Gypcrete or low-weight concrete) on top of the subfloor. Using a geothermal heat pump system with a stapled-under-the-floor type of system is generally not recommended, and if such a situation arises (for example, if the radiant system is already installed, and it is the intent to retrofit a GSHP system) the designer must do the legwork to determine if combining a GSHP system is still merited.
Radiant pipe diameter and spacing also play key roles with a successful system design when combining a GSHP system. With high heating load densities (BTU/hr-ft2), the correct radiant system output and a maximum floor surface temperature of between 78 degrees Fahrenheit and 82F (85F should be considered as the absolute maximum) can most often be achieved with 5/8-in pipe (no less than ½-in) and 9-in on-center spacing, though sometimes tighter six on-center spacing is required for applications with extreme heating load densities. Again, there are caveatsto this rule, and some systems have successfully worked with special plates or unique products that use radiant piping with as small as a 5/16” in diameter. In these types of circumstances, it is important that the engineering and contracting team work together to examine the application’s regional climate conditions, along with the radiant and geothermal heat pump products being used for the system. When heating load densities are low, on-center spacing can be stretched out to 12 inches. Those conditions would likely include something like a home’s garage, where desired temperatures are about 55F, or a basement (subgrade or walkout) where heating losses are significantly lower than an above-grade room that has numerous exposures and lots of glass. As a matter of point, one of the biggest rooms in a house that may a supplemental systems to deliver heat to the space would be a home’s “great room” that typically has high ceilings and a large overlook equating to a high percentage of glass exposure. In those instances, baseboard heating, fan coils or other HVAC system alternatives can certainly be used, although may not be necessary as today’s radiant systems can often deliver adequate capacity on their own, and can also be installed in walls and ceilings (i.e. not just floors). What is of upmost importance is to calculate system capacity and heat loss in this type of application, and notjust guess.
Insulation under the slab is an additional key element in the radiant system design that cannot be overlooked. For both walkout basements and full basements, significant heat loss can occur under the slab, sometimes accounting for half the total heating floor load. This becomes critical as a GSHP system does have a finite temperature limit between 120- and 130F before the GHSP units’ compressors may start to lock out on high head pressures. If insulating portions of the slab on grade or below grade is missed either in the design or installation, the GSHP system can be hindered from performing as intended.
Overall, when properly designed for the necessary heating load of the home, whether in portions of the house or via a whole-home zoned system, a combined GSHP and radiant system can provide ultimate comfort benefits for occupants. Both radiant and geothermal heat pump systems go virtually unnoticed, with quiet operation and the ability for a contractor to pre-set operation parameters for desired indoor temperatures in relation to those outdoors. Additionally, unlike traditional forced air systems, the operating temperatures of this type of combined system are significantly lower, which equates to enhanced comfort (as extremely hot or cold air is not being “blasted” at various intervals to achieve the desired indoor temperature). Remarkable energy savings can also be achieved with this type of system, thanks to both the lower operating temperatures and the year-round renewable energy sourcing which capitalizes on the consistent temperatures below the earth’s surface.
Case-in-Point: Combined Radiant/GHPS Retrofit Project at Ranch Residence in Bozeman, Mont.
Design/Build firm Energy 1 was presented with no small feat in the fall of 2011 when a local ranch owner requested assistance in creating a complete off-the-grid system for his existing property near Bozeman, Mont.
“This was a retrofit at a high-end ranch residence, surrounded by pristine landscape with mature vegetation,” said Leo Crane, vice president at Energy 1. “Maintaining a minimal level of disturbance, while achieving energy independence, was very important for the owner, and so these became key priorities for the Energy 1 team as well.”
The ranch owner, who also manages an international real estate brokerage company, was particularly interested in exploring geothermal heat pump options after learning more about this renewable energy technology from a number of his business deals.
“I grew up on a ranch and have always liked the idea of having a self-contained homestead,” the owner said. “Originally, I did what I thought was the right thing to do energy-wise, but after living here for a few years, I realized there was a lot more I could do. The original house had a tight energy envelope and was operating rather efficiently over the past 11 years, but I worried about curbing costs on energy sourcing. I had been using propane gas, but I started running into inefficiencies with the boiler. My plan was to replace it in the next five years anyway.”
Taking all aspects of the project into account, Energy 1 specified an open-loop geothermal heating and cooling system for the residence, which would minimize disturbance to the existing structure and surrounding landscape. The system included two 3-ton ClimateMaster Tranquility High-Temperature Series water-to-water units, and Energy 1 employed a unique direct, horizontal drilling technique to run well-water supply and return lines below-grade to the edge of the ranch. This was designed to feed into a radiant heating system installed throughout the ranch residence (staple-down in a 4-inch concrete slab on the ground level, and staple-up on the second floor).
To meet these increased demands for a self-contained, energy-independent property, Energy 1 also installed a propane-fired high-capacity generator for back-up power.
“Integrating the new mechanical system, while carefully protecting and maintaining the existing aesthetics, both inside and out, was critical,” Crane said. “To be successful with an energy retrofit project like this, the only long-term difference that the owner should experience is the slashing of their monthly energy bill.”
The work was completed in a short six-week timeframe in November of 2011, with some additional time for fine-tuning and system balancing.
“We have been conducting post-installation monitoring for the past two years to track efficiencies and the real-time savings with the new system,” said Crane. “Because this was a retrofit project, we could compare propane usage both pre- and post-geo installation. As far as the savings to date, the propane usage has been reduced by approximately 70 percent.”
From the ranch owner’s perspective, the project has been very successful. According to him, the goals of reducing the cost and dependency on propane, while also achieving short-term payback on a new mechanical system, maintaining the landscape and not disturbing the existing home, were clearly achieved. This system, combined with a new back-up power solution, has also allowed the owner to move towards his goal of a more self-contained homestead.
“Working with Energy 1 and ClimateMaster has been a very positive experience,” the homeowner said. “I am happy with the outcome.”