In the mid 1980s, the first serious market for geothermal heat pump systems sprang to life in the U.S. As was the case with floor heating, the availability of a highly reliable high-density polyethylene piping system instilled confidence that geothermal systems could operate without leaks in buried piping that would be very difficult and expensive to fix. This assurance allowed the market to gain momentum. And this was all before words like “green” and “sustainable” became engrained in the global mindset.

Today, the geothermal heat pump market is thriving in the U.S. and abroad. It’s driven by reliable technology, the ability to provide both heating and cooling, and a potential “amplifying” load for electricity produced from renewable sources such as solar photovoltaics, hydropower and combined heat/power systems. The recent enactment of a 30% uncapped federal tax credit for residential systems makes this technology a very appealing option.

Although the majority of residential and light commercial geothermal heat pump systems use water-to-air heat pumps with forced air delivery, there are many unique and profitable opportunities for combining geothermal heat pumps with hydronics.

Enlarge this picture Figure 1.

The type of geothermal unit used for supplying hydronic systems is called a water-to-water heat pump. It absorbs heat from a stream of low temperature water (or mix of water and antifreeze), increases the temperature of that heat using a standard vapor compression cycle, then delivers the higher temperature heat to another stream of water. A simplified concept is shown in Figure 1.

The source of the low temperature heat could be ground water from a lake, large pond or well. Systems that circulate ground water directly through the heat pump are called open loop systems. They have been used successfully provided there is an ample source of ground water, and that the quality of that water is such that it will not corrode or scale the heat pump’s internal heat exchanger.

The other option is a closed loop system. Here the water or mixture of water and antifreeze is completely contained in a buried piping system that operates under slight pressure. This fluid in the buried earth loop never directly contacts the soil, and should remain in the loop for decades. 

The chemistry of a closed loop system is far easier to control than the condition of ground water at a given site. Thus, closed loop systems are generally preferable from the standpoint of long system life with low maintenance.

The Lower The Better

Enlarge this picture Figure 2. Source: ClimateMaster Corp.

One of the most important aspects of merging a ground source heat pump to a hydronic distribution system is the required supply water temperature of the latter. The lower the water temperature requirement of the distribution system, the higher the heat pump’s efficiency.

Examples of low water temperature hydronic distribution systems include slab type radiant floors with low resistance coverings (or no coverings).  Properly designed radiant wall and ceiling panels are another possibility, as are generously sized panel radiators.  My suggestion is to draw the line at a supply water temperature no higher than 120°F under design load conditions. Although some heat pumps are capable of higher temperatures, their efficiency drops off significantly as the water temperature they are asked to supply increases above 120°F.

Heat emitters, such as fin-tube baseboard or plateless staple-up radiant tubing, are not suitable for use with geothermal heat pumps. The supply water temperature of such systems is often based on the use of water supplied from a boiler in the temperature range of 170-200°F. This is well beyond what any currently available geothermal heat pump can supply.  Adding enough baseboard length or bringing staple-up tubing closer together is just not practical (think almost four times more baseboard that would be installed in a typical system).

Buffering the Load

Enlarge this picture Figure 3.

A significant benefit of hydronics is the ability to easily divide a distribution system into zones. Retaining this benefit when the heat source is a geothermal heat pump requires a buffer tank. The water within this insulated tank separates the rate of heat production by the heat pump from the rate of heat dissipation by the distribution system. This is necessary because most current generation water-to-water heat pumps are on/off devices with very little internal thermal mass.

Unlike cast-iron boilers, they are highly “flow sensitive.” While operating, they must have a steady flow of fluid on both their source and load sides. The load side is usually where issues arise.  Reduced flow due to inactive zone circuits will cause the heat pump to shut off to protect itself from potentially dangerous internal pressures or temperatures. The schematic in Figure 2 shows an example of a water-to-water geothermal heat pump supplying a zoned heating system. 

In this system, the water-to-water heat pump is turned on and off by an outdoor reset controller — the same type of controller that you probably already sell to control boilers.  This controller calculates the necessary water temperature in the buffer tank based on outdoor temperature. As the outdoor temperature increases, this controller reduces the average temperature of the buffer tank and vice versa. The controller then turns the heat pump on and off as necessary to keep the buffer tank temperature close to this calculated target temperature. This action maximizes the efficiency of the heat pump and eliminates the need for any type of mixing device between the heat source and the load.

Enlarge this picture Figure 4.

Domestic water is heated by a “desuperheater” heat exchanger within the heat pump.  This desuperheater is heated by hot refrigerant directly from the compressor discharge.  It transfers this heat to a stream of domestic water passing through the other side of the heat exchanger. This steam carries the heat to a conventional electric water heater.  The electric element only operates as necessary to “top off” the temperature within the storage tank. Many geothermal heat pumps, including water-to-water units, can be ordered with this desuperheater. When this option is ordered, the heat pump usually includes a small bronze internal circulator to convey water between the desuperheater and tank.

Notice that the distribution system uses a variable speed pressure regulated circulator in combination with zone valves. This is just one example of how the latest hydronics hardware dovetails well with geothermal heat pumps.

Cool It

Enlarge this picture Figure 5. Source: ClimateMaster Corp.

Most water-to-water heat pumps can be ordered as “reversible” units. As such they come equipped with an electrically operated refrigerant reversing valve. When powered, this valve effectively swaps the function of the evaporator and condenser as shown in Figure 3. Chilled water is now produced on the load side of the heat pump, while the earth loop serves as the heat sink.

Water in the range of 40-50°F is ideal for cooling with chilled water air handlers. Some systems may use a single air handler that’s matched to the cooling output of the heat pump. However, just as hydronic heating can be zoned, so can hydronic cooling.  Multiple chilled water air handlers can be supplied from a single chilled water buffer tank. Smaller units, such as those shown in Figure 4, can be piped with 5/8-inch or 3/4-inch PEX or PEX-AL-PEX tubing wrapped with vapor-resistant insulation to prevent condensation. Each air handler can be equipped with a zone valve. These zones can then be supplied by a single variable speed, pressure regulated circulator — just like that used for the heating zones. A schematic showing the concept is given in Figure 5.

Beyond these basic designs are dozens of other possibilities. For example, larger systems can use multiple water-to-water heat pumps operated in stages. These heat pumps can be controlled by a multi-stage boiler controller, just like those used for multiple boilers. 

Systems can also be constructed using two buffer tanks: One for warm water and the other for chilled water. This approach is well suited for buildings where fall and spring mornings require heating, but increasing temperatures or increasing internal gains require cooling by afternoon. In some systems it’s even possible to use a water-to-water heat pump to simultaneously create both warm and chilled water in the two buffer tanks.

If you already sell state-of-the-art hydronics hardware for use in boiler-based systems, expanding into water-to-water based geothermal heat pumps is a natural. You probably already know much of the hydronics technology that holds a typical geothermal heat pump system together. As customers increasingly seek “green” solutions to heating and cooling, having the expertise to facilitate a “geodonic” system is a profitable opportunity.