Commercial First Cost - Ground Water (Open Loop)

Geothermal heat pump systems are easily confused with the more widely used water loop systems, often called water-source heat pumps. While the loop inside the building is similar, there are important differences.

With the conventional water loop systems, the heat pump units only deal with water in the narrow 60°F to 95°F temperature range. With geothermal systems, the heat pump must be able to function properly with lower water temperatures. In some cases, this can be down to 35°F outlet water in northern climates. The heat pump design to do this can result in a somewhat higher equipment first cost.

Another difference is the need for a well or ground water system, and its added first cost. Where an ample supply of water is already available, this may add very little cost….the cost of a return (disposal) well.

In most cases, you will have to install a well system, including the well pump, tank, piping to and from the building, and a disposal well. This cost can add $500 to $1,500 per ton of cooling capacity to the first cost.

Please note: ALL costs are rough estimates only. You must check locally for your costs.

In ground water studies done in 1994 by Kevin Rafferty and others at the Geo-Heat Center of the Oregon Institute of Technology, some general cost parameters were identified. Some key points of these studies follow. Capital cost comparisons associated with three ground water system designs are shown here. The first covers 50°F ground water or soil temperatures, and a range of 100 to 500 tons of building block load. The second covers 60 °F ground water or soil temperatures, with the building block load still at 100 to 500 tons. The ground water system includes production and injection wells, variable speed pump, heat exchanger, controls and buried piping to the building wall. This applies to both 1- and 2-well systems and groundwater depths of 200, 400, 600, and 800 feet.

The ground-coupled system includes installed vertical ground loops and header piping to the building wall, at a 1994 cost of $1,000 per ton, based on 200 feet of borehole per ton, at $5 per foot of borehole.

The hybrid system uses ground-coupled vertical loops and a cooling tower. Hybrid systems are used in buildings with low heating needs. The total cost per ton is shown at several heating to cooling ratios. This cost includes the indoor water loop system and heat pumps at the cost per ton shown in the lower curve.

The hybrid system costs include the installed vertical ground loops, header piping to the building wall, and closed circuit cooling tower, based on several heating to cooling ratios. This ratio is the feet of vertical loop piping needed for the heating load divided by the feet of loop needed to serve the cooling load.

For example: If the loop length to meet the peak cooling load is 10,000 feet, and only 4,000 feet is needed to serve the peak heating load, the ratio is 0.4. The cooling tower costs are based on 75°F wet bulb design. In all cases, the building loop temperature during the peak cooling load is 85°F supply water to and 95°F return from the heat pumps. Below 100 tons the costs rise rapidly.