Geothermal for Heating and Cooling (Main Residence and Pool Room) – Meredith, NH

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Project Summary

In an effort to reduce dependency on fossil fuel, our customer contracted with TJ’s Plumbing and Heating, Inc. to install a geothermal system for both heating and cooling.

This was no ordinary geothermal system. The challenge was effectively using controls to integrate three systems – geothermal for radiant, geothermal for air heating and cooling, and a backup oil boiler.

TJ’s Plumbing and Heating, Inc.’s custom design consisted of one 1,500-foot and two 1,000-foot standing column water wells. The wells were brought into the home in one location. The challenge was to stage the wells turning on without back feeding another well. We installed three 2-inch motorized valves on the well returns. This stopped any back flow into the other wells. TJ’s chose the valves and well pumps to be controlled by a Ranco set-point control, contacts and time delay relays.

The next step was to install a plate and frame heat exchanger. This separates the well water from the water running through the heat exchangers. The heart of the geothermal system is a Wilo Stratos circulator pump. This pump modulates its speed based on the number of zones calling, thus reducing electrical consumption.

The main house has radiant heating in concrete throughout the entire home. The system is now run by one ten-ton, two-stage Geo Comfort heat pump. This heat pump feeds its heated water into a 119-gallon Turbomax storage tank. The heat pump is controlled by a Honeywell two-stage controller. This controller allows for optimum efficiency. The stored water in the tank is now injected into the primary heating loop as needed. By injecting into the existing heating system this allows us to keep the oil-fired boiler as back up.

The control strategy for the injection of stored geothermal energy is quite elaborate. TJ’s installed a Honeywell set-point controller that senses primary loop temperature. If the loop temperature exceeds 120 degrees, the injection pump is turned off. This was necessary because we also heat domestic hot water storage tanks; and, they require water temperatures in excess of 120 degrees. The water heaters are the only thing in the system allowed to turn the boiler on. On a call for domestic hot water, the tank thermostat energizes a time delay relay, preventing the boiler from firing for 20 minutes. This allows geothermal system to pre-heat the hot water. Only then does the boiler turn on and top the tank off. This results in significant energy savings.

The cooling for the main area of the home was designed by TJ’s using one 5-ton Geo Comfort split system and two 1.5-ton Geo Comfort split systems. The existing cooling units were removed and replaced with the split systems. The air systems serve two functions – primary cooling and a third back-up heat. The primary heat is the geothermal radiant. If necessary, the air will back it up. And, as a last resort the oil boiler can be used.

One of the most interesting features of our geothermal design is the heating of the indoor pool.

TJ’s entirely removed the gas-fired boiler and replaced it with three Geo Comfort 8-ton, two-stage water to water units. They feed into one Turbomax 119-gallon storage tank. Six stages of heat pumps are controlled by a Tekmar staging control and separate Erie vales. The tank sends heated water to the plate heat exchanger for the pool area when needed and also through a coil in the ductwork to heat the space.

The control strategy for the pool area was specifically designed by TJ’s Plumbing and Heating, Inc. to save energy by cycling the Dextron unit on and off, along with the pool. The existing system has a Dextron pool conditioning unit. This unit was originally designed to run 24/7 and uses a lot of electricity. To lower the electricity consumption, TJ’s installed a digital 7-day programming unit. The system runs the entire pool and heat on a time cycle with the use of relays and contacts. This allows the building heat to energize the Dextron unit any time it needs heat, but still run all the necessary time cycles. Any time the the Dextron unit is on, the pool pump also runs, in case the Dextron unit needs to remove moisture. When the moisture is removed from the air, it is then sent to the pool. There is no wasted energy, or electricity.

TJ’s also installed hot tub heating using geothermal heat and Control4 automation with wireless and remote capabilities.

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