A ten ton closed loop horizontal geothermal field serves five water to air heat pumps (five forced air zones) and one water to water heat pump (three radiant floor warming zones and domestic hot water preheat). Individual loops in the field were increased in size to minimize pumping energy. The entire geothermal system uses one Grundfos circulator with a built-in pressure sensor to optimize pumping energy.

As motorized zone valves (connected to each heat pump) open and close the Grundfos circulator adjusts its speed and pumping energy based on system pressure drop.

During periods of sufficient system diversity (domestic hot water demand during the cooling season) a geothermal buffer tank and motorized valves can isolate the inside system from the outside field; this allows heat removed from rooms to be used to heat the domestic hot water and further reduces pumping energy.

The entire system is controlled, monitored, and accessed from an ENV direct digital control – this web based system allows the client to monitor and operate the system from the internet, sends status alarms, and trends energy use for system optimization.

Project Details

The mechanical system for the Rothmann residence was designed to be competitive in price with a conventional fossil fuel furnace system and yet more efficient than a standard geothermal installation. As a fundamental part of the LEED for Homes certification effort, this system shows that a green mechanical system does not have to cost more money to install than conventional equipment – and the green system will save energy over the life of the building.

Numerous features help to make this system green and more energy efficient:

The replacement of multiple circulators serving the heat pumps with a main circulator is made possible with the use of a modulating main circulator pump. The Grundfos CIRE pump has a built in PI control driving a VFD equipped motor to maintain the pressure setpoint.
We installed a time delay on each heat pump to ensure that its motorized zone valve is fully open before the main circulator pump is started. This further reduces system pressure variations and allows closer control of pumping energy.

We used geothermal loop sizing software to optimize loop spacing and loop tube diameter; balancing heat transfer (turbulent flow) with loop pressure drop (pumping energy) and pipe size (first cost).

The concentration of heat pumps in the mechanical room allowed us to send all heat pump returns to a geothermal buffer tank. Our digital control system watched the temperature of this tank to optimize system pumping energy; if the tank temperature falls below 40F or rises above 80F, then motorized valves open up to connect the inside geothermal system with the outside loop field. At tank temperatures between 40F and 80F, the valves to the outside field close, reducing pumping energy. At times of higher system diversity energy exchange can occur in the buffer tank instead of in the loop field.

We have explained to the client that as solar energy hits the house, warms the rooms, and is removed by the heat pump, we can harvest this solar energy to preheat his domestic hot water (and a future pool) instead of using energy to run the main circulator to send the energy underground.

The client was not sure if radiant floor warming or space heating would be more comfortable in conjunction with forced air zones. Through the use of the ENV control system, we were able to offer both control schemes. We installed slab sensors in the three zones for use with a floor warming control scheme and will use the room temp info collected from the forced air system communicating thermostats in a radiant space heating scheme.

This will allow the homeowner to easily converting between a floor warming radiant approach and a space heating radiant approach with a few mouse clicks.

We set up three radiant floor zones with two different mix temperatures for the two different floor types in the zones. As the ENV system directly controls the water temperature via the motorized mixing valves, it will be relatively simple to add radiant cooling in the future by simply reversing the heat pump operation and storing chilled water in the radiant buffer tank (and installing the appropriate relative humidity transmitters in living spaces for the dew point calculation).

For humidity control, the ENV is connected to a relative humidity transmitter in the basement zone to provide a relative humidity setpoint for the basement heat pump.
The ENV will also be able to stage radiant heat with dehumidifying operation during the winter – because of the proximity of the residence to the ocean, fog and high levels of humidity during the heating season often occur.

An important part of the control system is the ability to trend energy use and determine how close our initial system sizing was to the actual heat loss/gain of the building. The ENV system will allow us to isolate actual energy use and where it occurs in the mechanical system. This information will assist us not only in the commissioning process but also in setting a benchmark to maintain long term system efficiency.

Web access provided by the ENV system allows us to receive system alarms and the homeowner to adjust setpoints and occupancy status. We see the long term value in a control system like this in reducing winter energy use for many seasonal residences on the island.

The control system in this project is set up with numerous one click macros that perform multiple functions. Perhaps the most important is the home occupied macro – in the unoccupied mode, it shuts off domestic hot water preheat and drops the setpoints of the radiant and forced air systems. This saves energy and allows the homeowner to reduce his utility costs while protecting his home.