ASHRAE Journal - August 2014 - 64

2014 ASHRAE TEcHnology AwARd cASE STudiES

Domestic Hot Water

Warm Header

Domestic Hot
Water Heat
Pump
Cooling Mode

Heating Mode

Cooling Mode
Heating Mode

Five Geothermal Wells

Cool Header

Heating Mode

Domestic Cold Water

Domestic
Hot Water
Tank

Drinking Water

Cooling Mode

Radiant Floor

Radiant Floor
Heat Pump

Radiant
Water Tank
Heating Mode

Domestic Cold
Water Supply

Cooling Mode
Figure 1 Geothermal and heat pump flow diagram.

require around 110°F (43°C) supply air temperatures.
Due to the non-traditional requirements of the control
system and the desire to automatically gather large
volumes of data, a small programmable logic computer
controls all of the mechanical systems.
Several large, low-E windows face south. The roof eave
overhang is specifically designed to provide full window
shading in the summer and full sun in the winter. During
the heating season, the passive solar gain raises the living
area temperature up to 4.5°F (2.5°C) above normal with
much of the thermal energy absorbed by the dark-colored
tile into the floor mass. This thermal surge will carry elevated temperatures until very late in the evening without
any supplemental heating from the heat pump system.
Domestic hot water is generated by a 3 ton (10.5 kW),
two-speed water-to-water heat pump (operating only as a
1.5 ton [5.3 kW] unit) also served from the geothermal well
system. All domestic hot water is preheated via independent tubing in the upper floor structure. In the summer,
this design provides free cooling for the home and free
preheat for the water. In the winter, the radiant floor heat
pump, due to much lower lift requirements, operates at
least twice as efficiently as the domestic water heat pump.
Preheating domestic water with the radiant heat pump
system results in significantly reduced total energy consumption for domestic hot water heating as compared to
a single heat pump for water heating.
Home cooling as initially conceived was to be provided by direct circulation of cool geothermal well
water through the radiant floor system, a very efficient
and cost-effective means of cooling. Nonetheless, even
more efficient options were developed during the
house design process. With the exception of dedicated
64

ASHRAE JouRnAl

ashrae.org

Au g ust 2014

drinking water, in the cooling season all domestic
cold water (40°F to 70°F [4.5°C to 21°C]) from the local
municipal water supply is routed via dedicated PEX
tubing through the floor slab to extract free cooling
by increasing the water temperature up to near 74°F
(23.3°C), the temperature of the floor.
Next, heat is recovered from the floor mass to the
domestic hot water system via swapping valves on the
geothermal system. As shown in the flow diagram in Figure
1, this change allows the domestic hot water heat pump to
bypass the geothermal system and take heat directly from
the home via the radiant floor and delivering the heat
to the domestic hot water. Since the floor operates up to
20°F (11°C) above the geothermal well temperatures, this
design is over 30% more efficient than direct geothermal
coupling for the domestic hot water system and it provides almost free cooling to the house.
Because the large mass of the floor system acts as a thermal storage reservoir, and it takes multiple hours to actually change the floor temperature a single degree, the typical energy recovery timing problems due to generation
not coinciding with load usage are eliminated. Therefore,
using domestic hot water provides home cooling that will
be stored until it is needed. If any additional cooling is
needed, the control system will circulate cool water from
the geothermal wells directly through the radiant slab.
To prevent overcooling, if the cooling load is sufficiently
satisfied, the domestic heat pump will automatically swap
back to the geothermal wells for its heat source.

Indoor Air Quality
Indoor air quality to this home is provided via a geothermally conditioned 100% fresh air supply system.

ASHRAE Journal - August 2014

Table of Contents for the Digital Edition of ASHRAE Journal - August 2014