System information

ClimateMaster Geothermal Heat Pump Systems
27
Residential Products Technical Guide
Figure 16: Typical Radiant Floor Header System
The size of the buffer tank should be determined based upon
the predominant use of the equipment (heating or cooling). For
heating, buffer tanks should be sized at one U.S. gallon per 1,000
Btuh [13 liters per kW] of heating capacity at the maximum
entering source (loop) water temperature (EST) and the minimum
entering load (fl oor) water temperature (ELT), the point at which
the water-to-water unit has the highest heating capacity, usually
50-70°F [10-21°C] EST and 80-90°F [26-32°C] ELT. The minimum
buffer tank size is 40 U.S. gallons [36 liters] for any system.
Electric water heaters typically make good buffer tanks because
of the availability and relatively low cost. However, all local codes
and regulations must be followed. Insulation values of the tank
should be considered, especially when a buffer tank is used to
store chilled water due to the potential for condensation. A
minimum insulation value of R-12 [2.11 K-m
2
/W] is recommended
for storage tanks. Care must be taken when using the fi ttings
where the elements are threaded into the water heater for piping
connections. Typically, these fi ttings have very few threads, and
use a fl ange to seal against the water heater.
Equipment Sizing
Geothermal equipment sizing is particularly important not only for
comfort and IAQ (Indoor Air Quality) considerations, but also for
impact on installation costs. Since most geothermal installations
are closed loop, oversized equipment increases installation costs.
Undersized equipment may compromise occupant comfort and
even contribute to equipment operation issues. For example, an
undersized heat pump will run longer, which can potentially drive
the loop temperature too high or too low, causing even more run
time, and may eventually lead to operation at the extreme limits of
the heat pump.
Fortunately, equipment sizing procedures are well documented
and easily calculated using readily available computer software.
Heat loss loss/gain calculations for any residential HVAC design
should be performed using standard industry practices. Accepted
calculations include ACCA (Air Conditioning Contractors
of America) Manual J, HRAI (Heating, Refrigeration and Air
Conditioning Institute of Canada) and ASHRAE (American
Society of Heating Refrigeration and Air Conditioning Engineers)
manuals. Software versions of Manual J and other methods save
considerable design time. Either a whole house or room by room
calculation may be used for equipment sizing, but a room by room
calculation should be used for duct sizing.
Once the heat loss/gain has been determined, equipment should
be selected using the ClimateMaster GeoDesigner software.
Since the equipment capacity is directly related to the EWT
(Entering Water Temperature), the type of heat source/sink must
be considered when sizing equipment. For example, an open loop
system in the Northern U.S. will operate at approximately 50°F
[10°C] water year around, but a closed loop system in Georgia
may see temperatures ranging from 40°F [4°C] to 95°F [35°C],
which will affect the capacity of the heat pump in both heating and
cooling. GeoDesigner uses the heat loss/gain calculations along
with the loop type to determine heat pump capacity at design
conditions in both heating and cooling modes.
Figure 17: GeoDesigner Entry Screen For
Heat Loss/Gain
Because a heat pump operates in both heating and cooling, it’s rare
that a particular model will exactly match both the heating and
cooling loads. Sizing a heat pump for cooling is the best approach
in Southern locations, since the heating capacity is of little concern.
However, in a Northern location, equipment sized only for the
cooling load could cause excessive use of backup heat, increasing
operating costs. On the other hand, if a heat pump is sized for the
full heating load in a Northern climate, it will most likely be severely
oversized for cooling. In climates where relatively humidity is high
in the summer, oversized equipment can cause comfort and even
IAQ problems. A heat pump that is not running very often may not
provide suffi cient dehumidifi cation.
Newer technology has helped alleviate some of the sizing issues
mentioned above. Two-stage compressors, ECM fan motors and
whole house dehumidifi ers (see ClimaDry
®
II section) help provide
the appropriate capacities at design conditions and at part-load
conditions, and help keep relative humidity lower than single
speed systems or systems without dehumidifi cation mode. Proper
The Heating/Cooling Distribution System
and Equipment Sizing