Operating instructions

12
VI. UNIT SIZING
Selecting the unit capacity of a hydronic geothermal heat
pump requires four things:
A) Earth Loop Configuration and Design Water
Temperatures.
B) Hydronic Side Heat Exchanger Operating
Temperatures.
C) Building Heat Loss/Heat Gain.
D) Temperature Limitations
A. Earth Loop Configuration and
Design Water Temperatures
Loop configurations include the open and closed loop
varieties. Heat pump flow rate requirements vary
depending on loop configuration (see Table 1). Consult
ECONAR’s Engineering Specifications Manual for
capacities at different loop entering water temperatures
and hydronic leaving water temperatures.
1. Closed Loop Systems
Closed loop systems use a heat exchanger of high density
polyethylene pipe buried underground to supply a
tempered water solution back to the heat pump. Closed
loops operate at higher flow rates than open loops since
the entering water temperature (EWT) is lower. The loop
EWT supplied to the heat pump has a great effect on the
capacity of the unit in the heating mode. Earth loops in
cold climates are normally sized to supply a wintertime
EWT to the heat pump from 32
o
F down to 25
o
F, which
minimizes the installation cost of the earth loop and still
maintains proper system operation. The GPM
requirements and pressure drops for loop pump sizing are
shown in Table 1.
2. Open Loop Systems
On an open loop system the design water temperature will
be the well water temperature in your geographic region.
Many cold climates are in the 50
o
F range for well water
temperature. If your well water temperatures are lower
than 50
o
F, for instance Canadian well water can be as low
as 43
o
F, the flow rate must be increased to avoid leaving
water temperatures below the freezing point. If well
water temperatures are above 50
o
F, as in some southern
states where well water temperatures are above 70
o
F, the
flow rates may need to be increased to dump heat more
efficiently in the cooling mode.
Varying well water temperatures will have little effect on
unit capacity in the cooling mode (since the well is
connected to the heat pump condenser), but can have
large effects on the capacity in the heating mode (since
the well is connected to the evaporator). If well water
temperatures are to exceed 70
o
F, special considerations,
such as closed loop systems, should be addressed.
B. Hydronic Side Heat Exchanger
Operating Temperatures
The hydronic side heat exchangers discussed in section IV
are designed to operate at a specific fluid supply
temperature. This operating temperature will have to be
supplied to the selected space conditioning heat
exchanger by the hydronic heat pump. The manufacturers
or distributors of the hydronic side heat exchangers
publish the capacity of their equipment at different
operating temperatures and fluid flow rates. These
capacities and operating temperatures are required to
select the heat pump to be used in the system.
When selecting the heat pump, choose a unit that will
supply the necessary heating or cooling capacity at the
minimum and maximum hydronic loop
temperature
conditions respectively. Example; if a fan coil system
requires 35000 Btu/hr to cool a space with 45
o
F water
temperature entering the water-to-air fan coil, a GW42x
GeoSource 2000 heat pump is required to handle the
cooling load.
If an intermediate heat exchanger is used between the
storage tanks as pictured in Figure 6, expect a 10
o
F
operating temperature difference between the two tanks.
For example, if the direct coupled storage tank is at
120
o
F, expect the maximum operating temperature of the
tank connected through an intermediate heat exchanger to
be 110
o
F. This occurs when connecting open loop
applications to the closed loop systems with plate heat
exchangers or with indirect water heaters.
C. Building Heat Loss/Heat Gain
The space load must be estimated accurately for any
successful HVAC installation. There are many guides or
computer programs available for load estimation
including the ECONAR GeoSource Heat Pump
Handbook, Manual J, and others. After the heat loss/heat
gain is completed, loop EWT’s are established, and
hydronic side heat exchanger conditions are determined,
the heat pump can now be selected using the hydronic
heat pump data found in the Engineering Specifications.
Choose the capacity of the heat pump based on both
heating and cooling load.
D. Temperature Limitations
Be aware of the operating range of the geothermal system
when sizing the particular heat pump. An operating range
of 15
o
F (minimum for heating) to 110
o
F (maximum for
cooling) is required for the earth loop side
. These limits
have been established based on efficiency limitations and
safety pressure switch limits (25-psi low-pressure cutout
and 400-psi high-pressure cutout). Hydronic side
limitations in heating have a minimum of 50
o
F HYD
entering water temperature and a maximum of 130
o
F
HYD leaving water temperature range (entering water to
the hydronic side below 50
o
F gives low head pressures