Operating instructions

9
The entire system must be purged of air during initial
installation and pressurized to a 10-25 psi static pressure
to avoid air entering the system. This initial static
pressure may fluctuate when going from the heating to
cooling modes but should always remain above zero. If a
leak in the system allows the static pressure to drop, the
leak must be repaired to assure proper system operation.
Air continually entering the loop can cause corrosion,
bacteria, or pump cavitation.
The hydronic side circulator supplying the heat pump
should be controlled to run only when the compressor is
also running. If the pump is allowed to circulate cold
water through the system during off cycles, the refrigerant
in the heat pump will migrate to the hydronic side heat
exchanger. This can cause heat pump starting problems
(especially when this refrigerant migrates into the
condenser).
C. Circulation Fluid
The fluid circulating through the hydronic side of the
geothermal heat pump system is the transfer medium for
the heating and cooling being supplied to the conditioned
space. Selection of this fluid is very important. Water is
the most readily available fluid but has the drawback of
expansion during freezing which can damage the system.
System operation in the cooling mode, extended power
interruption to a structure, or disabling of an outside zone
(such as a garage floor) provides the opportunity for
freezing the circulating fluid.
Antifreeze must be used whenever the possibility of
freezing exists from the environment or from use of the
unit in the cooling mode. A propylene glycol based
antifreeze (readily available through HVAC wholesalers)
and water solution is recommended. Methanol based
antifreeze is not recommended for use on any hydronic
system where heat is being added to the system for
structural heating purposes. Freeze protection for the
hydronic side fluid down to 20
o
F (20% propylene glycol
by volume in water) is recommended for most indoor
applications (see Chart 1). Forty percent propylene glycol
in water (-5
o
F freeze protection) is recommended by
radiant tubing manufactures for snow melt applications, in
order to protect the tubing from expansion in outdoor
applications. Using over 40% in hydronic side
applications can cause pumping problems due to high
viscosity.
The water being added to the system should have 100-
PPM grain hardness or less. If poor water conditions
exist on the site, softened water is recommended, or
acceptable water should be brought in. Bacteria or algae
growth in the water is a possibility, especially bacteria or
algae that thrive at the particular temperatures produced in
the heating system. This growth can cause buildup on
hydronic side heat exchanger surfaces, reducing the
efficiency of the system or causing the heat pump to run
at higher head pressures and possibly lock out. Adding a
gallon of bleach or boiler system conditioner can reduce
the possibility of growth and clean up visual flow meters
and other components in the system.
Chart 1 -Propylene Glycol/Water Solution Freeze Point
D. Expansion Tanks
Expansion tanks must be used in the hydronic side of the
water-to-water system to absorb the change in pressure of
the closed system due to the change in temperature when
heat is supplied to the system. Diaphragm-type expansion
tanks should be used. The diaphragm in these tanks is
filled with pressurized air which expands or contracts to
maintain a constant overall system pressure as the fluid in
the system expands with increasing temperature. Use
EPDM diaphragm tanks because they are compatible with
glycol-based antifreeze fluids (butyl rubber diaphragms
will slowly dissolve with glycol-based antifreezes).
Tanks from 1 to 10 gallons are generally used with heat
pump systems in residential and light commercial
applications. Expansion tanks should be installed in the
system near the suction of the circulator pump whenever
possible. This maintains positive pressure at the
circulator pump and reduces the highest working pressure
of the system. A pressure gauge near the inlet of the
expansion tank gives a good indication of how the system
is operating.
Pressure relief valves are required on all hydronic
applications. A 30 psi relief is adequate if the system is
operated at 12 to 15 psi pressure. If a hot water heater is
used for a storage tank, the 150 psi pressure relief may be
acceptable (check local codes).
E. Application Diagrams
Figures 3 through 6 show the components of a hydronic
heat pump system discussed above used in some common
applications. These figures by no means represent all the
possible hydronic heat pump applications, but they do
show some important principals that can be applied to any
system.