Bulletin 30-20 — Page 24
© Copyright 2010, Unico, Inc.
If it is high, the system is overcharged and some refrige-
rant must be removed and collected in an empty refrige-
rant container. DO NOT RELEASE REFRIGERANT
INTO THE ATMOSPHERE.
In some cases, such as in a hot attic, the liquid line will
pick up heat and lose its sub-cooling. This will be appar-
ent if the sub-cooling at the evaporator is low. In these
cases, the liquid line should be insulated or strapped to
the suction line and both insulated. The same problem
can occur for long refrigerant lines; in this case, increase
the size of the liquid line to reduce the pressure drop.
CAUTION
TO MAINTAIN PROPER HEAT PUMP
OPERATION, DO NOT STRAP THE
LIQUID AND SUCTION LINES
TOGETHER FOR HEAT PUMP
SYSTEMS.
Superheat Method Do not charge the system based on
superheat. Superheat measurements should only be used
to verify that the expansion valve is working properly. If
is more than expected please refer to the Technote on
troubleshooting expansion valves.
The superheat should be between 8 to 12°F (4 to 7°C) at
the indoor coil. In some cases, particularly for the larger
capacity match-ups (i.e. 3 ton and 5 ton), a superheat of
15 to 18°F (8 to 10°C) is satisfactory. It is not uncom-
mon to measure a superheat above 20 to 25°F (11 to
14°C) at the condensing unit.
Be aware that the superheat value is also dependent on
the outdoor air temperature. At lower air temperatures
the superheat will be higher than at higher air tempera-
tures. If the condenser ambient temperature is between
75 and 85°F (24 to 29°C), superheat should be approx-
imately 10 to 12°F (5 to 7°C). If the outdoor temperature
is between 85 and 105°F (29 to 40°C), superheat should
be approximately 8 to 10°F (4 to 5°C).
To measure the superheat, use the following:
1. Measure and record the suction pressure at the eva-
porator outlet using an accurate refrigerant gauge. If
this is not possible, measure the pressure at the ser-
vice port on the suction valve fitting at the condens-
ing unit and add the estimated pressure loss in the
suction line between the condensing unit and evapo-
rator. Record the corresponding saturation tempera-
ture for this pressure (see Table 8).
2. Measure the suction line temperature at the evapora-
tor outlet using an accurate metal or glass thermome-
ter, or thermocouple. Insert the thermometer under
the insulation on the suction line and tape firmly
against the surface of the suction tube.
3. Determine the superheat with the following equation:
Suction Line Temperature
— Saturated Temperature
= Superheat
Charging by Gauge Pressures It is not possible to
charge the system by gauge pressures. Gauge pressure
should only be used to verify the system is working
properly
The Unico System will show a lower suction pressure
during the cooling mode than a conventional system.
Generally, it will be 10 to 15 psi (70 to 100 kPa) less.
For example, a normal suction pressure for the Unico
System
will be about 65 psig (450 kPa) with an 85 to
95°F (29 to 35°C) outdoor temperature. Expect lower
pressures when the outdoor temperatures are lower.
The head pressures should be similar to a conventional
system when in the cooling mode.
Using a Low Ambient Control Kit
Since the Unico System operates at colder coil tempera-
tures (in cooling mode), an anti-frost switch is installed
on the coil to prevent coil freeze-up. In certain instances,
such as when the outdoor ambient temperature is low,
the condensing unit will cycle on the anti-frost switch.
This may reduce the cooling capacity at a time when the
cooling load is still fairly high. To provide better control
and comfort, install a low ambient control on the con-
densing unit. Typically, a low ambient control is neces-
sary when operating the unit at outdoor temperatures
below 80°F (26.6 °C), especially for a 5-ton nominal
capacity system.
These controls come in different configurations such as
the Hoffman Controls Corp. series 800AA-head pressure
control. This control modulates the outdoor fan to main-
tain a minimum liquid line temperature. Other controls
may cycle the fan on/off. In either case check with the
condensing unit manufacturer to determine what controls
are compatible with the condensing unit.
R-410A, Puron
©
Refrigerant
Puron is a registered trade name for refrigerant R-410A
by Suva, a Dupont Company, which is an alternate to
refrigerant R-22.
In 1987, scientists and government officials met in Mon-
treal in response to the growing pressure to preserve the
earth's ozone layer. The outcome of the meeting was the
Montreal Protocol - an internationally binding action
plan to eliminate ozone-harming chemicals. Chlorofluo-
rocarbons (CFCs) - such as R-12 - were targeted first
since they caused the most damage to the environment.
A cap was placed on the production of CFC's and in
1996, by law, all manufacturing was required to cease.
The Montreal Protocol's next phase-out targets are hydro
chlorofluorocarbons (HCFCs), including R-22, the pri-
mary refrigerant in residential heating & air conditioning
products.
The 1990 Clean Air Act in conjunction with the Mon-
treal Protocol, established January 1, 2010 as the date
when the U.S. will ban the manufacturing of products
using HCFCs. The same document bans the manufacture
of R-22 in 2020. Thereafter, only recycled R-22 can be
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