Bulletin 30-015.003
Copyright © 2017 Unico Inc.
Page 14
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 temperatures (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 condensing unit. Typically, a low ambient control
is necessary when operating the unit at outdoor
temperatures below 70°F (21.1 °C).
These controls come in different configurations such
as the Hoffman Controls Corp. series 800AA head
pressure control. This control modulates the outdoor
blower to maintain 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.
CHARGING A HEAT PUMP SYSTEM
Charging in Cooling Mode.
Charging a heat pump,
by its nature, is more difficult than a cooling-only
refrigerant system. Quite often the ideal charge for
cooling is different than the ideal charge for heating,
making the system much more sensitive to the amount
of charge. In some cases, the compressor will trip on
high head pressure during the heating mode because it
is overcharged if the system was charged during
cooling. Likewise, the system may cycle on the anti-
frost control because of a low refrigerant charge if the
system was charged during heating.
To compensate for this charge difference some
outdoor unit manufacturers have a charge compensator
device that stores charge while in heating mode.
Unfortunately there are no add-on devices to
accomplish the same thing and only a few (usually the
most expensive) model lines will have one. For this
reason, it is often necessary to compromise the charge.
Although the unit can be charged in the heating mode,
it is best to charge the unit during the cooling mode as
described earlier. Then recheck the charge in the
heating mode to be sure the system is not over charged.
Charging in Heating Mode.
If the system is started
up on heating where the return air temperature is
significantly lower than the normal operating range of
65 to 75 °F (18.2 to 23.8 C), the suction pressure can
be very low. Operate the system to bring up the return
air temperature, using auxiliary heat if necessary,
before checking system charge.
In heating mode, the Unico System will have a
slightly higher discharge (LIQ.) pressure than a
conventional system, usually about 40 to 50 psig
higher. It is this higher pressure that produces a
warmer air temperature; preventing “cold blow”,
where the house is being heated with an air stream that
feels cold.
Some outdoor heat pump units include a manual high-
pressure switch. It is important that the system be
operated at pressures below the trip pressure of the
high-pressure switch to avoid nuisance shut downs. If
this occurs, use a high head kit as described below
To compensate for this charge difference some
outdoor unit manufacturers have a charge compensator
device that stores charge while in heating mode.
Unfortunately there are no add-on devices to
accomplish the same thing and only a few (usually the
most expensive) model lines will have one. For this
reason, it is often necessary to compromise the charge.
Although the unit can be charged in the heating mode,
it is best to charge the unit during the cooling mode as
described earlier. Then recheck the charge in the
heating mode to be sure the system is not over charged.
