Unit 2 — Introduction to Refrigerants & Handling Practices
Section 2 — Leak Testing, Evacuation and Charging
2.5 System Charge
Procedures for charging refrigerant and oil into a system, and the methods
used to verify that the correct charge has been achieved.
Jump to a section
2.5.1 — Charging with Refrigerant
Charging requires adding the correct type and quantity of refrigerant to a system
that has passed pressure and leak testing and reached the required deep vacuum.
The method used — liquid or vapour — depends on the refrigerant type, system size,
and manufacturer requirements. Using the wrong method or refrigerant can cause
compressor damage, fractionation of blends, or unsafe operating pressures.
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Liquid Charging
Refrigerant introduced as liquid — common for blended refrigerants and
larger systems. Cylinder must be inverted or positioned to deliver liquid.
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R-744 (CO₂) Charging
Requires specialized equipment and staged charging due to high operating
pressures and transcritical cycle characteristics.
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Vapour Charging
Refrigerant drawn as vapour into the low-pressure side. Used for fine-tuning
charge and for smaller systems — always with the system running.
Liquid charging is the standard method for refrigerant blends and is used
to quickly charge larger systems where vapour charging would take too long.
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Blended refrigerants must be charged as liquid.
Cylinders containing blends (e.g., R-410A, R-407C) must be positioned
or inverted to deliver liquid — drawing vapour from a blend cylinder
causes fractionation, changing the refrigerant composition in both the
cylinder and the system.
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Charge into the liquid line or receiver with the system off
for initial charging of dry systems. Open the liquid service valve slowly
and allow the system pressure to equalize.
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Liquid charging into the suction line is sometimes done
under controlled conditions with the system running, using a metering
device (flow restrictor or hand valve) to prevent liquid slugging the
compressor. This requires care and close monitoring.
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Use an accurate refrigerant scale to track the quantity of refrigerant
added by weight, especially for systems with a specified critical charge.
CO₂ systems operate at significantly higher pressures than conventional
refrigerants and follow transcritical cycle characteristics that require
specialized charging procedures.
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Charge in stages, ensuring the system pressure remains
within safe limits at all times. Never introduce refrigerant faster than
the system can safely absorb.
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Specialized equipment is mandatory — valves, gauges,
hoses, fittings, and over-pressure protection devices must all be rated
and designed specifically for CO₂ (R-744) service and the applicable
high-pressure range.
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Verify all connections and equipment ratings before beginning. CO₂
systems have design pressures that can exceed 130 bar (1,900 psi) on
the high side — standard manifold sets are not suitable.
Vapour charging is used to fine-tune a system's charge after an initial
liquid charge, or as the primary charging method for small systems using
single-component refrigerants.
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Refrigerant is drawn as vapour from the cylinder into the low-pressure
(suction) side of the running system. The pressure differential
between the cylinder and the low side draws refrigerant in.
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The system must be running during vapour charging — operating pressures
and temperatures are monitored continuously, and charging continues until
system performance parameters and manufacturer charge specifications are met.
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Do not use vapour charging for refrigerant blends as the
primary charging method — vapour drawn from a blend cylinder may not
match the correct refrigerant composition.
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Charging rate is naturally limited by the cylinder's vapour pressure and
the system's suction pressure — never heat a refrigerant cylinder to
accelerate vapour charging.
2.5.2 — Charging with Oil
Adding or replacing refrigerant oil in a system is required after major repairs,
component replacements, or when oil analysis indicates degradation. Oil is
introduced through dedicated oil ports or service connections and must be
compatible with the refrigerant and system materials.
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Vacuum charging — with the compressor or system under
vacuum, oil can be drawn into the system by opening an oil service valve
connected to an open container of oil at atmospheric pressure. The pressure
differential pulls the oil in. This method is simple but requires the
system to be in a deep vacuum before oil introduction.
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Oil injection pumps — manual or mechanical oil pumps allow
accurate dosing of oil into a pressurized system. Used where vacuum charging
is not practical, such as when adding oil to a running system through a
dedicated oil port or when precise quantities are required.
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Always use the correct oil type specified by the compressor and system
manufacturer — mixing incompatible oils (e.g., mineral oil with POE oil in
an HFC system) can cause sludge formation, foaming, and compressor damage.
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Keep oil containers sealed until immediately before use. Refrigerant oils —
especially polyolester (POE) oils — are highly hygroscopic and will rapidly
absorb moisture from the atmosphere, contaminating the system.
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Use the Correct Oil — No Substitutions
Each refrigerant and compressor type has a specified oil. POE (polyolester)
oils are required for most HFC and HFO refrigerants; mineral oil is used with
R-22 and some older systems. Using the wrong oil is not a minor issue — it
can void warranties, cause compressor failure, and create contaminants that
block the expansion device and damage system components.
2.5.3 — Verifying the System Charge
Once charging is complete, technicians must verify that the system has the correct
refrigerant charge. No single method is definitive on its own — a combination of
manufacturer data, instrument readings, and visual indicators is used to confirm
that the charge is correct before handing the system over to the customer.
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P/T Charts
Pressure/temperature charts relate refrigerant saturation pressure to
temperature — used to verify that measured pressures and temperatures are
consistent with normal system operation.
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Sight Glass
Installed in the liquid line — indicates liquid refrigerant flow and the
presence of bubbles that may suggest an undercharge or flashing caused by
other system issues.
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Subcooling
The difference between saturated condensing temperature and actual liquid
line temperature. Correct subcooling confirms the condenser is fully
condensing refrigerant and indicates charge level.
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Superheat
The difference between saturated evaporating temperature and actual suction
line temperature. Correct superheat protects the compressor from liquid slugging
while confirming the evaporator is fully utilized.
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Charge charts are provided by the equipment manufacturer
and relate ambient temperature and operating conditions to expected system
parameters such as suction pressure, discharge pressure, and subcooling.
They allow a technician to determine whether the system is operating within
its designed charge range for the current conditions.
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Critical charge systems — common in factory-sealed
equipment such as small split systems and packaged units — require a
precise refrigerant charge for correct operation. The charge is measured
and added by weight using a refrigerant scale. Adding even a small amount
above or below the specified charge can significantly affect performance
and efficiency.
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Always refer to the manufacturer's documentation for the specified charge
weight, acceptable operating parameters, and the correct verification
method for the specific equipment.
Subcooling
- Measure liquid line temperature at the condenser outlet
- Read high-side pressure and convert to saturation temperature using a P/T chart
- Subcooling = Saturation temp − Liquid line temp
- Typical target: 5–15°C (10–20°F) — refer to manufacturer spec
- Low subcooling may indicate undercharge or non-condensables
- High subcooling may indicate overcharge or a restricted liquid line
Superheat
- Measure suction line temperature at the evaporator outlet
- Read low-side pressure and convert to saturation temperature using a P/T chart
- Superheat = Suction line temp − Saturation temp
- Target varies by system type — TXV systems: 5–8°C; fixed orifice: per manufacturer
- High superheat may indicate undercharge or a restricted metering device
- Low superheat risks liquid refrigerant reaching the compressor
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No Single Method Is Sufficient
By combining manufacturer data (charge charts, specified weights), instrument
readings (subcooling, superheat, pressures), and visual indicators (sight glass),
technicians build a complete picture of system charge status. A sight glass alone
can be misleading — bubbles may indicate an undercharge or simply high subcooling
causing flash. Always cross-reference multiple methods before concluding that
a system is correctly charged.