In push-pull recovery the machine draws vapor off the recovery cylinder and pushes it into the appliance while liquid flows out of the appliance's liquid line into that cylinder. What condition has to be met before this setup will actually transfer refrigerant?
Correct Answer
D) The appliance must hold enough liquid refrigerant for the machine to build and hold a pressure difference between the appliance and the cylinder
Push-pull is a siphon driven by a pressure difference. The machine lowers pressure in the cylinder and raises it in the appliance, and that difference pushes a column of liquid through the transfer hose. It only pays off when there is a real body of liquid to move, which is why it is reserved for large charges. On a small charge the liquid runs out almost immediately, the pressure difference collapses, and the setup takes longer than a straight hookup. Once the liquid stops moving, the technician switches to normal vapor recovery to finish the job.
Why This Is the Correct Answer
The method works only while a pressure difference exists across a continuous column of liquid. A large liquid charge lets the machine establish and hold that difference long enough to move most of the refrigerant quickly, which is precisely why push-pull is a large-system technique.
Why the Other Options Are Wrong
Option A: The recovery cylinder must be warmed above the appliance's saturation temperature so the higher cylinder pressure drives the liquid through the transfer hose
Push-pull works by lowering cylinder pressure, not by raising it. Heating a cylinder to build pressure reverses the direction of the transfer and is a serious overpressure hazard on its own.
Option B: The appliance's own compressor must run for the whole transfer, since it is what keeps the liquid column moving to the cylinder
The recovery machine supplies the entire pressure difference. The appliance is normally off and locked out during a push-pull transfer, and a dead compressor does not stop the method from working.
Option C: The hookup must be made through the suction service valve, since liquid will not flow out through a liquid line service port
Liquid has to leave through a liquid line or receiver port, which is the whole point of the hookup. Pulling from the suction valve gives vapor, and there is no liquid column left to push.
Memory Technique
Push vapor in, pull liquid out, and only bother when there is a lake of liquid to move.
Reference Hint
Push-pull is a liquid recovery technique for large charges; finish with vapor recovery and keep the cylinder within its safe fill level.
More epa-608-type-2 Questions
A walk-in cooler condensing unit holds a full charge of twenty-five pounds of R-404A, which is below the fifty pound threshold that brings an appliance under the leak repair provisions. What still applies to this unit?
An office building has thirty water source heat pumps on a common water loop, and each one left the factory assembled, hermetically sealed and charged with four pounds of R-410A. How is one of those units classified?
A data sheet for an unfamiliar refrigerant lists a boiling point of about fifty-five degrees Fahrenheit at atmospheric pressure. What does that tell a technician holding Core and Type II certification?
A technician holds Core, Type I and Type III certification and is sent to recover the charge from a twenty ton rooftop unit charged with R-410A. What does that combination of sections allow on this job?
A fleet garage technician holds only the vehicle air conditioning certification issued under Section 609 and is asked to recover the charge from the shop's own rooftop air conditioner. What does that credential allow?
A split system's outdoor unit, indoor coil and the line set the installer ran and brazed between them are being considered. For the purposes of the refrigerant rules, what makes up the appliance here?
On a new installation the line set is brazed to a factory charged outdoor unit and an indoor coil shipped under a nitrogen holding charge, with the outdoor unit's service valves still closed. What does the recovery requirement call for before that brazing?
A rooftop unit's data plate has weathered until the charge figure cannot be read, and the technician needs the full charge to place the appliance under the evacuation and leak provisions. Where should that figure come from?
A computer room air conditioning unit holds forty pounds of R-407C and connects to a remote air cooled condenser through refrigerant piping run and brazed by the installer. How is that unit classified?
A shop's office manager, who holds no technician certification, is sent to a supply house to pick up a new recovery machine for the commercial crew. What does the sales restriction require at that counter?
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