How to Recover a Large Charge by Push-Pull

Why this matters

Vapor recovery through a recovery machine moves a residential split's few pounds in reasonable time. On a rooftop unit or a large split carrying twenty-plus pounds, straight vapor recovery can take most of an afternoon, because the machine has to boil every pound of liquid in the system into vapor before it can pump it. Push-pull moves the liquid as liquid: the machine draws vapor off the top of the recovery cylinder, pushes it into the system's vapor side, and the pressure difference shoves the system's liquid straight into the cylinder. On a large charge it routinely moves most of the refrigerant in a fraction of the time. It also fills a cylinder faster than any other method, which is why the 80 percent limit is the part of this job you watch, not the part you check at the end. The general recovery procedure and the machine's own operation are in the shelf's references (See related: Refrigerant Recovery Procedures Reference); the recovery levels and the record rules are owned by the 608 card (See related: EPA Section 608 Rules on a Residential Service Call).

When push-pull is the right method

Push-pull only moves liquid, so it needs a system holding a usable amount of liquid with an accessible liquid-side port: a liquid line service valve, a receiver outlet valve, or a liquid line access port upstream of the metering device. A common rule of thumb is that it pays off once the charge runs past about 15 to 20 lb; below that, the setup time eats the saving and plain vapor recovery is simpler.

It does not work, or works poorly, where the liquid is not where the port is: a system whose liquid has migrated in cold weather to the compressor shell or to a coil on the far side of a metering device from the port, a system so short of charge it holds mostly vapor, or a system where a check valve or a closed solenoid sits between the liquid port and the liquid. A system that has had its compressor burn out gets the same method but a dedicated cylinder for contaminated refrigerant, never the cylinder you reclaim clean product into.

Start state

Hazards, each with its control. Liquid refrigerant released from a hose or fitting freezes skin and eyes on contact: safety glasses and refrigerant-rated gloves for the whole job, and hoses with low-loss fittings so disconnects release grams, not ounces. The cylinder is the stored-energy hazard: an overfilled cylinder that warms has no vapor space, and liquid expansion can raise its pressure past its rating. The 80 percent limit and a working float switch are the control, and a cylinder is never warmed with a flame. In a mechanical room rather than on a roof, refrigerant displaces oxygen in a low, enclosed space; ventilate it and stop work if anyone feels lightheaded. If the refrigerant is an A2L, the machine, cylinder and procedure must all be rated for it (See related: A2L Refrigerant Handling (R-454B and R-32)).

Isolate the unit. The compressor must not start during recovery. Open the unit disconnect, lock and tag it, and prove the unit dead at the contactor with a tester proven on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024), with work practices at 29 CFR 1910.333(b)(2).

What you need:

• A recovery cylinder dedicated to this refrigerant, rated for it (400 psig service pressure for R-410A), inside its hydrostatic retest date stamped on the collar, with its water capacity (WC) and tare weight (TW) stamped.
• A recovery machine with a push-pull mode or valve setup, its float-switch cable connected to the cylinder's 80 percent float switch.
• A refrigerant scale under the cylinder, three hoses, and a sight glass to put in the liquid hose.

Know the cylinder's limit before you connect it. The common field rule caps net refrigerant at 80 percent of the stamped water capacity by weight. Treat it as a ceiling, not a target; where the cylinder maker states a refrigerant-specific maximum, the maker's figure governs. Maximum gross weight is tare weight plus that net limit. Write the maximum gross on a piece of tape on the scale display.

The setup

   SYSTEM, unit locked out
   LIQUID port              VAPOR port
       |                        ^
       | liquid pushed out      | vapor pushed in
       v                        |
  [ sight glass ]        [ MACHINE  OUTLET ]
       |                 [ RECOVERY        ]
       |                 [ MACHINE   INLET ]
       v                        ^
  +----------------------+      |
  | LIQUID port          |      | vapor drawn off
  |          VAPOR port  +------+ top of cylinder
  | RECOVERY CYLINDER    |
  | on scale, float      |
  | switch to machine    |
  +----------------------+

The loop runs in one direction: cylinder vapor into the machine, machine outlet into the system's vapor side, system liquid into the cylinder's liquid port. The machine is pumping vapor the whole time; the liquid moves because the system side is now at a higher pressure than the cylinder.

The steps

1. Weigh the cylinder and confirm room. Read gross weight, subtract tare to get the refrigerant already in it, and subtract that from the net limit. The room left must exceed the charge you expect to move, or you need a second cylinder staged before you start. Skip it and you find out at 80 percent with liquid still flowing.

2. Connect the loop and purge the hoses. Make the three connections in the diagram, sight glass in the liquid hose. Purge the hose air with a small burst of refrigerant from the system side as the machine maker directs, so you are not pumping air into a cylinder of clean product.

3. Start push-pull and watch two things. Open the valves in the order the machine maker gives and start the machine. Watch the sight glass: solid liquid moving means the method is working. Watch the scale: the gross weight climbs fast. Stop the machine when the gross reaches your taped maximum, whether or not liquid is still flowing.

4. Stop when liquid stops. When the sight glass goes from solid liquid to vapor and bubbles, and stays that way, the liquid is out. Push-pull will not move vapor efficiently; continuing just circulates it.

5. Switch to vapor recovery. Shut the machine down, close the valves, and re-plumb as a standard vapor recovery: system liquid and vapor ports together into the machine inlet, machine outlet into the cylinder. Recover to the level 40 CFR 82.156 requires for this appliance type and charge, which for an R-410A appliance under 200 lb with recovery equipment made after November 15, 1993 is a 10 in Hg vacuum. HFC recovery is also addressed under 40 CFR Part 84, Subpart C; follow the rule in force and check the current text.

6. Hold and check for rise. Isolate the system at the required vacuum and wait several minutes. A pressure that climbs back means refrigerant is still boiling out of oil or a pocket of liquid; resume recovery and repeat until the vacuum holds.

7. Clear the machine and close up. Run the machine's self-clear or purge mode into the cylinder so its compressor and condenser are not left full of refrigerant. Close the cylinder valves, weigh it, and label it with the refrigerant, the net weight, and whether it is clean or contaminated.

8. Tag the unit. The system is now empty and open to repair; it stays locked out, and the tag says recovered, with the date and your name. The restore of this unit is a repair, an evacuation and a weighed-in charge, not a step of this procedure.

Worked example: a 7.5-ton rooftop unit

A 7.5-ton rooftop unit on R-410A, nameplate charge 22 lb, compressor replacement scheduled. Recovery cylinder: WC 50 lb, TW 27.4 lb, dedicated R-410A, retest date current. Machine built after 1993.

Step 1. Net limit is 0.8 x 50 = 40 lb, so maximum gross is 27.4 + 40 = 67.4 lb, written on tape. Gross on the scale reads 35.4 lb, so the cylinder already holds 35.4 - 27.4 = 8.0 lb of R-410A. Room left is 40 - 8.0 = 32.0 lb, more than the 22 lb nameplate. One cylinder will do.

Step 3. Liquid ran solid in the sight glass almost immediately. Eleven minutes later the glass went to bubbles and then to clear vapor. Gross weight: 52.1 lb. Liquid moved: 52.1 - 35.4 = 16.7 lb, about three quarters of the nameplate charge (16.7 / 22 = 0.76).

Step 5. Re-plumbed for vapor recovery, the system pulled down to 10 in Hg in about 25 minutes.

Step 6. Isolated at 10 in Hg; in three minutes the gauge rose toward 4 in Hg. That is refrigerant coming out of the compressor oil, which is what a cold crankcase on a roof does. Recovery resumed, pulled to 10 in Hg again, isolated: held steady for five minutes.

Step 7. Machine cleared to the cylinder. Final gross: 56.9 lb. Recovered this job: 56.9 - 35.4 = 21.5 lb. Net in cylinder: 8.0 + 21.5 = 29.5 lb, under the 40 lb limit. The 0.5 lb short of nameplate is within what a charge adjustment years ago and a little refrigerant still dissolved in the compressor oil can account for; a shortfall of several pounds would instead be evidence the unit had been leaking, and that changes the conversation about the repair.

On the record: 21.5 lb R-410A recovered, the cylinder it went into, the date and the location, kept the way the 608 card describes for this charge band.

How to verify the recovery was right

• The cylinder's final gross is at or below the maximum gross written before you started, and the float switch was connected the whole time.
• The system reached the required level for its appliance type and charge and held there with the system isolated.
• The recovered weight is recorded against the nameplate charge, with any large difference explained.
• The machine was cleared, the cylinder labeled, and the unit left locked out and tagged as recovered.

References

• 40 CFR Part 82, Subpart F, section 82.156 (current eCFR text): required recovery levels
• 40 CFR Part 84, Subpart C (current eCFR text): HFC management, under the rule in force
• NFPA 70E, Standard for Electrical Safety in the Workplace, current edition, Article 120; 29 CFR 1910.333
• Recovery machine and recovery cylinder manufacturer instructions: push-pull setup, float switch, maximum fill
• See related: Refrigerant Recovery Procedures Reference; EPA Section 608 Rules on a Residential Service Call
• See related: A2L Refrigerant Handling (R-454B and R-32)