How to Pull a Vacuum on an AC System (and What Section 608 Actually Requires)

To pull a vacuum on an AC system, connect a vacuum pump to both the high and low side through large-bore hoses, fit a micron gauge at the system rather than at the pump, and run the pump until the reading falls below 500 microns. Then isolate the pump and watch for the pressure to hold. The 500-micron target is good service practice, not a Section 608 requirement.

That last sentence is where most guides on this topic go wrong, and the distinction is worth getting straight before you touch a hose. Search this subject and you will find automotive A/C pages, a Reddit thread and a handful of tool-brand blogs, several of which tell you EPA requires 500 microns. It does not. There are two separate vacuum operations in refrigeration work, they are measured in different units, and only one of them is set by federal rule.

What are the two different vacuums?

Confusing them is the single most common error among newer technicians, and it shows up on the exam.

Recovery evacuation happens before you open a system. You are removing refrigerant so you can legally break into the appliance, and the level you must reach is set by 40 CFR 82.156, measured in inches of mercury vacuum. Which number applies depends on the appliance's pressure class, its charge size, and when your recovery machine was manufactured.

Service evacuation — what people mean by "pulling a vacuum" — happens after the system is empty and before you charge it. You are boiling off moisture and removing non-condensables, and it is measured in microns of mercury absolute. No part of Section 608 specifies a micron figure for it.

	Recovery evacuation	Service evacuation
When	Before opening the appliance	After repair, before charging
Purpose	Remove refrigerant legally	Remove moisture and air
Units	Inches Hg vacuum	Microns Hg absolute
Typical target	0 to 25 in Hg by class	Below 500 microns
Set by	40 CFR 82.156 (federal rule)	Manufacturer and trade practice
Gauge	Compound gauge, or micron gauge for low-pressure	Micron gauge only

If you need the recovery side in detail, the required evacuation levels under 82.156 are tabulated in full — including the November 15, 1993 cut-off that describes your recovery machine's manufacture date, never the age of the system you are working on.

Why does a vacuum remove moisture at all?

Because lowering the pressure lowers the boiling point. At normal atmospheric pressure — roughly 760,000 microns — water boils at 212°F, so liquid water sitting in a line set stays liquid all day. Drop the pressure to around 20,000 microns and water boils at ordinary room temperature. It turns to vapor and the pump carries it out.

That is why 500 microns is the number the trade settled on. It is comfortably below the point where water flashes to vapor at any temperature you will encounter on a roof or in a crawl space, and it leaves margin for the gauge reading at the pump to differ from the reality inside the evaporator.

Moisture left in a system is not a cosmetic problem. Combined with refrigerant and oil it forms acids that attack windings and degrade lubricant, and it can freeze at the metering device and block flow. Non-condensable air raises head pressure and costs capacity.

What is the actual procedure?

1. Recover the existing refrigerant first to the level 82.156 requires for that appliance, and verify it before you open anything. Venting is prohibited outright under 40 CFR 82.154(a).
2. Make the repair, then replace the liquid line filter-drier. A drier that has been open to atmosphere has already begun loading with moisture.
3. Pressure test with dry nitrogen and find any leak before you evacuate. A system that leaks will never pull down, and you will waste an hour deciding whether the pump is at fault.
4. Connect to both service ports. Evacuating through the low side alone means everything has to migrate past the metering device, which is a restriction. Two-port evacuation is dramatically faster.
5. Use short, large-diameter hoses, and remove the valve cores if the system and your core removal tools allow it. A standard 1/4-inch hose with a Schrader core in the path is the biggest bottleneck in most evacuations.
6. Place the micron gauge at the system, as far from the pump as practical. A gauge at the pump inlet reads the pump, not the appliance, and will show you a number you have not achieved.
7. Run the pump with clean oil until the reading falls below 500 microns.
8. Valve off the pump and watch. This is the decay test, and it is the only part that actually proves anything.

What does the decay test tell you?

Everything. After you isolate the pump, the reading does one of three things.

It holds steady below about 500 microns — the system is dry and tight, and you can charge.

It rises and levels off somewhere under roughly 1,500 microns — there is still moisture boiling out, or refrigerant coming out of the oil. Keep pulling.

It rises steadily and does not stop, climbing past 2,000 microns and continuing — you have a leak, or a loose connection in your own hose set. Pumping longer will not fix it. Find the leak.

A rise that never stabilizes is the signature of a leak because atmosphere has an effectively infinite supply of air to push in. Moisture, by contrast, is a finite quantity that eventually finishes boiling.

Does the refrigerant change any of this?

The procedure is the same for R-410A, R-22 and the A2L refrigerants now taking over residential equipment. What changes is the handling around it. R-454B and the other A2Ls are mildly flammable, so ventilation and ignition-source control matter during the steps where you are opening the system, and you should be using recovery equipment rated for A2L service.

One point that trips people up: evacuation does not care about temperature glide or the composition of a blend. You are removing air and water, not refrigerant. The blend behavior matters at the charging step that follows, where a zeotropic blend has to be charged as a liquid.

What this means on the job

Two numbers, two jobs. Before you open a system, the figure you must hit comes out of Table 1 of 82.156 and it is measured in inches of mercury — for most residential high-pressure work under 200 pounds of charge, that figure is 0 inches. After you repair it and before you charge it, you pull to below 500 microns because the equipment manufacturer and basic physics say so, not because EPA does.

Knowing which is which protects you twice. It keeps you compliant when an inspector asks what level you evacuated to and how you verified it, and it keeps you from telling a customer that a federal rule requires something it does not. The exam tests the first one directly, usually by handing you an appliance class, a charge size and a machine vintage. Working through Type II practice questions is the fastest way to stop second-guessing the table, and the study guide walks the evacuation levels section by section.

Frequently asked questions

Which side do you pull a vacuum on?

Both. Connecting to the high and low side together lets air and water vapor leave through two paths instead of being forced through the metering device, which is a deliberate restriction. Single-side evacuation works on a small system but takes far longer, and on a system with a fixed orifice it can leave the opposite side barely evacuated at all.

How long should you pull a vacuum on an AC system?

There is no fixed time, and treating it as a stopwatch job is the mistake. A tight, dry residential split system with cores removed may reach 500 microns in fifteen minutes; a wet or large system can take hours. Pull until the micron gauge reads below 500 and then holds on the decay test. The gauge decides, not the clock.

Why won't my AC system pull a vacuum?

In order of likelihood: a leak in your hose set or at a Schrader connection, contaminated pump oil, a restriction from small hoses and installed valve cores, or a genuine system leak. Change the oil, isolate the pump with the gauge on it alone to confirm the pump and gauge are sound, then work outward toward the system.

Does EPA require 500 microns?

No. Section 608 sets recovery evacuation levels in 40 CFR 82.156, expressed in inches of mercury vacuum and, for low-pressure appliances, 25 mm Hg absolute. Nothing in the rule specifies a micron target for the dehydration you perform before charging. The 500-micron standard comes from equipment manufacturers and accepted service practice, and it is frequently a warranty condition.

Do you need a micron gauge, or will manifold gauges do?

You need a micron gauge. A compound manifold gauge cannot resolve anything meaningful below roughly 28 inches of mercury, and the last two inches on that dial cover the entire range that matters for dehydration. A manifold reading "29.9 and pegged" tells you nothing about whether you are at 300 microns or 15,000.

Sources

• 40 CFR 82.156 — Required practices
• 40 CFR 82.154 — Prohibitions and required practices
• EPA — Stationary Refrigeration and Air Conditioning
• EPA — Section 608 Technician Certification