The Science of AC Evacuation and On-Site Pull Down

Key Takeaways

• Modern evacuation requires specialized tools: Large-diameter hoses, core removal tools, and dedicated vacuum setups dramatically reduce evacuation time compared to traditional manifold gauge methods
• POE oil changes everything: Hygroscopic POE oil absorbs moisture readily, requiring deeper vacuum levels and longer evacuation times than systems with mineral oil
• Cold weather creates unique challenges: Moisture can freeze in systems during winter evacuations, requiring heaters and extended pull-down times to achieve proper micron levels
• Proper setup eliminates restrictions: Removing Schrader cores and using 3/8″ to 3/4″ vacuum-rated hoses instead of 1/4″ charging hoses can cut evacuation time by hours

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The Science of AC Evacuation and On-Site Pull Down

The science of how we pull an evacuation on air conditioning and refrigeration systems has changed dramatically over the years.

Back in the day, technicians would purge refrigerant through the system to remove air, moisture, and other contaminants. Today’s systems demand much more precision.

The introduction of POE oil has definitely changed the dynamic of evacuation procedures. POE is hygroscopic, meaning it absorbs moisture from the air like a sponge. This characteristic makes proper evacuation more critical than ever, especially when working with systems that have been opened to atmosphere.

As it pertains to this article, I’d like to focus on a particular job that was completed on a compressor trio (3 compressors piped in parallel to make up one circuit) on a McQuay air-cooled chiller. This real-world example demonstrates both the challenges and solutions for complex evacuation scenarios.

The Job: Compressor Replacement on a Mixed-Oil System

The system had a failed compressor that needed immediate replacement.

The compressor was replaced on this R22 system, which contained a mixture of mineral oil and POE oil. The POE oil was introduced from a previous compressor change, as both the previous replacement and the current new compressor were shipped with POE oil from the factory.

After the change, a pressure test was performed with nitrogen. The test held up over a 24-hour period, confirming system integrity before proceeding with evacuation.

At this point, an evacuation must be performed to remove air, moisture, and other system contaminants. Understanding how refrigerant works in the system helps explain why proper evacuation is so critical for system performance.

Many technicians use a manifold gauge to pull a vacuum on systems, but this traditional approach has significant drawbacks. Manifold gauges create more potential leak points and restrictions, while the 1/4″ charging hoses act as bottlenecks that can extend evacuation time by hours or even days on larger systems.

The Modern Evacuation Setup: Eliminating Restrictions

We can eliminate these restrictions from our setups by employing large-diameter hoses, removing Schrader cores, and eliminating manifold gauges entirely.

Many technicians I speak to get confused about how this setup works. “How do I put the cores back in?” is often the first question.

Core Removal Tools: The Game Changer

Using a core removal tool allows techs to remove cores and re-install them when needed without breaking the vacuum.

The core removal tool is a complete assembly that includes:

• A core remover stem
• A ball valve for system isolation
• Sometimes an auxiliary tee for additional connections

The ball valve allows you to isolate the system from the pump. This has many advantages, including:

• Performing decay or rise tests without disconnecting equipment
• Removing or installing cores under pressure or vacuum
• Protecting your vacuum pump from system contamination

Check Out The Video Of A Schrader Core Removal Tool In Action

Vacuum Hose Selection and Setup

A surefire way to eliminate a manifold is to run vacuum-rated hoses directly from the system to the pump.

Many vacuum pumps are manufactured with several ports built into them, usually in different sizes as well. This allows for multiple connection points and maximum flow rates.

Vacuum-rated hoses range in size, but the most common are:

• 3/8″ diameter (good baseline)
• 1/2″ diameter (better flow rate)
• 3/4″ TruBlu hoses by Accutools (maximum flow rate)

You will need to use a micron gauge placed somewhere on the system side, not on the pump side, to monitor evacuation performance. It’s advised to keep the gauge upright to avoid any system contaminants from entering the sensor.

Check Out The Video Of The TruBlu Hose Racing A 1/4″ Hose To 500 Microns

Step-by-Step Evacuation Procedure

After the pressure test is complete and you’ve confirmed system integrity:

✅ Note: It is vital to ensure your vacuum pump oil is fresh and free of contaminants. A simple oil change can remedy this and dramatically improve pump performance.

Also, it’s advised to periodically perform an ultimate vacuum test on your pump prior to evacuation. This is done by placing your micron gauge directly onto the pump itself. Turn on the pump and ensure the micron reading reaches the nameplate ultimate vacuum of the pump.

The Evacuation Steps:

1. Install core removal assemblies to the system service ports and remove the Schrader cores

1. Attach large diameter vacuum hoses to the core removal assemblies and run them to the vacuum pump

1. Place a micron gauge on the system side (not the pump side) to monitor evacuation performance

1. Start the pump and open up the core remover ball valves

1. Monitor the evacuation until reaching 500 microns or below (some industry experts now recommend even lower levels for POE oil systems)

1. Perform a rise test by closing off the ball valves on the core tools and monitoring the micron gauge:

• A fast, sharp rise may indicate a leak in the system
• A slow, gradual rise that flattens may indicate further evacuation is needed as contaminants still exist

Once the rise test is complete, we must add the refrigerant charge either from a tank or by opening up system service valves.

As the refrigerant is added to slight positive pressure, it’s advised to remove the micron gauge, re-install Schrader cores, and take down the evacuation setup. Then add the remaining charge to the system following proper refrigerant leak checking procedures.

Job Site Evacuation: Real-World Challenges

The steps above are generalized and don’t always reflect job site conditions. Let’s examine the compressor trio job from above to understand real-world challenges.

The same tactics were employed as previously stated, but after a 20-hour evacuation, the system was just under 1000 microns. This was far from acceptable.

Keep in mind that the pump ran overnight with temperatures below freezing. An evacuation can struggle in colder temperatures as moisture can freeze within a system. Small ice droplets can form and significantly slow down the process.

Sublimation (the transition from solid directly to gas) will take place under these conditions but can dramatically increase evacuation times, especially in colder ambient conditions.

Troubleshooting the Slow Evacuation

To address the slow pull-down, I implemented several strategies:

1. Performed a nitrogen sweep – purging the system with nitrogen to help carry out moisture
2. Changed the vacuum pump oil – fresh oil has better moisture-carrying capacity
3. Activated system heaters – turned on both the receiver and evaporator bundle heaters

The heaters are typically used in extreme cold temperatures to keep system pressures up during operation. In this case, they served a different purpose: raising internal system temperatures to help vaporize trapped moisture.

The combination of these strategies worked:

• The heaters raised system internal temperatures above freezing
• The new oil grabbed more contaminants effectively
• The nitrogen potentially melted formed ice droplets as it passed by, allowing them to vaporize more easily during the next evacuation pull down

The pump ran for another 48 hours, and on arrival at the site, the system had achieved 335 microns – well below the target of 500 microns.

When tackling complex commercial jobs like this, proper scheduling becomes critical. Consider implementing strategies from The September Sweet Spot for commercial HVAC maintenance to avoid rushing through critical procedures during peak season.

Check Out The Video From The Job Above, Which Explains In Detail The Outlined Steps

Best Practices and Professional Tips

Some systems require significantly more attention, especially larger machines that have been in service for years. Smaller, newer systems typically won’t have issues pulling down as long as the system is properly sealed.

Key Success Factors:

• Large-diameter hoses – Reduce restriction and evacuation time
• Core removal tools – Eliminate the biggest restriction in the system
• Proper micron gauge placement – Always on the system side for accurate readings
• Fresh vacuum pump oil – Change before critical evacuations
• Temperature considerations – Use heaters in cold weather to assist moisture removal

Triple sweeps with nitrogen may help in some cases, particularly with contaminated systems, but I find it’s not usually needed for standard evacuations. The key is understanding when to employ additional techniques based on system conditions.

Remember to remove your micron gauge at a slight positive pressure while charging. This ensures the gauge doesn’t suffer any damage from oil or refrigerant contamination.

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Summary

Proper evacuation has evolved from simple purging to a precise science requiring specialized tools and techniques. The introduction of POE oil and modern environmental regulations demand that we achieve deeper vacuum levels than ever before.

Implementation of large-diameter hoses, removing restrictions, and eliminating potential leak points will ensure your evacuation is as efficient as possible. Understanding when and how to employ additional techniques like nitrogen sweeps, oil changes, and system heating can mean the difference between a successful evacuation and a callback.

For more advanced troubleshooting techniques and system-specific guidance, check out my YouTube channel for tips, tricks, and troubleshooting videos, and listen to The HVAC Know It All podcast here or on your favorite podcast app.