How long do you evacuate HVAC systems post repair. And Why?

Jim Curriculum Developer Frederick, Maryland Posted   Latest  

I have been wanting to ask this question for a while in hopes of having a discussion based on technical accuracy. So how long do you evacuate and HVAC system prior to charging? Is it different if you have opened the system as opposed to a recover/charge cycle? Please remember to include your "why" in responses. I have found there are many reasons for techniques to develop and be used. If the answer is "that is what I was told years ago" or "I live in a swamp" or whatever the reason I would like to know.

Thanks!

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Bill Owner/Technician
Jackson, Michigan
Bill Default
 

with my new 12324yf machine the machine is in 100% control of Evac time.... with my 134A machine usually until i can reach -15 inhg and i verify removed weight is stable/ not increasing at all

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Jim Curriculum Developer
Frederick, Maryland
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Just to clarify, are you speaking of Recovery Time or Evacuation Time where the machine is in complete control? I am asking about pulling a vacuum on the system post recovery/repair.

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Robert Owner
Kennett Square, Pennsylvania
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Jim, The RRR machine determines recovery time as needed by watching system pressures for five minutes. If the pressures rise, the pump kicks back on and will recover again until pressure rise doesn't occur for two minutes This is per the SAE spec. My original response pertained to vacuum time.

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Jim Curriculum Developer
Frederick, Maryland
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I understood your response. Thank you. My question was a reply to Bill Kenel above. He seems to imply that his machine is 100% in control of EVACUATION time as opposed to RECOVERY time.... Every unit I have driven has at least some control over vacuum times. Did the system send a "new comment" notice to you or did it send a "reply to you" notice on the thread. I am asking because we should…

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Steve Technical Support Specialist
Gainesville, Florida
Steve Default
 

The concept of evacuation before charging is based on the fact that the boiling point of water changes with atmospheric pressure. Water will boil at close to 30 inches of water vacuum at room temp. So the idea is to get TT he vacuum below the boiling point and hold it there long enough to boil all the water out of the system. As you might guess no one knows how much water there is nor how much…

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Allan Instructor
Winnipeg, Manitoba
Allan Default
 

The purpose of evacuating the system is to remove air and perhaps more importantly, to lower the boiling point of any moisture trapped in the system so it can be boiled off and removed with the air. The length of time required to achieve this will vary depending on factors such as, how long the system was open, ambient temperature, relative humidity, size of the system etc, etc. Most…

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Eric Owner/Technician
Edgerton, Wisconsin
Eric Default
 

I will evacuate a freshly opened non leaking system (opened to replace an engine for example) for 30 minutes using a micron gauge to verify a deep enough vacuum was reached. A freshly repaired system gets evacuated for an hour minimum with a micron gauge to verify. With the vacuum pump off I monitor the micron gauge to see if it rises too much and if it does then I will evacuate for 15 minutes…

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Robert Owner
Kennett Square, Pennsylvania
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Hello Jim, I base vacuum time on the length of the hoses. When teaching AC, I advocate 30 minutes for the typical system where the condensor is in front of th radiator, the compressor is belt-driven by the engine, and the evaporatpr is under the dash. If, for example, the condenser is on the side of the vehicle (school bus), the vehicle has rear air (Dodge Caravan, Chev Sunurban), or perhaps a…

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Chris Technical Support Specialist
Commack, New York
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Typically 20-30 minutes on a evacuated (came in with a charge maybe a quick repair) as a base procedure. If the system has been empty or open for prolonged periods (multi day engine repairs) I was taught, in Ford factory training, to evacuate for 45 minutes. Later years dealing with Carrier bus AC systems I was told to perform evacuation with Micron AC gauges and test for loss of vacuum. If…

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Bill Technician
Rosetown, Saskatchewan
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I am not typically rushed and have noticed it usually takes about 1 hour for my micron gauge to stabilize or reach its lowest point with my Model 93560 2 stage pump through a gauge set (fieldpiece sman360). Some take longer. Cannot remember one that has taken less. I am not high volume AC and the season hasn't started here yet. The micron gauge seems to be more of an art than a science and its…

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Andrew Technician
Commack, New York
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I would structure work so that the evac could run more than an hour, by working another car in the meantime or going to lunch. I was strongly encouraged to give it one hour in school, I think my teacher wanted 30 minutes minimum, one hour preferably. I forget what the warranty policy is at the dealer regarding evac time and clock time but there was one. As for the actual theory behind it and…

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Robert Owner
Kennett Square, Pennsylvania
Robert Default
 

+1 on HECAT!

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Rudy Technician
Montebello, California
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I was taught that 45 mins is the minimum amount needed to boil off moisture and remove air. I tend to go for an hour if there's time. (Usually make time)

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Glen Owner/Technician
Arthur, Illinois
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It depends on how long it takes the micron gauge to reach what might be called bench marks 2000, 1500, 1000, 800, 500 or about 3 tone changes on the vacuum pump :) Then the goal is under 200

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Bill Technician
Rosetown, Saskatchewan
Bill Default
 

Glen, what type of systems do you work on that are pulling down to 200 during evacuation and what type of rig are you using to pull that. I have been toying with the idea of making up a few things to aid evac time, but wasn't expecting to hit below 200 for automotive.

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Glen Owner/Technician
Arthur, Illinois
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Bill, For this discussion MVAC. 7 CFM JB DV-200N vac pump with big vac hose. Other things that I feel are important or that I have managed to learn from people much smarter then me are. Seal the system in the best possible way when replacing components and the use of N2 if needed. The vac pump oil and how often it's replaced The oil that is added to the system and how fresh it is I…

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