The Critcal Importance of “Pulling a Vacuum”

Posted: August 18, 2015 in Uncategorized

Greetings & salutations from the Comfort Crew of Comfort Heating & Cooling!  We are family-owned HVAC contractor based in Fredericksburg, VA.   Our goal with these blogs is to help educate homeowners about their HVAC systems–as with anything, knowledge is power.  Understanding your heating and air system will help you make wise decisions regarding maintenance and expenditures.

In this exciting episode, we are going to discuss an extremely important aspect to HVAC system repair and installation–pulling a vacuum.

To best explain this concept, let’s back up a step and cover some necessary HVAC 101 basics.  Imagine taking a glass of ice water outside on a steamy summer day.  You will notice condensation forming on the glass, correct?   Now imagine how much condensation would form if that ice water was negative 400 degrees Fahrenheit!

Flash Fact:  The Freon in your HVAC system is well BELOW negative 400 degrees Fahrenheit.

Condensation forms every time your unit’s copper lines are opened to the atmosphere.  New system installation obviously involves open copper lines, but numerous repairs also require opening the lines–examples include replacement of an evaporator coil or compressor.

Condensation in a HVAC system is verboten–a dangerous no-no that acts rather like a blood clot in your body.  It is absolutely crucial that this moisture be completely removed.  The method by which moisture (called “non-condensables”) is removed is via a vacuum pump.  As the name implies, these devices pull moisture out of the system.  Vacuum pumps measure their performance in units called “Microns.”  Every manufacturer of HVAC system has a set Micron level that must be achieved to ensure proper performance.

Pulling a vacuum also guarantees there are no leaks in the system.  Even a pinhole leak will prohibit pulling a vacuum to a certain level and holding at that level.  Rule of thumb for technicians that know there stuff–pulling a system down to 350 microns (and holding at that number) ensures that all non-condensables are out, and there are no leaks anywhere in the lines.

The problem for techs is time.  The time it takes to pull a proper vacuum is affected by a number of variables, the first of which is the application–new system installs require far less time to pull a good vacuum as opposed to a part replacement.  Other factors that effect time are: the size of the vacuum pump being used; the size/tonnage of the unit; the length of copper lines connecting indoor and outdoor units, and the amount of moisture in the system–higher humidity days will generate more moisture than dry days.  New system installations (and any repair requiring opening of the copper lines) should NEVER be done on misty or rainy days.  As an example of time–a small vacuum pump pulling down a large 5-ton unit with 75 foot copper lines on a high humidity day….pulling a good vacuum could easily require a full day, if not more!  To add to their woes, technicians can’t just leave the pump to do its thing and return later–part of the process involves routinely changing the vacuum pump’s oil.

In the heat of the summer technicians often get over-booked and can be forced to cut corners on their work.  The biggest time suck in this industry is pulling a good vacuum, so it is almost always the first area where quality suffers.

So what can happen with a badly pulled vacuum?   There’s a chance that nothing could happen–there’s a chance that a short vacuum time is enough to get moisture out, and assuming no leaks your system could be fine (albeit possibly running at less than peak performance).  Going back to the analogy that non-condensables are akin to blood clots in your body…a person with a blood clot could be fine for years and years with no issue.  There’s also a good chance the clot will hit a critical area, potentially causing tremendous harm–death is not beyond the realm of possibility.  It’s the same for your HVAC system–a system with moisture in the lines could run for weeks, months, even years with no issues (although performance will almost certainly be affected)…and this is the big problem with a badly pulled vacuum–you may not have a problem until well after the labor (and even parts) warranty has been exhausted.  Non-condensable damage isn’t easy to fix, and of course the repair could be a full system replacement.

How do you protect yourself?  Ask questions BEFORE technicians put their hands on your unit.  Ask “to what microns do you pull a vacuum?”  Remember–350 microns is the magic number to absolutely guarantee no leaks and removal of all moisture.  You want to hear AT LEAST 500 microns–this is usually enough to ensure no moisture, and an experienced tech will be good enough to install the unit with no leaks.  Make sure the technician actually owns a micron gauge (called a micrometer).  Many technicians feel they know when a system has been adequately pulled down, and that’s really impossible without a gauge–as mentioned before the number of time variables involved make every system different.

That’s all for today–we hope this has been useful!  For DIY tips and our “Diagnosis Dictionary,” visit our website at


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