: Air Conditioning Experts - Vacuum?

04-24-13, 01:08 PM
So I know you need to "vacuum down" the A/C system before you charge it. At 30 in/hg of vacuum, the boiling point of water is 70 degrees F. I've read that keeping a vacuum on it for 30 minutes will do it. So my question is, do you draw it down to 30 in/hg, close the system and then shut off the source of vacuum? Or do you keep the vacuum source on for the full 30 minutes?

Here's why I ask. I've also read not to use venturi-type vacuum pumps - the kind that operate off of compressed air. I don't know why, but that's the type I have and it will draw the system down to 30 in/hg easily. But, to keep it "running" so to speak, my air compressor runs non-stop. So as I think about it, once it's drawn down to a vacuum is there any benefit to keeping the pump running? It's not sucking anything out - it's at a vacuum. There's no circulation.

04-24-13, 02:40 PM
Those venturi type pumps can't pull a system down to the same micron-vacuum level as a vane type pump. Some vacuum is better than none, but if you don't have a regular pump then I suppose letting it sit with vacuum for an hour, then pulling vacuum again, would be the next best thing.

It's a good idea to leave the systme under vacuum for a while after any evacuation, so you can see that it maintains the vac level without any leaks.

04-24-13, 06:36 PM
The vacuum pump should be allowed to work for 30 minutes continuously. I have one of the higher level vane type pumps from Harbor freight and it has worked flawlessly on the few systems i have used it on. Although it was more expensive than the air powered ones, It was still cheaper to buy it than to have a shop vacuum/recharge the system too.

04-25-13, 09:23 AM
I've had great success with my cheap venturi pump, too, the the question of whether the pump should be running or not was bugging me. I figured a vacuum was a vacuum and once you get to 30 in/hg, why continue to run the pump?

I may invest in a "real" pump at some point just from an efficiency perspective. Running a huge air compressor to keep the venturi pump going seems kind of silly.

04-28-13, 03:30 PM
Just from the manuals i have read over, i think their reasoning on leaving it going so long is to just allow it to boil out any moisture that may have built up deep in the system. weather this in precautionary or if it's really what i needs, i couldn't be 100% on, but hey, if you can do it, why not?

The Ape Man
04-28-13, 04:52 PM
Boiling moisture takes a little time. Picture the sun drying the fog from your windshield. The vacuum was run for 45 minutes at max pull in places I worked. That was with electric high vacuum pumps.

The venturi unit I used for only one season would use a bazillion watts of electricity due to the airflow needed to make it work.

05-11-13, 08:25 AM
I bought an "FJP 6905" vacuum pump. It was the least expensive I could find, and Amazon had it for $83 shipped. It has mostly good ratings from people using it to vacuum down refrigerant systems. It's whisper quiet compared the the circus that the venturi pump was - between the sucking sound and the compressor running it was deafening.

Still waiting on a replacement condenser line and I can put it all back together. I can't wait to have A/C again.