Careful — a critical tool on your service truck may not be doing its job. How long has it been since you checked the performance of your vacuum pump?

Many vacuum pumps in the field are not working well enough to do their intended job. Vacuum pumps can easily be underappreciated. Their contribution to the reliability of air conditioning equipment is not easily observed. However, moisture in an air conditioning or heat pump system can wreak havoc.

Moisture enters a refrigeration system anytime it is opened up for repair, or when the low side of a system with a leak runs at a vacuum. Mixed with refrigerant, moisture creates acid that attacks motor windings and compressor bearing surfaces. It also mixes with compressor oil, creating a sludge that can restrict strainers and expansion valves and cause a reversing valve to stick.

Your vacuum pump must be working properly to prevent all of these malfunctions.

When using your vacuum pump to evacuate a system, some acid and moisture pulled from the system is trapped in the vacuum pump oil. This contamination has the same destructive effect on your vacuum pump as it does on refrigeration systems. This, in conjunction with inadequate maintenance, slowly degrades pump efficiency, rendering it increasingly ineffective.

There are so many ineffective vacuum pumps in the field today that your pump’s condition could make all the difference for a customer. If the system has been opened one or more times by others, it may be in terrible condition in terms of its moisture content.

If your pump is working properly when you service that system, you may be able to completely arrest any damage that was taking place prior to your arrival. If your pump is working poorly, you may not be able to stop the damage in the system — or worse, you may contribute to it.

If you aren’t sure that your pump is working properly, check its performance with an electronic vacuum gauge. If you don’t have one, check with your local parts suppliers. Often they have vacuum pump clinics for testing.

The 15600 "Cool Tech" vacuum pump from SPX Robinair offers a gas ballast feature that is said to keep the oil clean for a longer time. Regular oil changes are still recommended, of course.


Below are some tips regarding vacuum pumps and moisture in systems.

Determining when you should have your vacuum pump rebuilt is somewhat arbitrary. Your pump must pull a vacuum deep enough to boil any moisture that may be in the system, but the boiling point of moisture changes with its temperature.

In 86 degree F weather, for example, your pump need only pump to 32,000 microns to begin boiling water (remove moisture from the system). In 21 degree weather, your pump must be able to reach 2,500 microns to begin to remove moisture. A lower micron reading means a deeper vacuum. Zero microns, if it were possible to create a pressure this low, would be a perfect vacuum.

  • The deeper your pump will evacuate, the faster and more thoroughly you can dehydrate a system. If you were evacuating a system in 86 degree weather with a pump that would pull down to 50 microns, you would be off the job faster, with better results than if you were using a pump that would only pull down to 32,000 microns.

    A pump rebuilder I talked to recently said that if your pump will achieve a 50-micron vacuum, it’s in great shape. However, he was unable to give me a definite micron level at which a pump must be rebuilt.

  • My “must-rebuild” level is 500 microns.

  • Don’t go too low in the field. Your pump might be able to pull a 50-micron vacuum, but you never want to evacuate a system too low because compressor oil boils at about 200 microns. You don’t want to suck the oil out of the compressor.

    The primary cause of poor pump performance is failure to change the oil on a regular basis.

  • Replacing the oil every time you use the pump is optimal.

  • Always change the oil after pumping down a system that has had a burnout. But even if you take excellent care of your pump, it will still wear out.

    Your manifold gauge set and hoses can prevent you from attaining adequate vacuum even if your pump is working properly. Your refrigerant hoses and manifold can appear not to leak when they are under pressure, but may leak when under a vacuum.

  • Test your gauges and hoses separately from your pump.

  • Use hoses specifically designed for evacuation. Standard hoses are not always suitable.

  • Storing them properly between uses will extend their life.

    Vacuum through both the high and low sides of the system to minimize evacuation time.

    Consider buying an electronic vacuum gauge for your own use. Using this tool during an evacuation is the only way to ensure an effective job. It tells you if you have removed all moisture from the system, as well as if you have a leak. It also speeds up your work by preventing you from leaving the pump on too long.

    When using an electronic vacuum gauge, pump the system down to 500 microns. Valve off the system from your hoses and manifold and allow the system to sit for about 5 min:

  • If the system remains at about the same micron level, it is properly evacuated.

  • If it rises to about 1,500 microns, moisture is still present.

  • If the system rises rapidly up to atmospheric pressure (0 lb gauge pressure), oops, you should have done that leak check.

    Many technicians leave the old liquid-line drier in the system when they repair it. I have seen systems that were opened three or four times without replacing the drier.

    When a drier is left in the system, if it does not become restricted, it eventually absorbs too much moisture and will come apart.

    If you have ever worked on a system where a drier came apart, you know that it is one of the worst things that can happen to an air conditioner. The drier’s desiccant circulates through the system, lodging in all the refrigerant system components and causing myriad malfunctions.

    Always replace the liquid-line drier when you open a system. The modest cost of a drier to the customer is a small price to pay for the reliability it provides.

    Never add a liquid-line drier to a system without ensuring that any other liquid driers already in the system get removed. Two driers in series with each other will cause a partial liquid-line restriction and degrade the system’s performance in hot weather.

    Howard Leonard is president of Total Tech HVACR Training, Phoenix, AZ. His firm specializes in training for service technicians. He can be reached at 602-943-2517.

    Publication date: 06/17/2002