I want to use a 2.5 CFM vacuum pump (from Pittsburgh Automotive... http://www.harborfreight.com/25-cfm-vacuum-pump-98076.html) to pull a vacuum on a system in my house. I am a bit confused, though, as to how the pump works since there is only an inlet valve but no outlet? I will suck the gas into the pump to create vacuum in the system, but where does that gas then go?
My pump has a small outlet that is covered to capture the oil that may spray out if the pump is turned on without a load, or when first pumping down a system so the oil doesn't get blown out of the pump but is drained back into the pump. Most mechanical high vacuum pumps are rotary vane these use a fiber plate and the oil to make the seal. If there is no load the pump pulls more oil in and pushes it out the exhaust (some pumps do make big oil messes if not loaded for long or allowed full flow at the start of a pump down. Some less expensive pumps have a port to direct the oil into a container (vacuum pump oil can get expensive). There should be a warning if the pump doesn't have an oil reclaim system because it can be very messy.