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?
How does an electric vacuum pump work
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OK... so the mystery has been solved. :-)
The problem got so bad today that the failure rate reached 100% (every time the pump came on it would churn for >30 seconds)... I got a lot of text messages. When I got to the pump, I noticed that during these failures there didn't seem to be any flow at all, and I could feel the pressure build up around the coupler just below the flow control check valve (the one above the ejector pump).
I began to suspect the issue was with this valve, so I took it off and found it was basically just a disk inside a cylinder. It was not attached to anything (no hinge to keep it from flipping over freely). This effectively made the valve stop flow in either direction. I guess that the valve was originally something like this:
The issue may be similar to the issue described in this video, except my valve (American Granby SPCV200) was a swing check valve:
To awnser your first question I would advise checking your local plumbing and electrical code before relocating the switch as it may be required to be within sight of the pump (this is so a service tech can easily see that the powers is off and no one has turned it on when they weren't looking) Instead you could add another switch in series and locate that beside the heater.
Isolation valves are always a good idea (as you said for future service work)
Instead of getting a pump with a thermostat built into it you could buy a clip-on aquastat and mount that on the copper piping in the kitchen and run a cable back to the pump as it may be cheaper. And then there will be no need for a timer.
For your second question there should (hopefully) already be a check valve after the pump but before the tee fitting
This is to prevent the cold water going back through the pump (one this will make your hot water cold as you said and two it can spin the impeller backwards and damage the motor). If there is one it might be worn down and have failed in which case you will need to replace it. If there isn't one then you might want to add one.
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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.