What to set valve to when using 1 tank for portable grill?

The automatic changeover is also the regulator. If you want a high pressure connection to your Weber, (which is what your Weber expects) you are going to have to plumb into the line(s) between the tank(s) and the regulator. Too problematic, in my opinion. You'd have to tee off both tanks because you'd never know in advance which tank was full.

Much easier to tap in after the regulator. You can tee off anywhere, but understand, that you now have low pressure gas and you need to remove the regulator from the Weber. There are numerous threads on this forum and elsewhere on doing this. You can't have two regulators.

Just get a small refillable tank and be done.

Yes. How do you go get your cylinder filled when at the RV park? Flip the changeover, disconnect the empty cylinder, and get it filled. You can do the same if you just want to disconnect a cylinder. There have been times my 5lb cylinder runs empty and I just grab one of the 20lb cylinders off the trailer.

What about the changeover valve?
If I don't have any T adapter. Do I set the changeover to draw from right tank and then I can disconnect the left? Should I turn off both tanks entirely and never user changeover valve at the same time as portable grill?

What if I have the T adapter? Can I just keep the changeover valve as it and connect to one of the two for the portable grill? Or even maybe at the exit of the changeover?

Basically my question is: can I have the left not connected to anything while the changeover valve is drawing from the right? I'm worried it will create a leak in the system and empty my right tank into the air.

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Depending if you have the new style or old style connections. Either this:

Adapter New Style


Adapter Old Style


And a Hose

Looks like with an extra hose this might do what you want:
Extend a flow

Link:

http://www.amazon.com/gp/product/B00025LR7S/ref=oh_details_o08_s00_i00?ie=UTF8&psc=1

Search Amazon for this:
Weber 41455 20 Pound Tank Adapter Hose for Use with Weber Q and Go-Anywhere Gas Grill

I have one for my Q, you can still use the little bottles if you want or hook up this hose and use a 20lb tank.