AC/Heat Pump Questions

Hello all!I've been transitioning out thermostats and run into some oddities, that lead to questions. Bear in mind, I'm new to all of this...1.) There are two copper lines running to where my air handler is, one smaller and one larger with black insulation around it. What do these do, exactly? I've noticed that they are cool to the touch when AC is called for, and warm to the touch when heat is called for. Logically, I can figure they have something to do with controlling the temperature of the air being blown, but what exactly is happening inside those pipes?2.) I can hear "liquid" (I'm assuming refrigerant) running through them when cold air is called for. Is this okay? Should I also be able to hear the same "liquid" when heat is called for? I would assume the freon is liquid when cold and gas when hot (because science) and so I wouldn't hear "liquid" on hot.3.) Is there a basic way for me to use my multimeter at the thermostat/air handler/condenser unit to diagnose issues or just to ensure everything checks out? Where do I test? Circuitry is not something I understand super well, so I'm not sure where/what terminals on the three units I can hold the meter to and see if everything checks out.Thanks in advance for any info! via /r/DIY


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