Replace shut-off valves

I am planning on replacing the shut-off valves and supply lines on my bathroom lavatory faucets. The current shut-off valves are sweat soldered on. In order to preserve the existing copper tubing (2 - 3" stub), I would like to remove the existing shut-off valves by unsweating them rather than cutting them off. I have a few questions.

1. I have zero experience sweating or unsweating joints. Besides getting the water out of the line, what are the keys to success for unsweating a shut-off valve?
2. How long should I have to heat the shut-off value before it breaks loose?
3. I plan on installing compression shut-off valves. What type of "dressing" do I need to use on the ferrell and nut, if any?

Thanks,
Bertman48

The key to unsweating valves is to remove all of the water from the area around the valve, and prevent any trickles into the area. You will want to open any faucets possible to admit air and let water drain. You might have to blow air through the pipes or use a vacuum to suck the last water out at a faucet. It may help if you can take the bonnet off the valve.

Ferrule type valves are often longer than the sweat type valves. Check the length to see if you can cut at least one of the lines off flush with the valve body to avoid unsoldering.

If you are using ferrule type valves to get the union effect, I have seen a sweat valve by Watts, sold at some HDs, that have union type couplings soldered onto the pipe and nuts to connect the valve. They cost about the same as soldered valves.

#1 Remove the supply tube from the valve.

#2 Remove the nut behind the handle, it is called a packing nut.

This should allow the water to drain out.

#3 Heat the fittings until they come off.

If you have straight stops you can use a straw to remove enough water to unsweat the valve.

Then heat and wipe the pipe, with a cotton rag, until the pipe is clean.

Install new compression fittings or sweat on male adaptors and use a threaded fitting.