hot water heater died

[rigcranop]

As BrianR said, cleanliness of the pipe & the inside of the fitting is important. They make wire brushes for that purpose or you can use steel wool. Flint is also correct about too much heat. You need to heat evenly around the circumference of the fitting also. Shortly after the flux has melted you can test the heat by touching the solder to the joint. If it melts make a pass around the fitting. If you bend the solder into a semi-circle, you can get the side of the fitting that is away from you much easier.

[xoxoxoBruce]

there are only two pipes: incoming cold water and outgoing hot water. how frickin' hard can it be?

It's not. If the replacement heater is identical it's even easier. But they seldom are, so it might take a couple fittings.

Yes, dry, clean, flux, and if the copper starts to discolor, it's too hot.
When you finish, don't touch it... unlike electronic soldering it takes time to cool. Solder goes through a plastic state between liquid and solid. If you move it during that transition you can ruin the joint and burn the hell out of your hand. It only takes a minute or so to be safe to move, but it's easy to forget how hot it is, because there's no visual clue.

Shop vac the rugs, turn up (down) the A/C, and fans, for the carpeting. A dehumidifier would help, too.

[Flint]

That's a key point there about making a hook at the end of the solder, for quickly getting around to the other side of the pipe.