Acid core solder IS normally used for plumbing work.
It is NOT used for electrical work.

I just read several caveats (on the web) not to use acid core, but instead use resin core with copper. Not that I'm disagreeing, just pointing out.

So I thought I had committed a plumbing sin - maybe not.

Here's one of those web sources:
http://plumbing.hardwarestore.com/learning/how-to-install-copper-pipes.aspx

I'm about to install 3/4 copper in our basement walls for basebaord heat. (Is type L the right one?)

Any suggestions for which flux & solder to use? I have Oatey's green can, as I recall. As for solder, acid or resin, or either?

In plumbing we normally do not use a core type solder.

Resin core solder is principally used for electrical and electronics because the resin flux does not promote corrosion on the connections.

Acid core solder is principally used in the sheet metal trade for doing very intricate work.

The flux in acid core solder is primarily the same flux as we use in plumbing but the preferred method in plumbing is to apply the flux directly to the joint with a flux brush then use solid solder. In this manner the flux has slightly more time to react with the metal and perform its cleaning process before the solder is applied.

Some plumbing codes are now moving away from the petroleum based acid fluxes entirely and opting for the water based flux.

Acid flux is actually very aggressive stuff and more forgiving of poor cleaning.

Rosin flux is always used in electrical work, and is very important n electronic work. Acid flux is so aggressive it can eat the thin layers of copper used to make printed circuit boards, and the salt residues it leaves behind are conductive if they get moist (and since they are hygroscopic they tend to stay moist).
The rosin fluxes have different levels of 'activation' that indicate how aggressive they are.
Some of the finest geometry printed boards use immersion gold plating (a few micro-inches thick) and can use the least aggressive fluxes.

The carrier for the flux can be petroleum based or water based (actually just not petroleum so it cleans up with water), but the actual active component remains the same.
The electronic industry has been moving to water cleanup and low activity fluxes for many years, particularly after 1,1,1-trichloroethane was pretty much banned as a board cleaner.
Just about everyone had a vapor phase degreaser, a tank that held hot vaporized 1,1,1. When a cool object (room temperature) was placed in the vapor it condensed on the object and dripped off cleaning the item.
Now glorified dishwashers are used.
By measuring the conductance of the wash water you can tell if the items have been cleaned of ionic contaminants. Really pure water is a very good insulator.

Thanks all.

Based on your most excellent commentary, it seems I can leave my little hot water repair alone - but get some other stuff for the future. Is that correct?

Acid core solder is fine for Plumbing work providing that it is a certified "Lead Free" solder but be very careful here because nearly all resin core solder and most acid core solder is 60/40 lead solder. Check the labels carefully.