Psarinuk:
Please don't use compression fittings. Soldered joints are the most reliable joints you can have, and I'd advise you to solder all your piping joints. If you go with compression fittings, you'll be kicking yourself for doing that because you'll be wanting to go back and redo it all in soldered joints once you learn to solder.
Just go out and buy yourself a few feet of the size of piping you'll need to solder (probably 3/4 inch) and practice on some 3/4 inch pipe fittings. In the course of a weekend of practice, you'll be confident to start soldering your copper pipe radiators.
Just remember the most common cause of leaking solder joints:
1. The joint not being hot enough (often because of water in the piping).
What I do is apply heat to one side of the joint and the solder to the other. By the time the solder melts on the far side of the joint, everything closer to the flame will be hotter, and the solder will fill the whole joint rapidly.
Another reason why joints might not be hot enough is because of the use of "pencil tip" torches, so named because they don't mix the fuel gas with air before burning it. The result is that such torches produce a darker blue flame of the shape of a pencil tip inside a lighter blue flame. If you buy ANY torch that mixes the fuel gas with air in a short tube before igniting it, you will have a solid color transparent blue flame that will be much hotter than any pencil tip torch flame.
2. Not allowing any way for the expanding air inside the pipe you're soldering to escape.
What draws solder into the joint is capillary pressure. But, if you're heating a copper pipe and haven't allowed any way for the expanding heated air inside the pipe to escape, then the capillary pressure drawing molten solder INTO the joint has to overcome the force of the pressurized hot air inside the pipe wanting to escape through that same joint. And, usually the result is that air pressure trumps capillary pressure, and the joint won't take solder. So, allow an escape route OTHER than the joint you're soldering, and the Gods will smile on your solder joints.
3. People not understanding what the heck they flux does.
Copper rusts, but the oxide layer it forms is highly impermeable to air and water, and as that brown oxide layer forms on this orange metal, it better and better protects that copper from further oxidation. This is why the copper water piping in a building will generally outlast the building.
At room temperatures, the rate at which copper oxidizes is very slow. However, at soldering temperatures, it's almost instantaneous. And, solder simply won't stick to that oxide film. It'll stick to the bare metal, but not the oxide film.
The whole idea behind sanding the copper pipe, brushing out the copper fittings, and fluxing both is to:
a. first remove any oxide film from the OD of the pipe and the ID of the fitting by sanding or brushing with a steel wire brush
b. putting on an grease with a low melting point that will act as a physical barrier between the air in the room and the bare copper that was exposed by sanding or brushing. So, try to flux as quickly after sanding or brushing as possible. Once a joint is fluxed, you can leave it form months before soldering it.
Once you do those two things, and provide a hot enough flame and allow for some way for the heated air to escape (besides through the joint you're trying to solder) then you're almost certain to bat 1000 in your soldering. (no leaking joints)
Some tips:
a) Even though I heat the joint from one side and apply the solder from the other, I cheat a bit and melt a bit of solder into the joint on the flame side first. That molten solder on the flame side allows for good heat transfer between the fitting and the pipe, thereby ensuring that the pipe inside the fitting is just as hot as the fitting. Once I melt a bit of solder into the joint on the flame side, I move the solder to the far side of the joint and wait for it to start melting on the far side. Once it does, then I know that not only the fitting is hot enough, but the pipe inside the fitting is also hot enough, all the way around the joint for the solder to take everywhere.
b) you'll notice that solder flux (or solder "paste") will often say that it's corrosive. They add zinc chloride to solder flux because at soldering temperatures it breaks down and reacts with the water vapour in the air to form hydrogen cloride (or hydrochloric acid). Hydrochloric acid is much more agressive at dissolving the oxide copper forms than the bare metal, so the whole idea of adding the zinc cloride to solder paste is so that the hot acid will dissolve any oxide in the joint that wasn't removed by sanding the pipe or brushing out the fitting. The reason they tell you to remove any flux from the solder joints after the joint is cool is because of this concern that the acid will attack the copper pipe. At room temperatures that acid is of no real concern, and not removing the old flux is not going to cause the joint to leak or the copper metal to dissolve. Still, it's just good practice to remove the old soldering flux cuz it's messy stuff.
c) Knowing what the flux does helps you understand what to do in some circumstances. For example, if you unsolder a fitting from the end of a one pipe and want to solder that same fitting onto the end of a different pipe, must you remove the old solder from the fitting, brush out that fitting and then flux both the pipe and that fitting before re-soldering it?
Answer: NO. The solder on the inside of the old fitting will protect the metal of the fitting from oxidizing as well as any solder flux will. So, all you need to do is sand and flux the pipe, heat the fitting until the solder inside it melts and slip that fitting over the fluxed pipe end, and then just heat and add some solder to ensure the joint is full. That joint will be AS STRONG as if you'd brushed and fluxed a new fitting and soldered it. And, of course, the reason is because the old solder was just as effective as flux at preventing any oxygen from getting at the metal during the soldering process.
Stick with soldered joints, and you won't regret it.
