If you're new to plumbing, you will want to practice soldering pipes before you try your skills on an actual solar thermal installation. It's not difficult but it's extremely important to do the job well or you'll get leaks and other problems. Practice makes perfect!

Apply the flux

You cannot solder without flux, as the solder will simply run off. Most fluxes come with a small brush which is used to apply the liquid to the surfaces. Use this brush to apply flux to the copper pipe and the fitting, covering the entire area of the join on both parts, then slide the pipe into the fitting.

Excess flux in the pipe is not good for the system (it is acidic and will cause your solar fluid to deteriorate), so be generous but not excessive in its application.

Heat the joint

Before you begin heating the joint, unravel a couple of inches of solder from your roll and place it to hand. You will need it ready when the joint is heated.

Next, you need to heat the entire fitting and part of the tubing, hitting the flux directly with the flame from your blowtorch. It normally takes between 15 and 30 seconds for a propane torch to heat the join sufficiently that the flux starts to sizzle and boil – that's hot enough. Do not overheat, as the flux will burn and stop the solder from sticking.

If you are soldering pipes in an enclosed space or near anything even slightly flammable, place an insulated heat-resistant pad or small sheet of metal between the joint and the environs. If possible, fix the pad in place so you can keep both hands free to work.

Apply the solder

Take the solder you prepared. You will need about as much solder as the size of the pipe (an inch of solder for one-inch pipe, a half inch for half-inch pipe, etc.) and you should direct it to the opposite side of the pipe that you are heating. If it will melt on that side, it'll melt on the side where the heat is!

Apply the solder to where the pipe meets the fitting. If the joint is hot enough, the solder will melt and be drawn into the joint – even if it moves vertically against gravity. As soon as the solder starts flowing into the joint, you can stop heating.

Leave it alone

Although it's tempting to fiddle with the joint, to check that the solder has completely filled it and that everything is perfect, leave it alone. Moving, jolting or rattling the joint may cause the solder to cool unevenly and ruin your work.

You may have seen professional plumbers wipe joints with a damp cloth while they are still hot, to remove the flux. This is not necessary; they usually do it because they work one joint at a time and it saves them going back over the pipe run when it's finished. You can wait until the joint cools and then wipe off any excess flux, which prevents the