The big payday on HVAC jobs is when you get to replace a whole system. However there are many simple fixes, both temporary and permanent that can be used to save customers a great deal of money. One of these is soldering metal.
One of the first things that hvacr techs learn about soldering is that you can not weld copper pipe with water still in the pipe. Before any repairs can start, the pipe needs to be drained, closed and dried. This causes problems because sometimes if there are multiple leaks say in a water cooling system, water might keep migrating down as you try and repair the pipe.
Fortunately there are a few tricks to try and work around this problem. One is called the bread trick. Take some white bread without the crust and stuff it into the pipe above the area you need to solder. This only works for very slow leaks, but it is effective because the bread swells as it absorbs water. Once water pressure is restored, the bread will dissolve. However you may need to remove strainers along the system.
The more technical method that I have seen HVAC in Denver workers use are pipe freezing machines. This freezes the pipe and its contents solid so that it can be cut. This is much more expensive and uses highly specific equipment, however it works perfectly, unlike the jerry rigged bread method. Just be careful to only freeze the area you are working on. Freezing the whole pipe can cause it to burst.
When working on pipes that contain coolant, Denver air conditioning technicians use different technology called brazing. Brazing uses oxyacetylene for refrigeration. Instead of lead, use silver soldering. Silver chemically binds with copper to make a leak free joint. The oxyacetylene torches are used to heat the whole pipe up to above five thousand degrees Fahrenheit. Of course this means you need to use the proper safety equipment for these kinds of torches. This especially means a welding mask to save your eyesight.
To properly braze, you need to clean the pipe. Once it sets it should be solid. The inner and outer joints need flux applied before heat. The torch should not heat the solder. Instead evenly heat the pipe so that the solder makes a uniform fit. Use just enough solder for a fit fit. Too much and it could break up and foul up equipment. The final step is to perform a pressure test. Apply soap to the outside of the pipe and look for bubbles indicating an improper seal. Once pressure is applied, any drop indicates a leak.