More work is involved when making joints with this type of fitting, but they have the advantage of being cheaper and less bulky than compression fittings. There are two types: one has a ring or reservoir of solder incorporated during the manufacturing process and simply needs the application of heat to seal the joint ; the other type, known as ‘end feed’, has to have molten solder introduced at the fitting mouth.
Before you start to make the joint, make sure that the surrounding area will not catch fire when you use the blow torch. Use asbestos mats (if you do not have them, spare pieces of ceramic tile will do) to cover any adjacent material liable to catch fire. To make a capillary joint, alter cutting and preparing the pipe, thoroughly clean the inside of the fitting and the surfaces of the pipes, otherwise the solder will not adhere. Use a rag to remove any grease or dirt and then burnish them to a bright finish with either fine wire wool or fine glasspaper.
A suitable flux recommended by the fitting manufacturer should then be smeared over the outside of the pipes and the inside of the fitting. Take care that no areas are left bare. Now insert the ends of the pipes into the fitting, ensuring that they reach the integral stop, and make a scratch mark on the pipes next to the fitting to indicate any unwanted movement. Either the fitting or the pipes should then be twisted to help ‘bed in’ and distribute the flux evenly. Wipe off any excess left outside the joint. Heat should now be evenly applied to the whole fitting area with the blow torch.
With pre-soldering fittings it is only necessary to continue heating until a ring of solder appears around the circumference of the fitting mouth. While not strictly necessary, an added precaution against leaking is to run a piece of cored solder around the mouth of the still-hot joint. End-feed fittings should be heated until they are sufficiently hot to melt the solder wire, which is introduced around the mouth of the fitting. Once the solder starts to melt, slowly take it round the fitting mouth until the fitting overflows and will accept no more. Leave a ring of solder around the edge of the fitting as before.
Once the joint has been completed, using either fitting, leave it for at least two minutes to cool and harden. Finally wipe off any flux left on the exterior of the joint. If you are using stainless-steel pipe, special precautions must be taken to make a satisfactory joint. Ordinary flux used for copper is not satisfactory; an ‘aggressive’ flux is necessary to remove the oxide which forms more quickly on stainless steel than on other metals. Several types of flux, in liquid and paste form, are suitable, most of which contain an acid base. As these are highly corrosive, they should be handled with care, and any excess left on the external surface of the pipes and fittings ,must be removed.
The pipes should also be flushed out as soon as possible to remove any traces of flux from the inside of the pipe bore. The other difference concerns the application of heat. This should be directed on the fitting only, and not on the pipes. This is because the thermal conductivity of stainless steel is lower than copper and any heat applied to the pipe will not be effective.