Sweat fitting

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Sweat fitting is a means of joining copper tubing, using solder as a filler metal. Sweat connections are rapid to create, and offer a high degree of reliability. Until the mid 20th century, solder contained lead. Due to its toxicity even in trace amounts, now all solder used for potable water is required to be lead free.

[edit] Making a sweat connection

Sweat fittings are smooth, and slide onto the outside of a tubing section. In the UK, there are two types of such fittings; end feed fittings which contain no solder, and solder ring fittings in which there is a ring of solder in a small circular recess inside the fitting. The areas where the tubing and fitting will join are cleaned with an abrasive cloth to expose the bare metal. In the case of wire (soft) solder, they are treated with flux paste, a petroleum or water based compound which cleans the metal and helps solder to adhere. The joint is then heated with a propane torch and, and in the case of an End Feed fitting, solder is melted onto the joint. The molten solder is drawn into the joint by capillary action, filling the joint. With a Solder Ring joint, the joint is heated until a ring of molten solder is visible around the edge of the fitting. The joint is then allowed to cool, and is fully hardened within minutes.

Sweat connections are usually considered the most difficult of the three methods of connecting copper tubing, but sweating copper is a very simple process, provided some basic conditions are provided:

The tubing and fittings must be cleaned to bare metal with no tarnish
Any pressure which is formed by heating of the tubing must have an outlet

If these conditions are met, the joint is heated thoroughly, and solder is applied all the way around the joint, the connection should be solid and leak-free.

Sweating is also sometimes used for other workpiece shapes and other metals.

[edit] Types of solder

Two varieties of solder are used currently, hard solder and soft solder.

Soft solder comes on a wire spool and is easily bendable. Soft solder is used in combination with flux paste on joints which supply potable water. It is non-toxic. In the refrigeration trade there is also a silver bearing solder which is commonly used. This solder is used because it is resistant to vibration. The vibration of refrigeration equipment would tend to crack standard solder.

Hard solder, also called silver solder (sold under the brand Sil-Foss), comes in rigid rods and contains a phosphorus flux mixture in the rod. No flux paste is used with hard solder. Due to the toxicity of phosphorus, hard solder should not be used for potable water lines. It is a popular choice for refrigerant lines in HVAC/R systems.