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Soldered Capillary Plumbing Joints for Copper Pipes

A capillary plumbing joint consists of a copper sleeve with socket outlets into which the pipe ends are soldered. It is neater and smaller than a compression joint and forms a strong connection that will not easily pull apart.

Because a capillary plumbing fitting costs much less than a compression fitting they are often used when a number of joints have to be made, such as when installing a new central heating system. They are also useful in cramped spaces, where it would be impossible to use wrenches to tighten a compression joint.

end-feed capillary plumbing fitting
An end-feed capillary fitting


Many people are put off using capillary fittings because of the need to use a blow-torch. However, modern gas-canister blow-torches are easy to use and are not dangerous if used correctly.

How a capillary joint works

If you joined two lengths of copper pipe by just soldering them end to end the join would be very weak because the contact area between solder and copper would be very small. Because the sleeve on a capillary fitting increases the contact area and also acts as a brace to strengthen the connection, a much stronger join is formed.

Molten solder is sucked into the space between the pipe and fitting by capillary action, and this combines with a thin layer of copper at the contact surface to bond the pipe to the fitting. To help the solder to 'take' the copper needs to be clean and shining. Flux is applied before soldering to prevent oxides forming which would impair the solder-copper bond.



Types of capillary joint

The most common type of capillary joint has a ring of solder pre-loaded into the sleeve. It is known as an integral ring or 'Yorkshire' fitting (the name of a leading brand).

The 'end feed' type of capillary joint is virtually the same as an integral ring fitting, except that you have to add the solder when you make the joint. The sleeve is slightly larger than the pipe and liquid solder is drawn into the space between by capillary action.

Preparing the copper pipes

You need to make sure that the end of the pipe is square and doesn't have any burrs. Clean it and the inner rim of the fitting with steel wool or emery paper until they are both shining. Apply a coating of flux to the end of the pipe where it is to be soldered, and to the inside of the fitting. Push the pipe into the fitting until it rests against the pipe stop inside the socket. Twisting it a little will help spread the flux. Remove any excess flux a cloth.

Applying the heat to make the joint with a blow-torch

Light the blow-torch and gradually build up the temperature of the copper by playing the flame over the pipe and the fitting. If you are using an end-feed fitting, feed solder into the gap between the pipe and the fitting. When the metal is hot enough the solder will melt. When you can see a bright ring of solder all round the end of the fitting the joint is complete. If you need to make two or more capillary joints with one fitting, for example the three ends of a tee joint, they should all be made at the same time. If this is not possible you should wrap a damp rag round any joints already made to help prevent the solder in them being melted.