Tank connector

tank connector or tank coupler is a fitting which connects a pipe to a cistern, allowing water to be drawn from the cistern, and in some circumstances, allowing water to enter it.

Tank connectors are typically found on cold water storage cisterns and feed and expansion cisterns. They are usually made out of brass and accommodate the pipe via a compression fitting, although plastic push-fit tank connectors can be purchased. Tank connectors are typically available as 15mm, 22mm, and 28mm fittings.

A tank connector typically consists of a flange at one end and a compression fitting on the other. A washer is usually supplied with the fitting and should be secured via the flange on the inside cistern wall. A second washer for the outer cistern wall is recommended. Ideally, the flange will have a hexagonal shape so that a pair of adjustable grips can easily be used to secure the fitting in place.

Cold water storage cistern

A cold water storage cistern will have at least two tank connectors: one for the overflow, and one for drawing off cold water to supply the hot water cylinder. The connector for the overflow is normally a 22mm fitting in accordance with Byelaw regulations, or 28mm under Scottish regulations.

The connector for the hot water cylinder supply is usually a 22mm fitting, although a 28mm fitting may be necessary for larger households. Where the cold water storage cistern supplies cold water outputs, it is vital that the outlet for the hot water cylinder is fitted higher than the cold outlet, so that if the water in the cistern is exhausted, the hot water will stop flowing before the cold does. This means that an individual taking a shower will not be scalded if the water in the cistern runs out: the water will run cold, not hot.

In order to prevent the ingestion of sediment or limescale into the plumbing system, tank connectors should be fitted at least 50 – 75mm above the base of the cistern. However, water regulations recommend that, where possible, outlets should be fitted to the bottom of the cistern in order to prevent sediment from gathering in the first place. Tank connectors should be fitted on the opposite side of a cistern to the inlet, ensuring the circulation of water through the cistern in order to prevent stagnation.

Feed and expansion cistern

A feed and expansion cistern will also have two tank connectors: one for the overflow, and one for connecting the cistern to a 15mm feed pipe which supplies central heating system with water and allows for the water, when heated, to flow back up into the cistern as it expands. For this reason, this pipe should never be fitted with an isolation valve.

The water level in a feed and expansion cistern is adjusted so that when the central heating system is cool, the water level is only slightly higher than the outlet. This is so that the water in the central heating system has the space to expand when heated. The cistern must be able to accommodate the expansion of the water by approximately 4% of its original volume, and the elevated water level must be at least 25 mm / 1 inch below the overflow.

How to fit a tank connector

Installing a tank connector on a cistern is a simple task.

  1. Using a marker pen or crayon, mark the location of the tank connector on the cistern wall.
  2. Drill the hole using a 29mm hole saw for a 22mm tank connector, or a 22mm hole saw for a 15mm tank connector.
  3. Remove any swarf created by the drill.
  4. Insert the tank connector into the cistern wall, making sure that the rubber washer is on the inside. Fitting a washer to the outside of the cistern wall is recommend, and will offer an extra layer of security.
  5. Tighten the fitting to create a watertight seal.
  6. Use your fingers to check the fitting for leaks while the cistern is filling. If there are any leaks, drain the cistern and tighten the fitting as necessary. Tightening joints with water in the cistern is poor practice and could lead to disaster if the cistern ruptures.


  • Tank connectors and the adjacent plumbing of a cistern are usually brass compression fittings. In the event that a blowtorch has to be used in the vicinity of a plastic cistern, take exceptional care to protect the cistern from heat, including the transfer of heat along the pipework to the cistern wall. The use of blowtorches in loft spaces is not advisable due to the fire hazard.
  • In the event that a tank connector must be fitted to a galvanised cistern, the tank connector must separate the cistern wall from any copper pipework. This is to prevent a corrosive electrolysing reaction from occurring.
  • Never use oil-based jointing compounds to make cistern connections. Not only are they likely to contaminate the water in the cistern, they may cause an unwelcome chemical reaction with the plastic of some cisterns.
  • Never make a hole in a plastic cistern by heating a piece of pipework with a blowtorch and pushing it through the cistern wall. This may weaken the surrounding area of the cistern wall and affect its structural integrity. It is also liable to void the manufacturer’s warranty of the cistern.