Fortic cylinder: what is it & how does it work?

A Fortic cylinder or Fortic hot water tank; alternatively, a combination cylinder, is a vessel designed for heating and storing hot water. It’s typically made out of copper, with a layer of foam insulation on the outside.

What makes a Fortic cylinder unique is that it has its own built-in cold water storage cistern. This is in contrast to a standard vented cylinder, which gets it supply from a 25 – 50 gallon cistern in the loft.

The cistern section of a Fortic tank is directly above the hot water cylinder section. It has exactly the same purposes as a conventional cold water cistern: to replenish the cylinder with cold water, and to accommodate the expansion of the water as it heats up.

Fortic cylinder overview

The supply for the storage cistern will be a 15 mm compression fitting from the mains. A ball valve, ideally a Part 2 ball valve, controls the water level in the cistern. The cistern must also have an overflow pipe.

When the cistern is full, the water level should be at least 25 mm below the bottom of the overflow pipe. A Fortic cylinder should also come with a close-fitting lid to prevent the ingestion of dirt and debris in the cistern.


Water does not flow directly into the top of the hot cylinder from a hole in the bottom of the cold cistern, because hot water rises. If this were to happen, the cold cistern would contain the hottest water. Instead, cold water is delivered to the cylinder via a pipe which runs from the base of the cistern to the base of the cylinder. This pipe is often partly visible underneath the insulation, running down the length of the cylinder.


Another pipe runs from the top of the cylinder to the top of the cold water cistern. It enters the cistern higher than both the ball valve and the overflow. This is the expansion pipe, which accommodates the expansion of the water as it’s heated, and allows it to vent into the cistern if necessary. Hot water should never be contained without accommodating its expansion, as this would be dangerous.


In some Fortic cylinders, there may be a galvanic/sacrificial anode in both the cylinder and the cold cistern. This is a piece of metal which is more certain to corrode than copper, thus protecting the cylinder from corrosion. In the cold cistern, the anode may be contained in a small piece of copper or plastic pipe attached to the bottom.

Fortic cylinder types

Fortic cylinders are available in several different arrangements. They are available as both direct and indirect cylinders. In a direct Fortic cylinder, the water is heated via two immersion heaters. One is situated near the bottom, and the other about two thirds up. The lower immersion heater will typically operate on a cheaper, night electricity tariff (Economy 7). The top one uses more expensive, day-time electricity to boost the temperature of the water as it is drawn off during the day. The Economy 7 immersion heater works during the night to heat up the water for the day ahead.

In an indirect Fortic cylinder, the water is heated by a heat exchanger. The heat exchanger is normally in the form of a coil of copper pipe, which carries hot water from the boiler. Water enters the top of the coil via the “flow”, and exits at the bottom by the “return”. These are typically 22 mm fittings.

It is possible to buy primatic Fortic cylinders. These are cylinders whose heat exchanger also facilitates the supply of water to the central heating circuit. This eliminates the need for a separate feed and expansion tank. The two bodies of water are separated via an airlock which forms inside the heat exchanger. However, not only are primatic Fortic cylinders very rare, they have big disadvantages. The main one is that you can’t use inhibitor chemicals in a primatic system. This makes them much more vulnerable to corrosion.

Pros & cons

The main benefit of a Fortic cylinder is that there is no need for a separate cold water storage cistern. For this reason, direct Fortic cylinders are commonly installed in flats and smaller properties. They are an effective solution for providing hot water in properties without a loft or gas supply.

However, Fortic cylinders do have their drawbacks. Flow rates at hot outlets will be poorer than systems with a separate loft cistern due to the lower pressure head, and even poorer than combi boiler and pressurised cylinder systems. They are also generally incompatible with pumped showers, as most of these require a minimum cold cistern capacity of 50 gallons (227 litres). Even a 180 cm tall Fortic cylinder will only have a cold cistern capacity of around 40 litres. Finally, there will always be the potentially annoying sound of running water within the living area of the property when the cistern fills.