Primatic cylinder

Primatic cylinder, alternatively a self-priming cylinderis a type of indirect hot water cylinder in which the water for the central heating is separated from the domestic hot water by an air lock inside the heat exchanger. This is in contrast to a standard indirect cylinder, in which the heat exchanger is typically a coil of copper pipe.

Primatic cylinders are supplied by the cold water storage cistern in the loft. Their design means that a separate feed and expansion tank is not necessary. However, they come with several disadvantages that make them inferior to standard indirect cylinders.

Primatic cylinder overview

In a primatic cylinder, the heat exchanger also facilitates the supply of water for the primary circuit, i.e. the central heating. When the cylinder is filled, water flows through the heat exchanger allowing water to pass into the primary circuit. As the cylinder fills, the shape of the heat exchanger traps air inside itself – imagine holding a bowl upside-down at the bottom of the kitchen sink, and turning on the taps. This is the priming of the cylinder – the formation of the bubble.

When the cylinder is full and the boiler is activated by the cylinder thermostat, hot water from the boiler flows through the heat exchanger. Heat is transferred by the heat exchanger to the rest of the water in the cylinder, providing hot water for the taps in the home. An immersion heater will usually be fitted so that the household will still have hot water even in the event that the boiler isn’t working.

A pipe rising up from the heat exchanger and bending down in the shape of a sharp loop allows any air or gases which form in the primary circuit to discharge into the cylinder. The gases will then rise up and out of the hot water cylinder, and out of the expansion pipe in the loft.

Since both hot water cylinder and central heating system are supplied with water by the cold water storage cistern in the loft, there is no need for a separate feed and expansion tank and its associated pipework. The expansion of the water in the primary circuit when it’s heated by the boiler is also accommodated by the heat exchanger – the bubble is compressed and changes position.

If you want to know why you have a cold water storage cistern, a hot water cylinder, and a boiler, but no feed and expansion tank or expansion vessel, then it’s quite possible that you have a primatic cylinder. It is also possible to buy primatic fortic cylinders, i.e. fortic cylinders with their own integrated cold water storage cistern. These remove the need for loft cisterns altogether, although they are not frequently encountered.


In terms of construction, a primatic cylinder is similar in many respects to a conventional indirect hot water cylinder. Both are generally made out of copper, with a layer of foam insulation. Older cylinders may have a separate insulation jacket, which is tied around the cylinder. The top of the cylinder is bell-shaped to prevent air locks, and the bottom of the cylinder will usually be concave to improve its structural integrity. The cylinder should be situated on a flat, stable, continuous wooden base – ideally ¾ inch plywood across three timber bearers. Air gaps between the timber bearers will permit the circulation of air under the cylinder, limiting the formation of condensation.


While primatic cylinders remove the need for a separate feed and expansion tank in the loft, they come with several significant disadvantages. The main disadvantage is that any loss of the air bubble will cause the domestic hot water for the taps and the bath to merge with the murky stale water inside the radiators. An obvious indication that the two bodies of water have merged will be brown-tinged water coming from the hot taps – hardly the ideal sort of water for doing the washing up. The only way to correct this problem is to drain the cylinder down and fill it up carefully, allowing the air bubble to form again and hoping that it stays in place.

The possibility for these two waters to merge also means that one cannot put any inhibitor chemicals inside the primary circuit in order to protect the system. As a result, the radiators and the boiler are much more liable to corrosion damage and blocking, which may prove expensive. Also, it is not possible to fit a shower pump on a system with a primatic cylinder, as this may cause the loss of the airlock and the merging of the two waters.

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