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

Primatic cylinder or 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 standard indirect cylinders, in which the heat exchanger is typically a coil of copper pipe.

Primatic cylinders are supplied by a cold water storage cistern. 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 and passes 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, heating the water in the cylinder. An immersion heater should also be fitted as standard. This ensures there’s still hot water even if 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 cylinder, and out of the expansion pipe in the loft.

Use with other cisterns

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

If you have a cold water storage cistern, a hot water cylinder, and a boiler, but no feed and expansion tank or expansion vessel, it’s quite possible that you have a primatic cylinder.

It’s also possible to buy primatic fortic cylinders. These have their own built-in cold storage cistern. This means there is no need at all for cisterns in the loft. However, such cylinders are very rare. As long as there is a gas supply to the property, a combi boiler system is likely to be a better choice.


Primatic cylinders are similar in many ways to standard indirect cylinders. Both are generally made out of copper, with a layer of foam insulation. Older cylinders may have a separate insulation jacket tied around them. The top of the cylinder is bell-shaped to prevent air locks. The bottom of the cylinder is usually concave to improve its structural integrity. The cylinder should be situated on a flat, stable, continuous wooden base. Ideally this should be ¾ inch plywood across three timber bearers. Air gaps between the bearers will allow air to circulate under the cylinder. This limits the formation of condensation.

Primatic cylinder disadvantages

While primatic cylinders don’t require a feed and expansion tank, they have several big disadvantages. The main one is that the loss of the airlock will result in the hot water being contaminated by water from the radiators. This typically results in brown-tinged water coming from the hot taps. The only way to fix this is to drain the cylinder down and carefully refill it. This will allow the air bubble to form again.

The risk of losing the airlock also means that you can’t put any inhibitor in your radiators. This makes your central heating system much more susceptible to corrosion and blockages. Needless to say, primatic systems may prove to be very expensive in the long-term. Also, shower pumps can’t be used them, as they can cause the loss of the airlock.