Fortic & Primatic cylinders: what’s the difference?

The hot water cylinder is one of the most important components in your home. Hot water cylinders come in different types as well as in different sizes. But what’s the difference between a fortic tank and a primatic tank?

The difference between these two tanks relate to two different design aspects. Two things they do have in common, though, is that they are both vented and low pressure. By vented, these means that they are open to atmospheric pressure. And by low pressure, this means that they are supplied by a storage cistern.

But what makes a fortic tank unique is that unlike a conventional hot water cylinder, a fortic tank has its own integrated cold water storage cistern, instead of having a separate cold water storage cistern in the loft.

Fortic cylinders are ideal for flats or smaller properties where it wouldn’t be practical or possible to install a bulky cold water storage cistern. You can also buy both direct and indirect fortic cylinders.

  • An indirect fortic cylinder contains an internal heat exchanger, which consists of a coil of copper pipework. Water heated by the boiler from the central heating circuit is pumped through this coil, heating the water in the cylinder indirectly.
  • A direct fortic cylinder does not contain a coil. The water is instead directly heated by two immersion heaters. One of these heaters is typically on a cheaper night tariff of electricity (Economy 7); the other is used as a heat boost during the day.

So, what’s a primatic cylinder? The answer lies in the heat exchanger. You see, in a typical indirect hot water cylinder – fortic or not – the heat exchanger consists of this coil of copper pipework we mentioned in earlier. Water in the coil and your radiators is supplied by a feed and expansion tank in the loft.

However, in a primatic cylinder, the cold water storage cistern not only supplies the cold water to be heated for the taps, it also supplies the water for your central heating. When the cylinder fills up, an air bubble is formed inside the heat exchanger. It is this air bubble which keeps the domestic hot water and the central heating water separate.

For this reason, primatic cylinders have serious disadvantages in comparison to cylinders with a coil and a feed and expansion tank in the loft. You can’t fit a shower pump to them, and most importantly, you can’t put any inhibitor chemicals inside the central heating in case the bubble is lost and the two bodies of water merge. The end result of the latter is rusty brown radiator water coming out of your hot taps, and shorter radiator life, since you can’t put any chemicals inside them to stop them from corroding.

So there you have it – that’s the difference between a fortic tank and a primatic tank. There is actually quite a lot of difference, albeit in very different respects. It’s actually possible to purchase hot water cylinders which are both fortic and primatic, although these are very expensive and much less frequently encountered.

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