Is this how a Primatic system works?

Can anbody enlighten me cuz there isn't much info out there on primatics.

Here goes:

Inside the hotwater tank is another cylinder . On the underside of the cylinder is a dome which is connected to a shaft which moves up and down, this allows the dome to move up into the cylinder.

When the the tank fills the Central heating system starts to fill first when it becomes full the HW tank starts to fill. As it does the air in the dome is pushed up by the water forcing the dome to move upwards and seal the cylinder and thus isolating the Central heating from the hotwater.

Is this correct or am I way off?

One other thing. How is the water topped up in a primatic CH system.
If the HW tank is full this means the CH cylinder has been sealed off, so how does extra water get added if needed?

Thanks much.

Iv'e always believed there are no moving parts on a primatic and the heating water and hot water are seperated by an air bubble.
You might find this of interest www.purimachos.info/faq.htm

Strangely enough I already looked at this site and saw the pictures which is why i came to the conclusion above.

As you can see in the cut away of the tank there is a dome shaped piece of metal and a shaft above it.

To me this is how it works, the dome seals of the sides of the cylinder by using the pressure of the air or air bubble which is formed in the underside of the dome.
Thus as the HW tank fills the water forms the air bubble in the dome which is forced up by pressure of water and seals off the CH cylinder.

If it was simply a case of an air bubble alone I would not think this would work. Water is denser and therefore heavier than air, so while water would not be able to rise up through the air bubble because of the fact that water is heavier than air and the air pressure would force the water down, water from the CH would most certainly be able to drop through this air bubble from above into the HW tank.

As you can see in the diagram on the right in the above site the dome at the bottom is only half full with air (air bubble).

This is why I assume that the dome must rise up into the cylinder from as you say pressure by the air bubble which is formed inside it and thus seal the system.

I am only speculating as it seems logical to me, but I'm open to offers of explanations to put me right if I'm wrong.

just found this

Primatic (self priming) - only for use with gravity systems because the water for the central heating is taken from within the cylinder, and is only separated by an air lock (system additives must not be used with this type of cylinder as this will come out through the hot taps)

My thoughts are it does work without any moving parts.
Your 'shaft' I believe is a tube which extends almost to the top of the dome, which water goes up when it's being filled or topped up.
This water becomes heating water and can never get back down the tube again because of the bubble formed.
I agree the water is heavier than air, but the pressure created by the head of water is pushing the water up the tube, but of course the bubble of compressed air is counterbalancing it and holds it in a state of equalibrium.
When the water gets hot and expands the bubble absorbs it and if there is need to bleed radiators the state of equalibrium is broken by opening the bleed valve, so any water lost is replaced as it goes up the tube and overflows to the heating side.
I also think moving parts would make it so costly to produce it would have no advantage and impossible to repair.
This is my interpretation, It doesn't mean I'm right.

Mandate I think your explanation is perfectly plausable and sounds more likely than mine.

Now that you have pointed out the part about the tube it does indeed seem the way it works.

Thanks for clearing that up, your help is much appreciated.

Mandate has it about right. We had one split down the middle when I learned. What they didn't tell us was that they are unadulterated crap which will trash your heating system eventually because of unavoidable corrosion.