Expansion tank pipes

[Loubanez]

Hello, I'm an electrcian who is just trying to grasp some knowledge of heating systems as I've got some S plan jobs coming up.
On older installations what is the pipe used for that runs from the header tank into the flow pipe? Expanding water??
How come this set up is not required on combi boiler installations?
Does an expansion vessel overcome this?
I often see systems with the cold water tank still in place but the heating tank removed, how is this possible?
Sorry for the endless questions!
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Also the cold pipe that runs from the header tank to the return?

 

[SJB060685]

With a feed and expansion tank in the roof for heating circuit the minimum 22mm vent is for safety purposes and the cold feed (commonly 15mm) is for filling and expansion.
Combi systems these days are sealed systems without a tank in the roof, a pressure relief valve is the safety device and an expansion vessel takes the place of the header tank and does what its called.
The systems you've seen with heating circuit tank removed are sealed systems but still using open vented hot water system.

 

[Loubanez]

With a feed and expansion tank in the roof for heating circuit the minimum 22mm vent is for safety purposes and the cold feed (commonly 15mm) is for filling and expansion.
Combi systems these days are sealed systems without a tank in the roof, a pressure relief valve is the safety device and an expansion vessel takes the place of the header tank and does what its called.
The systems you've seen with heating circuit tank removed are sealed systems but still using open vented hot water system.

Regarding your last sentence. A sealed system (combi), is still using the heating loop (flow and return) for the hot water?
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Regarding your last sentence. A sealed system (combi), is still using the heating loop (flow and return) for the hot water?

Is sealed another way of saying unvented?
Sorry for all the dumb questions

 

[SJB060685]

Sealed is another term for unvented yes.
A combination boiler provides instantaneous hot water through a plate heat exchanger and no hot water cylinder is needed, although a hot water cylinder can be plumbed into a combi system. I hope that answers your question?