If the system already contains fresh(ish) inhibitor there is probably enough in the system already, but yes you could add more via the header tank. Do read the warnings on the inhibitor bottle threating the end of the world if you mix different inhibitors.
Skids, you are getting confused with Combi boilers, which are sealed systems.
Condensing boilers come in both sealed and open vented variants.
Utility, If you are going to add inhibitor to the expansion tank, drain sufficient water out of the system to empty the tank, and them some more. Add inhibitor to empty tank, then turn valve back on to fill up. This will ensure that the inhibitor circulates around the pipework. If you just add it to the tank with the water already in it, then the inhibitor will just sit in the tank and not do any good.
A conventional condensing boiler is 'exactly what it says on the tin'.
Condensing is where the flue gases are reused to take as much heat from the combustion process as possible and transfer this to the water in the system, therefore higher efficiency. Most boilers will say Open vent model also available even condensing boilers.
They do not have to be a combi or a sealed system boiler to be a condensing boiler.
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