Your hydronic heating system is (normally, and yours almost certainly is) a closed system under pressure (typically somewhere around 15psi to keep the water from boiling at a low temp like the cap on the radiator in your car). Because your boiler is heating your domestic hot water, it probably maintains some nominal temp all year long. That hot water circulates through a heat exchanger (either a coil or plate) in the indirect WH, when the aquastat for the IWH drops below a certain point. This also triggers the boiler to likely raise it's temperature and turn on its pump until the IWH has been reheated. There's likely separate valves to prevent that hot water from circulating into your baseboards, and that may have a separate pump to circulate it as well.
Unless your heating system has a leak, the water that gets put in to fill it the first time is the water that continues to circulate until you trash the whole thing.
Your system MIGHT have a automatic fill valve, and if it has a leak, might be adding new water over time. That can mask any problem until that new water which contains dissolved minerals and oxygen rusts out the system! So, from a practical matter, it's sometimes better to shut the valve to the automatic fill. If there's a leak, the pressure will drop and the boiler will stop, but you'll know it and then be alerted to fix the system rather than it running for years, adding water and corroding things inside along the way. After the original fresh water was installed, eventually, it reaches an equilibrium, all the oxygen is used up (things rust) and the minerals deposited somewhere in the system, then there's essentially only H20. It will have some dissolved metals and things in it, so you wouldn't want to drink it, but it is in equilibrium with the system, and things aren't changing anymore.
Hope this helps...