Central heating header tank

At what rate are you using water - in litres per week, or whatever?

Is there not a ballvalve which automatically tops up the tank?

If your level is dropping your ball valve is probably siezed.

Usually there is so little flow that it locks up in the closed position after a year or so and then the header tank slowly empties.

Try waggling the ball valve to confirm that water comes in when you press the ball down and then stops when you release it (wouldn't want a sticky valve to stick open).

Servowarm used to fit open systems with no water supply the tank. You had to top up with a bucket when needed. Usually every few years. When mine was open vented I used to turn off the cold water supply to the ball valve - in case of a leak or whatever when away. Don't think I ever had to top it up between inhibitor changes etc which required a drain down.

How often should inhibitor be changed?