How often is it normal to fill the heating circuit in a new building with underfloor heating?

Hello everyone,
how often nowadays does water still need to be refilled in a new building (built 2016) with underfloor heating?
We have a Viessmann gas boiler and about 140 sqm of heated area.

Now I have to refill water about once a year because the pressure drops from 1.5 bar to about 0.9 bar. Although the underfloor heating only runs about 7.5 months out of 12 and the rest is only for hot water production.

In your experience, is this still within the normal range or should one already start to be concerned?

You have a leak.

The underfloor heating is maintenance-free in this regard and should be left alone as much as possible. Because refilling with new water always introduces new oxygen into the system, which promotes corrosion and sludge buildup.

oh oh oh this will be expensive

Your heating system is definitely leaking. If you're lucky, only a fitting in the manifold isn't tightened properly, maybe wipe everything there with a dry cloth after you've refilled it. But I think you would have noticed that already...

My tip.. defective expansion vessel.

Or somewhere there is still air in the system. For us, it was in the heat exchanger of the hot water tank that I had not properly vented.

Otherwise, I had to top up a little only once after a year (1.6 to 1.8 or so).