ᐅ Odor coming from the underfloor heating manifold

Hello,

we started building our single-family house with a basement in 2017 and moved in November 2018.

Now to our problem:

On the ground floor, a musty smell comes ONLY from the right corner of the underfloor heating manifold. It is particularly noticeable when the outside temperature drops significantly. Currently, this happens in the evening from about 9 p.m. when there is more outdoor humidity/dew. The problem occurs in both summer and winter. I should also mention that there is always a slight odor in the guest bathroom. We have no pressure loss in the underfloor heating system and no visible damp spots in the house. I sealed the empty conduits once, but the smell did not go away.

We have mainly heated with our stove and hardly used the underfloor heating, but the problem exists just the same in summer and winter. Our contractors are currently a bit puzzled.

I have attached a picture.

Best regards

It somehow sounds like moisture beneath the screed that rises up into the distribution box.

And only on the right side then? We still want to check the wastewater pipe in the guest bathroom. We have no apparent issues with water from the outside.

Would I suspect it if there is a smell coming from the distribution box? Is there a way to check with a cloth or paper to see if anything is damp? The screed is open to the floor slab there, so maybe there is something in between after all.