Odor from underfloor heating manifold

Hello,

we started building our single-family house with a basement in 2017, and we moved in 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, noticeably stronger when the outside temperature drops significantly. Currently in the evening from about 9 p.m. when there is also more outside air humidity/dew. The problem exists in both summer and winter. I should also mention that there is always a slight smell in the guest bathroom. We have no pressure loss in the underfloor heating and no visible damp spots in the house. I sealed the empty conduits, but the smell did not go away.

However, we have heated a lot with our stove and hardly used the underfloor heating, but the problem exists just as much in summer as in winter. Our craftsmen are currently somewhat puzzled.

I have attached a picture.

Best regards

Sounds somehow like moisture under the screed, rising up in the distribution box.

And then only on the right? We still want to check the sewage pipe in the guest bathroom. We have no obvious problems with water from the outside

Would I suspect if it smells like it’s coming from the distribution box? Is there a chance to reach inside with a cloth or paper to check if something is damp? The screed is open to the base plate there, maybe there is something in between after all.