manhole or inspection point
- is perhaps what you are looking for.

Hovmand & Nancy Andersen's Illustrated Building Dictionary suggests 'manhole' for a brønd, but not directly in connection with district heating.
They also suggest 'inspection point' for what I think you mean.

Just outside our house there is a small 'manhole' (not big enough to get into, but it gives access to the meter from just above where the district heating pipe enters the house.)
This is used for reading the meter and possibly other checks.

There is a similar 'brønd' with a stopcock for the ordinary water supply a few metres away, and in Danish I would call the district heating one a 'brønd' too.