For several years my sister has had radiant heating using polybutylene
in concrete downstairs and copper behind baseboards upstairs. Lately
she has been smelling antifreeze upstairs and in the vestibule at the
foot of the stairs.

Is it ethylene glycol that makes antifreeze smell? That molecule is
twice as heavy as air, which makes me think the leak is upstairs. An
antifreeze leak can be smelled long before there is enough moisture to
see. What's a good way to locate it?

Her husband thinks the leak is downstairs near the pressure-relief
valve. She smells other odors there, but not antifreeze. If her
husband is not sensitive to the odor of antifreeze, that explains it.
Do some people tend not to notice the smell of antifreeze?

Choreboy