hydronic radiant heating leak

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
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If you think you have a leak, turn off the fill and check the pressure gauge often. If it is a leak, you'll lose pressure.
You need to get a professional leak detection company out to find the leak. Leakbusters comes to mind....

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Choreboy wrote:

Regular auto antifreeze is put in heating systems that run "outside". This happens when a radiant system (copper pipes in a concrete slab) develops leaks and is replaced by baseboard (SlantFin) units. The runs go through the attic space which is unheated. With a setback thermostat you have to worry about it freezing, so antifreeze is used.
I imagine that a variety of odors might eminate from the antifreeze, but ethylene glycol would be the dominant one.
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