DIY & Home Improvement

They paint a liquid over the joints in the heating system, which then dries to a solid compound. It's also frequently used to seal the boot (the part of the vent behind the drywall) to the wall.

If it is duct sealant, the worst effect would be a slight reduction in the efficiency of your heating system - a little bit of air could leak from the boot into the space behind the drywall, instead of being pushed into the room. If this is the case, it's a very small problem that's not worth worrying about. Since it's falling off, the installer probably used too much and what remains on the boot may keep it sealed. If there is a leak, it would be very small.

Another candidate is drywall compound or plaster, but that usually falls off as a large chunk or as a powder - meaning a powder before you touch it.

But back when I built houses we left the HVAC work to our really good HVAC contractor, so I could be wrong.