Compare two 1' x 1' fire dampers. One is German made:
and one is domestic:
The 90 minute German damper is mortared in to the fire separation that contains it. There is no gap around the outside of the damper. The mortar conducts operational heat into the surrounding structure. The domestic damper has to breathe around the outside. UL555 testing is conducted under negative pressure, which means that cool laboratory air is drawn past the system (thus cooling it down), and into the furnace. It's a clean test to witness, unlike some firestop tests, under positive pressure, where off-gasing can occur, depending on conditions. In the eighties, an enterprising indiviual in The Canadian Province of Alberta, found out about the damper-exterior gap, and saw an opportunity to expand firestop sales, by persuading consultants to mandate firestop caulking around the sheet metal angle lips surrounding each domestic fire damper. The sales pitch was to stop smoke migration. However, in so doing, he voided the UL55 damper listing, as SMACNA shows here: https://www.smacna.org/docs/default-source/technical-resources/improperfiredamperinstallation.pdf?sfvrsn=d121fda5_2 . If the sheet metal angles, which are to cover the gap and hold the damper in place whilst a weight is dropped onto the ductwork (part of the UL555 test procedure), are all caulked in, the no cooling air can rush past the damper into the furnace, to cool that damper down. If the sheet metal damper cannot be so cooled down, then reality no longer matches the test conditions, which means the listing is invalid. This means that when the damper is bounded (field installed per UL555-based listing), smoke migration around the outside of the damper can occur, when the portion of the engaged compartment, which contains the damper, is under positive pressure and immersed in smoke, resulting from a fire. This means that a code compliant damper only works as intended, provided the dampers in the field are under negative compartment pressure, which is difficult to guarantee, being that heat rises. Germans have a saying for this: "Operation: erfolgreich. Patient: tot" (Translation: Operation: successful. Patient: dead.) Here's another myth debunked: "Gotta have the drop test to prove that the dampers are not torn out when the duct reacts to fire!" In the case of UL555 dampers, that may be somewhat relevant, except for one thing: Only UL555 dampers have and need that gap, which keeps the dampers light (compare the weights and measures in the two aforementioned categories). But mortared firestops have been proven to hold moving cable trays under 3400N or 765 pounds of force by Otto-Graf-Institut in Stuttgart, in 1978, under project F 80 448/Tei/tr. I will predict that critics will say that this is not a representative comparison, as cable trays are not ducts. Yeah, the're actually tougher and generate more longitudinal force and firestop mortar defeated this force. So this is tougher and works anyway.