Two different seals sit in the edge of a fire door and they do opposite jobs. The intumescent strip does nothing at all until the fire is well developed, then expands to seal the gap. The smoke seal, usually a brush or a fin, works from the first minute, when smoke is what is actually killing people.
The intumescent strip
A strip of material set into a groove in the door edge or the frame that expands many times its own volume when it reaches a temperature typical of a developing fire, closing the gap between leaf and frame so hot gases cannot pass. Until that temperature is reached it is inert. That is why a strip can look perfect and still have been painted, filled or compressed into uselessness.
The smoke seal
Usually a brush or a flexible fin, often combined with the intumescent strip in one product, which closes the gap against cold smoke from the outset. Smoke moves ahead of fire and is what makes escape routes unusable, so on doors protecting escape routes the smoke seal is doing the work long before the intumescent strip has warmed up at all.
Drop down seals
The gap under a door cannot take a fixed brush without the door fouling the floor, so a drop down seal, a mechanism in the bottom edge that lowers as the door closes, is used where a threshold seal is required. They need the door to close fully to operate, they can be knocked out of adjustment, and a seal that no longer reaches the floor is a common finding that a visual check from standing height will miss entirely.
What ruins seals
Paint. Filler. Sealant used to make a gap look tidy. Replacement seals of a different type from the one certificated for that door. Strips removed during redecoration and never refitted. All of these are common, all of them are invisible to anybody not looking closely, and all of them turn a certified fire door into a decorative one.
Questions people ask about fire door seals
What is the difference between intumescent and smoke seals?
The intumescent strip expands in heat to seal the gap during a developed fire. The smoke seal, usually a brush or fin, blocks cold smoke from the start. Many products combine both.
Can fire door seals be painted?
No. Paint can prevent the strip expanding properly and is one of the most common reasons a certified door fails an inspection. Seals are masked or removed and refitted during decoration.
Does every fire door need a drop down seal?
No. Whether a threshold seal is required depends on the door's certification and the gap under it. Where one is fitted it needs to be checked for adjustment, not just for presence.