NFPA 72 forbids detectors too close to the corner where the ceiling meets the wall. I have seen the reason for this in fire models- coming across the ceiling, the heat curves down before it hits the wall, and the reverse- coming up the wall, it curves out before it hits the ceiling. Instead of a model, imagine a stream in a park, fabricated for a rubber duck race. If there was a 90 degree turn, some ducks would get stuck, and move erratically in the corner. Just like the stream is full of water, the corner is full of heat, but it is 'stagnant', so activation is delayed. For water curtains NFPA 13 requires 'detectors' where 72 forbids them. (The draft-stop acts like a wall). If the fire is under the draft-stop, the above reverse flow is not as bad, but in this case, half the fire is under the opening, and although the head might not cover all the floor under an opening, a full pattern is always twice as good as half. The activation difference is only a few seconds but I can see a fire fighters OK dream turn into a nightmare over a few seconds! I think draft-stops should go the way of "heat collectors", which BTW, would work if we turned them vertical, extended them from the deflector to the bottom of the deck, and called them "heat diverters" :-). We have the same 'upper corner problem' at beams, and the MUCH wider application-sidewall heads. Sorry for the non question/answer post- I guess I just needed to talk it out.
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