Well, I witnessed it today. I don't wire it this way but I did a takeover for a friend. House full of ESL 2-wire smokes with sounders. Started testing and the first one goes temporal and the rest kick in steady. Then I test the heat detector in the kitchen, which is on the same circuit, dead short so no sounders and all it does is place the zone in trouble. Even better the trouble report is not enabled. So here it goes, if the fire starts in the kitchen, nobody gets woke up and the smokes and sounders don't work any more. Congratulations, your all dead. Now I have to explain to the home owner how screwed up his wiring, that I can't get back to, is. This is best example I can give to show why we don't mix heats and smokes on a two wire powered smoke loop.
Im actually surprised I didnt toast my Elk main board.....I had the mechanical heat detectors on the same loop as electronic smokes. I didnt realize exactly what the RRSMOD (reversing relay) would actually do once it kicked in, reversing power across everything. The heat detectors could create a short and blow up power supplies, fuses, or maybe just trip a PTC. Looks like Elk builds their stuff for newbies like me, because its still all up and running.
I have the heat-only detectors on the other 2 conductors of the 18/4 now, running to a separate zone on the board but defined as a 4 fire wire so my "if any fire zone goes into alarm, then turn on output 3" rule fires to get the RRSMOD to sound the sounders in the smokes. I need to test this all of course, but with a mechanical heat detector not sure I have an easy way of making sure all I do is trip the rate of rise and not the heat portion (since only the RoR portion is resettable)