I searched the board, and saw no posts about this.

I have a 25-year-old house, with AC wired smoke detectors and (what I assume to be) heat detectors located in pairs around the house (5 of each). One of the smoke detectors was false alarming (even though I vacuumed it, etc.) and since they looked pretty ancient, I went through the house and replaced all the smoke detectors with wired units with battery backups. I alternated around with combo ionization/carbon monoxide detectors and photoelectric detectors. So, all's good there - I know they're new, they look nice, and I know that they work.

Now I still have these 5 ancient (and f-ugly) heat detectors, that I'm not sure even work.

How important is it to have heat detectors, especially if each one is sitting right next to a smoke detector anyway? Should I A) replace them (at ~$25/ea) or B) remove them, cap off the wires in the junction boxes, and make a cosmetically acceptable repair (a flat plate on the wall to cover the holes or repair the drywall).

On a related note, it looks like there's one sprinkler head in the ceiling of the kitchen (it looks like a little round button about 1" around and 1" tall - no spindly things sticking out of it though. Looks like maybe something pops out of it?), but I have no idea how this is hooked up and I see no sensor near it. How do I figure out what this really is and if/how it works? Do these sense by themselves and then start spouting water, or do they get a signal from somewhere else?