and I end up installing them because they are a pain to install and I have an HVAC License and Fire Alarm training .But current job I am on the HVAC contractor insisted on installing them and did so against codes and regulations.
First off he installed them on the roof with out a reset station placed inside to be able to test and rest them if they activate as required by code. Which means crawling up on roof or going to basement and shutting off breaker. 2nd he installed them in these plastic enclosures again a code violation these units are not designed to be installed outside as it gets too cold to function properly in Pa. 3rd no matter how well you seal these boxes they eventually get moisture into them and will cause problem.If he had bothered to read the instructions he would have known this.
and that it violates several different codes. But because again this is one of those community's with no Fire marshal or other official to properly inspect things you just do what you want and send in letter saying its installed and tested.
Had the HVAC installer really wanted to put them on roof and do so properly they make a proper weather tight duct detector for the job.
But you can not tell this ignorant know it all HVAC installer.