I mainly deal with commercial refrigeration, but was called out for a fire alarm caused by an HVAC smoke detector. When I arrived the audible alarm had obviously gone off because the fire dept. Was on-site. I found that only one of five AH's smoke detector had tripped on the boiler sys. I removed the clear plastic cover and checked the filters, screen, and tubes; all were clean. The detectors are located in the supply air plenum three feet from each AH. After speaking with the customer I found out that this wasn't the first time that that exact unit had tripped. Any ideas on what else to troubleshoot would be greatly appreciated.