My post below didn't say anything about the IR. I suggested that the occurrence of the problem across multiple sensors suggests to me a wiring or brain fault. Period.
I didn't say the installer was "faulty", I said his troubleshooting was half-assed.
I detect an extreme lack of reading and compresension skills on your part. Go back and re-read my post and tell me where I said anything about the physics of infrared detection.
I'm not blaming the installer (who in this case is apparently NOT the ORIGINAL installer either) for the problem, I'm saying he didn't diagnose it properly and was likely more interested in simply selling the OP new hardware.
Believe it or not, this DOES happen.
Of course it is. The ensuing discussion is ostensibly about whether the bill he's been handed is legitimate.
My take is, the installer who's billing him has failed to correctly diagnose the problem and has merely treated the symptoms, and should not be paid for an incomplete job.
He's been sold equipment he probably doesn't NEED, and he's being billed for work that wasn't properly done (ie. determining and FIXING the problem).
In fact, the company I work for specializes more in CCTV, although we do do the odd alarm.
Experience has nothing to do with common sense. Common sense says that this installer has done a lousy job of troubleshooting, has apparently not found the actual cause of the problem and is merely patching up the symptoms, and from that may simply be interested in selling the OP new hardware (probably with a healthy markup) as a "quick fix".