I never knew that. Is this stated in the manual that is in each detector box?
When I stated this I was not clear. They are only rated for single family dwellings. These dwellings can be in a multifamily building but the interconnected units can only be in a single family dwelling. This is an example from a BKR manual.
Compatible Interconnected Units
Interconnect units within a single family residence only. Otherwise allhouseholds will experience unwanted alarms when you test any unitin the series. Interconnected units will only work if they are wired tocompatible units and all requirements are met. This unit is designed tobe compatible with:
BRK Electronics? Smoke Alarm Models 9120,9120B, SC9120B, 7010, 7010B, 4120, 4120B, 4120SB, 4919, 2002RAC,100S, 5919, 5919TH; BRK Electronics? Heat Alarm Models HD6135F,HD6135FB; BRK Electronics? CO Alarm Models CO5120BN,CO5120PDBN; Smoke/CO Alarm Model SC6120B, SC7010BV,SC7010B; and First Alert? Smoke Alarm Models SA4120, SA4120B,
SA4121B, SA4919B, SA100B.