I had to put 9 of these into my house to get a building consent. I was pleasantly surprised how easy they are to install and connect together. The instructions were clear.
Had a good run with these, unlike a similar brand that kept bleeping after about 3 yrs.
I bought from PSL because they has reputable interconnected alarms at a good price. Interconnected means if one goes off, they all go off. That reduces the chance you don't hear an alarm in the living area or garage because there are a couple of shut doors in-between. The ionisation alarms in the house proper, connect to the heat alarm in the garage.