We offer both two wire and four wire type smoke detectors in a variety of popular brands. All of the hardwire alarm systems we sell can accept either 2 wire or 4 wire type, but there are some things to keep in mind when making this decision. A couple of things are:
1. Four wire type can be zoned out by area, 2 wire type cannot and all must be ran on one loop.
2. Generally speaking, you are limited to a maximum of about 8 two wire smokes per system (exact amount is determined by loop resistance), four wire type allow you to have many times this amount, only limited by the current available from the board or power supply.
3. Two wire type offer full supervision of the line between the panel and smoke, whereas four wire type only supervise the detection circuit by default. Two wire type are the latest technology, however four wire type are the universal standard and should be used when in doubt.