Variables when using existing equipment
We would prefer to install new equipment with new wire when at all possible although sometimes the owner or the job calls for us to reuse existing equipment.
When using existing cable we cannot guarantee that there are not shorts or ground faults on the wire. If there are trouble shooting will be required and depending on the building often it is less expensive to install new wire right from the beginning.
When trying to use existing equipment such as door contacts or motions it is best to not only tone out the cable to make sure but also power up each device that is powered and test each contact, glass break, motion, panic, keypad etc….
When using existing camera wires such as the older installations with RG59 and 18g2c cable we can install new specialized equipment with high definition cameras that will use the older cable. RG59 will allow much longer runs than CAT6 or CAT5 cable for the high definition cameras.
Know that technology has changed since your old equipment was installed because now we have TRI TECH motion detectors that have modern technology to help reduce false alarms. Changing out just the headend and reusing the equipment in the field may sound fine and may seem like you are saving money by doing so although if there are false alarm issues that savings may not be what you thought it was.
When trying to reuse fire alarm equipment understand that many of the old horn/strobes were 12 volt and now almost everything is 24 volt and most of the time the old horn/strobes will not sync with the new ones and many of the old ones had a higher current draw and the strobe light was very different and many of the old appliances do not meet current code. Smoke detectors normally should be changed after 10 years, if you have been cleaning them every six months as per the manufactures recommendations you may get a longer life out of them although technology is improving and the new smoke detectors have many more features than the old ones did.
Call an expert and take a good look at the existing equipment to see what should be reused if anything and compare the cost of hardwired verses wireless and make a decision based on the expert opinion of an expert considering your budget.