Ok, so I have to run two 2″ pvc pipes for my new ac/heating system. Can i use the chimney that is already there the heat water is venting throw it now..? If it make a differance I live in Philadelphia, PA.
Unfortunately you cannot use your existing, still used for water heater venting chimney as a chase to run PVC vent pipes. Hot exhaust fumes from your water heater could melt furnace’s PVC pipe. In case you’d have an unused flue inside the chimney walls (for example one used for the water heater and second one with nothing connected to it) you could run those PVC pipes inside it.
I’m assuming that you’ve upgraded your regular furnace that utilized chimney for venting with a high efficiency one. If so I’d recommend to check this post – http://checkthishouse.com/1887/gas-water-heater-and-furnace-chimney-flue-sizing.html – to make sure that it is not oversized to handle single water heater.