Our 18 year old Firex smoke alarms are due to be replaced (we have changed the batteries at least once a year all that time.) The plug in to the hard wire is different now. Is there a replacement smoke detector that we can get that will be compatible with the 18 yr. old hard wires? Your prompt help will be very much appreciated. Jann Tilzey, Sparks, NV janntilzey@gmail.com Thanks!