Why does my hot water direct vent heater reset light come on after I turn on the hot water?
It is often neccessary to repeat the ignition sequence several times, especially when the unit has been stored for a time. It takes a while for the gas to get to the burner.
If you have been tapping the unit outside to get results, this may be indicative of an electrical problem. Pull off the electrical terminals, one by one, and make sure they are tight and clean. You can crimp the connector a little with a pair of needle-nose pliers to tighten it up. Do each one individually and reconnect so that you don't mix them up.