Found furnace going through startup. Gas valve will not open. Gas valve has voltage to valve. Customer needs new gas valve. Also exhaust pipe is back-pitched. Also intake air for furnace and water heater are tied together this is a no no for safety, cannot do this. Per inspection of venting system for furnace and Navien heater and intake system I do not see a way to vent equipment per manufacture install book or city and state code. I would reinstall 80% furnace and standard heater.