That's enough time to crack the base on a cold start. Using the ashpan door causes a forge-like condition to develop at the grate. The sudden heat up in the center while the outer edges of the base are cold can crack it due to the differing rates of expansion. With a top down start air at the base of the fire will be irrelevant for starting purposes.Yes, I do understand the damage it could do if left unattended. Therefore I always set a timer on Alexa or my phone to remind me to return in 10 minutes to check on it and close the door.