Water heater flame "rollout" on firing
I'm having central heating installed and the water heater vent tied in to the new flue. The heating guy, when he turned the gas back on and relit the water heater after installing the new vent pipe and a new supply valve, noticed that on starting up, there was a several second hesitation and then the flame "rolled out" forcefully, flashing momentarily out the opening that he had uncovered to view what was going on.
He says that is not normal, he's not a water heater guy, but that maybe the burners are dirty or something and I probably need a new water heater. He doesn't think it's a drafting issue (the flue is not connected to the vent yet), because it's just on startup.
What do you guys think are causes to be considered? Am I probably looking at a new water heater? This one is probably about 10 years old, has a bathtub ring 8" off the floor and peeling paint from previous basement flooding (doesn't flood any more), but has otherwise functioned OK since I bought the house a year and a half ago.