The flue gas temperature will be around 160 F when the water heater is new if it is set to store water at 140 F. The flue gas temperatures can reach over 350 degrees F as scale builds up.

How do you know if your hot water heater is venting properly?

Quote from the video:
Quote from Youtube video: Into the vent and out the chimney. That's good that means we're venting if for any reason. You feel any warm or moist air or coming back at you that means this vent is not venting properly.