Delayed ignition does not soot up a water heater. It soots up from constant burning with a lack of sufficient ventilation or flue draft. If the area does not have good ventilation, there will not be air for combustion. If there is not adequate positive draft in the flue, then the gases will not go to the outside of the structure and can stay in the firebox causing poor combustion. If you have exhaust fans, no firplace, and tight windows, then the flue may be the only place air can get into the house and therefore there is negative gas flow in the flue. These are some possibilities, there are others, but the water heater being defective is not one of them.