Gas Smell

Anyone have an idea on why my direct vent (baxi Luna HT380) upon its morning startup seems to exhaust a gas smell?  It happens every morning when someone takes the first shower of the day.  My research turns up a possibility of a delayed start...We had recently had an issue with getting no direct hot water and have had sensors replaced as well as cleaning out the heat exchanger with a flush.  But we still get a gas smell upon its first start up of the day.  Any help?? Or Ideas???

Thanks