As I was unable to correct the negative pressure causing my atmospherically vented natural gas water to back draft at start, I decided to replace it. The 15 year old AO Smith 40 gallon, 40K BTU water heater had absolutely no issues, except that it left our house hold yearning for more hot water.

Obviously a new atmospheric vented water heater was out as it would exhibit the same problem as the old one. What other options are there?

In a previous posting I mentioned running into an issue with our water heater failing to draft when all the exhaust fans and clothes dryer running.  I did some research and through testing I thought the entire issue could be resolved by adding more “make-up air”.   Thus I added a 6″ duct into our mechanical room.  This was in addition to our existing 5″. You would think that after having ~48″ sq. of open hole to the outside would mitigate any possible negative pressure, but you would be wrong.  The water heater still failed to draft in worst case CAZ testing. I called a local HVAC contractor to evaluate.  They spent 3 hours trying different things, including adding 3′ to our chimney, but no joy!

Over Thanksgiving I discovered a torrent of cold air pouring behind our shower in the basement and tracked it to a large opening in our attic. That same day I went and picked up some additional insulation and proceeded to close the opening. Problem solved right? Nope, instead I found out by sealing this rather large hole I made our home too tight. The problem is that in worst case testing (all exhaust fans running and the clothes dryer) a significant enough negative pressure was created which prevented our water heater from drafting correctly. If you have one or more atmospheric vented gas appliances, make sure to test after home improvements!