Gas water heaters use a special flue connection, called a draft hood, that allows indoor or surrounding air to induce a draft up the flue increasing speed and efficiency of the exhaust of the combustion gasses.
That is the theory anyway.
If the flue is installed properly it should draft properly but there can be problems that can be hidden that can cause problems with proper draw.
When the draft hood does not function properly you get what is called backdrafting. This is where combustion air would rather spill back into the home or garage instead of getting effectively sucked up the chimney. This is potentially dangerous because sometimes gas appliances do not burn as clean as they should and monoxide is a by-product of improper combustion.
On a recent home inspection I noted issues with draft on the water heater. The combustion air was spilling into the home. So much so the plastic trim on the water pipes were melted! I suggested that this was potentially dangerous and a licensed professional plumbing/heating and air contractor repair as necessary.
Several weeks later I was asked by my client to re-inspect the work that was done. Everything looked great until I got to the water heater draft hood. No change to the back drafting condition could be noted.
One of the problems with repairs done by the seller is that they usually want to meet the agreed upon conditions for the least amount of money as possible!
A week later I was called back to inspect the back draft once again. This time I met the heating and air contractor who was involved in the repair. He told me how he began to fix sections of the flue and he kept discovering problems. He ended up replacing the entire flue all to way up to the roof line due to the deteriorated, unlined masonry chimney that was at the root of all of the issues.