During my inspections rounds, I uncovered a situation not discussed before on this forum. The issue is replacing your old gas burner with a high efficiencies PVC vented units. Many times the old Gas burner and gas hot water heater shared the same vent into a masonry chimney . The gas hot water heater’s 3 or 4” vent alone will never draft correctly, in that setup. It used the existing burners heat to draft the chimney. Again the cross-sectional code applies. There is no way a 3” line is going to produce a positive draft in an 8/8 clay masonry chimney. That’s drafting into an area 7 times larger and to make matters worse most chimneys are exposed with 3 walls facing outside. If your installer had read his manual, It clearly states that the gas hot water heaters draft situation is the responsibility of the furnace installer. He has to address the situation. To prove my point there was a Carbon Monoxide sensor laying on top of boxes. No battery installed I asked the owner if she had a battery, which she did. Within a minute it began chirping like a canary. The Gas hot water heater was back drafting Carbon monoxides into the living space. There are 3 ways to correct this issue. #1. Run a full length 3 or 4” liner all the way to the top of the chimney, plate the top and attach a termination cap. #2. install an inline Draft inducer wired into the controls of the gas burner head. ( forcing or pushing the exhaust out) #3. Remove the gas hot water heater and install an electric one. This is another example of installers not reading the installation manuals, Not know the responsibilities of the code, or are being to lazy to complete the job correctly. Never having to address the issues because they did the job without a permit. Unfortunately have made the same mistake over and over. In emergency situations, furnaces \ boilers can be installed without a permit, providing they obtain one as expeditiously as possible. Most do not and John q public knows nothing till they read somewhere that they could be in a similar situation. If you have a setup like I just described, read the manual and call the installer to right the situation he should have done the first place. If you do not have a monoxide sensor, this is just one other reason you should have one.