The only way to really be able to seal a house that has a gas-burning appliance on the inside, whether it is a water heater or a furnace, is to use one of the more modern high-efficiency furnaces that have a dedicated and sealed combustion air intake.
- These appliances have a (typically 2" PVC) supply and exhaust line, which is typically routed straight to the outside through a wall.
- All of the newer 95% efficient gas furnaces work that way, and high-efficiency water heaters, both in standard tank and tankless flavors, are available this way.
All high-efficiency gas-burning appliances are pricey though.
- The recommended tank water heater A.O. Smith Vertex costs nearly $2000 before rebates, and the tankless Rinnai heaters in high-efficiency are about $1200.
- This excludes installation costs, which tend to be much higher for tankless water heaters.
- High-efficiency gas furnaces can run between $3000-$5000 installed before rebates, depending on home size and your location.
You would probably rather look for a workaround how to seal the air duct but stay within code, but even if there would be such a way you would either cheat yourself or endanger yourself. If you desire to seal your house (which I fully support and which makes a lot of sense to save energy) you simply cannot have an oxygen-eating heating appliance inside. Not only do you not want to lower the oxygen content in your home, but also you risk that the appliance starts emitting carbon monoxide if supplied with insufficient oxygen.
If code would allow you to install some sort of one-way damper in the combustion air duct that would allow air in but not out, you would simply be cheating yourself with the blower door test results. The test measures air leakage from inside to outside because this is the simplest way to do it and it is assumed that air leakage of a house is the same in or out. Improving your results in the test by limiting air leakage from inside to outside doesn't mean your house seals better in real world performance.