The efficiency of a 90+ AFUE condensing furnace is not affected by obtaining the combustion air needed from the outside of the house.
However the overall efficiency of the house is. Very few people realize that a certain volume of air is required for proper combustion of the furnace and gas water heater for that matter, and not many contractors pay attention to this simple code requirement, especially when replacing old furnaces.
Check out Chapter 17 of the residential building code. The formula is pretty simple; you need 50 cubic feet of free area for every 1000 Btu/h of input for your gas furnace and water heater.
- So as an example if you have a 60,000 Btu/h furnace and 40,000 Btu/h water heater you need 5000 cubic feet of free area. [(100,000Btu/h / 1000) x 50 = 5000 cubic feet]
- So with the average Ohio basement height of 7' that is an empty 714 square foot room to allow for proper combustion.
By drawing fresh air from the outside directly into the furnace cabinet you are removing the volume of space inside the home needed for proper combustion. As a result by drawing fresh air from the outside to the furnace you ARE increasing the overall efficiency of your home because you do not need that volume of air in the house for combustion and you are reducing infiltration into the home because of that. The furnace is now a closed loop system pulling air from the outside and then sending it back out with the combustion gases.
On the other hand, if you were using the inside volume of air in the house for combustion, it would be pulling it into the furnace and sending it right outside. For every Cubic Feet per Minute (CFM) of air pulled out of your house by the furnace, a CFM of air needs to be replaced.
Therefore to sum everything up, no outside air does not affect the efficiency of the furnace, but it does affect the efficiency of the home.