With these higher efficiency furnaces, came some other considerations. The 90% implies that 90% of the heat that is put into the furnace is transferred into the duct system and subsequently into the homes living space leaving only 10% going out the exhaust.
Below is brief explanation of how a high-efficiency furnace works and the difference in the exhaust, but for those to just don’t care, here is a PDF put out by Bryant that gives more information on the proper venting configuration for a high-efficiency furnace. There are many other approved configurations, but I am of the opinion that the concentric vent is among the best.
This higher efficiency is accomplished is by using a principle similar to that used in the air conditioning mode. Energy transfer takes place when ever a liquid or gas changes state. In the AC system, the refrigerant “flashes” from a liquid to a gas at the “evaporator” unit and it requires energy in the form of heat to do so. As the house air, forced by a blower, flows across the coil, heat is absorbed into evaporator and is then taken out to the “condenser” unit by the refrigerant, where it turns from a gas back into a liquid. As it turns back into a liquid, it releases the energy that it previously consumed in the form of heat.
Now back to the furnace. . . The same principles hold true here but are being used a little differently. A lower efficiency gas furnace creates water vapor in the form of steam that goes out the exhaust. The Higher efficiency furnaces condense the steam forcing it to release the energy that it consumed when turning from liquid into gas and in so doing, add more heat to the house instead of sending it out the exhaust.
Where does this steam come from? The combustion takes place when natural gas, which is actually methane, is mixed with oxygen in the combustion air.
In other words, CH4 + 2 O2 = 2 H2O + CO2. The = sign is where the combustion takes place and the result is heat, plus water vapor and CO2. A high-efficiency furnace captures the energy within the water vapor by harnessing the latent heat of vaporization. In other words, it forces this water vapor to condense into a liquid and as it does, it is forced to release the heat that was required to keep it a vapor.
For this reason, the high-efficiency furnaces are sometimes referred to as “condensing gas furnaces” as that is what separates them from the previous configurations. With this in mind, there is no longer the need for exhausting through a B-vent but instead the exhaust can be directly through PVC pipe. One of the reason that PVC is used is due to the fact that water that can collect in the pipe and if it were metal, the chances are good that it would corrode the pipe as this moisture/water is thought to be somewhat acidic and corrosive.
For this reason, it is important that the vent line has a constant slope back to the furnace so this water can drain into the collection system of the furnace. If as a low spot and water collects enough to block the air flow through the pipe, the furnace will not light. So this can be an easy first check if you hear the furnace try to light but doesn’t. You will here the “induced” air fan turn on but then it will turn off without igniting the furnace. It checks for any excess back pressure in the exhaust prior to ignition and if it senses that the exhaust is obstructed, it will not ignite.
In any event, this is not meant to make HVAC techs out of home owners, but to help them understand basically how the system works and how the install should look.
Here is a picture of an alternate venting configuration. The exhaust is open at the front and the combustion air intake opens to the bottom. This is to prevent anything from dropping into the intake line.
Note that this appears to be close to ground level, indicating that this unit is probably installed in a basement.
If the system is installed using the single-pipe method, the combustion intake air should still have something that resembles this configuration close to the unit.
There are many benefits to using a two pipe installation but installers wanting to cut cost, usually try to just do a single-pipe install.
Here is a video advertising a concentric vent, but it shows how the vent works and installs.