Since the mid 1980’s high efficiency furnaces have seen a popular growth in part due to government requirements to reduce energy cost and green house gases.
The early models of high efficiency furnaces required the furnace have both an exhaust and a single combustion air intake as a requirement of installation. Late models through the 90’s until present allow for the furnace to have either a single pipe exhaust or a two-pipe system described above. 
The addition of a combustion air intake to a system doesn’t improve the efficiency of the furnace or degrade the efficiency when not use, however the home efficiency will be improved by two piping the system and we will explain below both the Pro’s and Con’s.
What is an Exhaust Vent?
Much like your car exhaust the exhaust vent on the furnace removes the harmful combustion (Carbon Monoxide) products from the furnace and exhausts them to the outside, where they are diluted by the open air.
What is a Combustion Air Vent?
Again, much like your car engine air intake, the furnace needs air to burn with the natural gas and propane to burn properly and efficiently. So, this pipe allows exterior air to travel into the furnace to be use for the combustion process.
Can we install a in-take vent?
All Vents (exhaust or in-take) require manufacturer or regulatory distances from windows, doors, intakes and exhaust to other appliance etc. This will impact the decision to use a two-pipe venting system or not firstly. Most home furnaces will be able to “exhaust” vent pretty easily, but sometimes the “in-take” vent installation requirements can not be met, this is common on many town homes and row housing where there is only one or two exterior walls and many items are already exhausting to the exterior.
The nature of a “in-take” vent means it moves air from one location to furnace so this air can not be toxic or explosive, keeping it away from gas meters, car exhaust and other fossil burning appliance are just a few of the requirement your heating contractor will review to determine if two pipe venting is possible.
Finally, we get to the Pro’s and Con’s
So, you are good to go with a two pipe system, “but if its doesn’t impact furnace operation then why the added expense and another hole in the wall you ask?
Pro’s
- A more reliable system. Using free fresh air from outside ensures the furnace is getting good clean air to burn and will help ensure a good flame. It has been proven that flame sensing via flame rods stay cleaner when a two-pipe system is implemented. (Some homes with single pipe venting can see flame rod fouling more than once a season meaning more down time and annoying outages)
- A more efficient home. When using a two pipe system the air used to burn is free, on a single pipe system you are using air from the interior of the home and it must be replaced by air entering the home (air exchange) through cracks, windows and doors etc, this is cooler often low humidity untreated air that now needs to be warmed up. When two-pipes are implemented the air entering the home doesn’t need to be replaced by the air leaving the exhaust. Keeping the warmer air in and costing less.
- A Higher humidity levels. Less Air exchanges from a two-pipe system means less colder and low humidity air entering the building keeping humidity levels higher.
Con’s
- Another hole in the wall. Yes, believe it or not many people hate seeing more thing sticking out of their home and when the vents are located in a visible area they can be unsightly.
- A Colder Basement. With the two-pipe system implement there are now fewer air exchanges happening within the basement area. This can often make a basement feel cooler the simple solution is to add addition seasonal return air, but that adds some cost. In our experience this is not an overly common concern, but it has arisen and should be discussed as part of the process.
- Stale Air. This is where the fewer air exchanges hits home again, because the home is seeing fewer air exchanges per hour compared to a single pipe system sometimes the air in the home can feel “stuffy” because of the lack of fresh air. A balance to this is installing a Heat Recovery Ventilator (HRV), again this doesn’t appear often but should be discussed on some new homes that already have high air sealing done.
- Blocked vent. As we are now using an air in-take this pipe can be prone to blockage from snow, pest or child play. Unlike a single vented system where these rarely become an issue, air in-takes should be installed at a minimum of 12” above the anticipated snow height for your area, here in Niagara 9-10” so the in-take should be a minimum 22” above grade, even at this height drifting snow can come into play. Most intakes have screens installed which prevents most pest entry, but the occasion hornet nest may be found. When the screen is either not installed, fallen out or installed down by the furnace then children can place small items into the vent, yes it happens we have pulled all kinds of things from vents.
Our conclusion, we feel the gold standard is to always install the two-pipe system when possible.