During a thermal infrared inspection today, among other things I was there to investigate, I thought this would be an interesting picture.
This is a thermal image of a high efficiency condensing gas furnace. It is the one you have seen with PVC tubing exiting the unit. They are terrific furnaces. The one pictured here is 6 years old, which makes it the fourth generation. This later generation has worked out all the kinks. They work great.
Many gas furnaces, like my own, have flues which exit through the roof. My furnace loses a lot of its heat through that flue. Of the heat remaining to send through my house, I am gaining only a portion of that. Therefore, a small portion of the dollar I am spending for natural gas is actually converted into heat for my house. My furnace is "85% efficient," but that does not mean I am getting $.85 worth of heat for every dollar I spend on gas.
This unit eliminates the need for the flue that exhausts through the roof because it captures and re-burns that exhaust! So for every dollar spent on natural gas, it is getting nearly a dollar's worth of heat. And the exhaust is so cool, a PVC tube can handle it and you could virtually blow dry your hair with it (well, except for that pesky carbon monoxide presence).
Another terrific advantage it offers is that it brings in fresh air every time the system turns on. It exchanges 10-12% of the air in the house as it works to heat the house. So it is not simply reheating the same stale air.
Today, the outdoor temperature was 56 degrees F during my inspection.
This furnace is working as intended. The purple line just to the left of center is the air intake tube. The vertical yellow line to its right is the exhaust tube. The purple intake is 57.6F, and the yellow exhaust 95.7F. That is really efficient! And pretty cool, too, so to speak...
My recommendation: If you have a gas furnace and are contemplating replacement, look into a high efficiency condensing gas furnace. They are more expensive, to be sure, but very, very efficient. You will notice the difference on your first gas bill!