Heat Exchanger When I purchased my current house it came with a fairly new furnace. Unfortunately this was a mid efficiency furnace so it wastes fuel. I don’t know why someone wouldn’t install a furnace of best efficiency other than cost but regardless it was done and I needed to do something about it. I didn’t want to replace this furnace as it was almost new so I decided to increase efficiency. New high efficiency furnaces are designed with multiple heat exchanging stages. Air is pushed by a air pump into the burner chamber where it is heated up. It than passes through each stage transferring a certain percentage of heat from the burner side to the fresh air side. A mid efficiency furnace like my with only one heat exchanger allows a lot of this heat to escape up the chimney. The setup in my house is that I have a chimney liner and my furnace has a air pump forcing air through the system. This system is sealed and all fumes are forced up the chimney. The design I came up with is an in series system that replaces a portion of the external exhaust line between the furnace and chimney. Fresh air is brought in from the outside preheated by the heat exchanger and injected into the cold air return just ahead of the air filter. Moisture that builds up in the heat exchanger during operations runs to the base of the exchanger closest to the furnace where it then drains to the furnaces waste water pump while the exhaust flows up the chimney. The difference in temperature on the fresh air side between both ends of the heat exchanger is quite good. During our cold Canadian winters the air temperature has dropped bellow -30dC where it felt like -40dC or there about. With those temperatures the fresh air out of the heat exchanger would still be approximately 20-40dC warmer depending on the outside temperature when it went in. A side benifit to this is that I always have fresh air pumped into the house. The design of the heat exchanger is quite simple. The main chamber comprises of an outer shell that has a thicker wall thickness than the internal heat exchanger with a series of tubes running down the length of the main chamber. The total area of the ID of all the tubes is close to double to that of the area that the furnace exhaust system came with. This should theoretically slow the flow of hot exhaust giving more time for the heat exchanger to work and warm up the fresh cold air.