September 14, 2013

HVAC: Condensing Furnace

Furnace Diagram
A furnace provides heat through the exchange of heat through a intermediary fluid movement such as air, steam or hot water. A furnace can use different fuel including natural gas, LPG, oil, coal or wood. Combustion furnaces always need to be vented outside. Condensing furnaces are essentially high-efficiency furnaces with a AFUE (Annual Fuel Utilization Efficiency) starting at 90% or greater, verses mid-efficiency furnaces with a AFUE in the 80's. This difference in efficiency is the result of heat lost to the flue, or the loss of of some of the heat produced during combustion.

A condensing furnace takes the exhaust and puts it through a second heat exchanger where the furnace takes additional heat out. The furnace pulls so much heat out that the water in the exhaust condenses, thus giving it its name. The small amount of water also contains some chemicals, is a little acidic, and can be drained into the home's sewer. Furthermore, since the exhaust is no longer hot (very little heat is now lost to the flue), the flue pipe can be Schedule 40 PVC verses a metal vent pipe. Additional PVC piping brings in fresh air from the outside of the home directly to the furnace so no heated air from the home is vented outside, helping to maximize efficiency.

Air Flow Diagram

Condensing Furnace Implementation Diagram

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