High Efficiency Furnace Exhaust Temperature

Due to the condensing nature of a high efficiency furnace its venting must be made of a material that is resistant to corrosion.

High efficiency furnace exhaust temperature.

Even when designed correctly a natural draft flue system has certain limitations in terms of drafting speed. These days 95 afue is a common rating among new gas powered furnaces. Differences between 80 and 95 furnaces. In a great majority of cases plastic piping is used to vent high efficiency equipment.

It is classified as type bh venting. A high efficiency condensing furnace can achieve efficiency ratings of 90 or above. When outdoor temperatures drop into the 20 s or below. It uses a venting system to extract that heat before venting.

To make it simple the typical combustion gas temperature for a 90 efficient furnace should be in the 100 to 130 degree f range. High efficiency furnaces use draft inducers. In this type of furnace 20 of the energy produced by natural gas is expelled as exhaust. When this temperature starts to exceed 130 degree f it is usually an indication that there is a problem.

These highly efficient furnaces have a heat exchanger and a condensing chamber that help maintain a considerably low temperature on the pipes. What is vented is a residual corrosive condensate of water and carbon dioxide. This usually equates to a median pipe material temperature value of 86 to 100 degree f. The high temperature of flue gases is a major concern to me due to the fact that schedule 40 pvc pipe and fittings are only rated for 140 degrees fahrenheit.

A conventional furnace vents these combustion gases through a vertical exhaust system that is attached to the furnace. Venting for high efficiency gas furnaces materials. As a result natural draft furnaces struggle to reach efficiency levels above 70 percent. Conventional low efficiency furnaces have exhaust temperatures commonly in the many hundreds of degrees and conditioned air exit temps at the up to 250 300 degree range at times whereas high efficiency furnaces extract more of the heat from the combustion so the exit temperature is much lower to the point that in very high efficiency furnaces the exiting combustion gases and conditioned air may both be around 100 150 degrees.

A standard efficiency furnace provides 80 annual fuel utilization efficiency afue. You can capture that heat with a high efficiency condensing furnace. I think we in new england are going to see pvc failures in the near future directly related to the high flue gas temperatures of the new high efficiency boilers. High efficiency furnaces offer 90 or more afue.

High efficiency or condensing furnaces exhaust the combustion gases and bring in fresh air through pvc pipes to the outside usually routed through the sidewall of the house.