High Efficiency Furnace Exhaust Temperature

Differences between 80 and 95 furnaces.

High efficiency furnace exhaust temperature.

You can capture that heat with a high efficiency condensing furnace. Such furnaces cost more to run since they fail to capitalize on a significant amount of energy. When this temperature starts to exceed 130 degree f it is usually an indication that there is a problem. 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.

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. High efficiency furnaces use draft inducers. What is vented is a residual corrosive condensate of water and carbon dioxide. Venting for high efficiency gas furnaces materials.

As a result natural draft furnaces struggle to reach efficiency levels above 70 percent. Even when designed correctly a natural draft flue system has certain limitations in terms of drafting speed. When outdoor temperatures drop into the 20 s or below. High efficiency furnaces offer 90 or more afue.

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 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. This usually equates to a median pipe material temperature value of 86 to 100 degree f. A high efficiency condensing furnace can achieve efficiency ratings of 90 or above.

It is classified as type bh venting. A standard efficiency furnace provides 80 annual fuel utilization efficiency afue. 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. A conventional furnace vents these combustion gases through a vertical exhaust system that is attached to the furnace.

These highly efficient furnaces have a heat exchanger and a condensing chamber that help maintain a considerably low temperature on the pipes. In a great majority of cases plastic piping is used to vent high efficiency equipment. A high efficiency furnace uses a different type of venting system because it extracts the heat that remains in those combustion gases before venting the resultant mixture of water and carbon dioxide. These days 95 afue is a common rating among new gas powered furnaces.

A higher efficiency furnace with 90 to 98 efficiency is significantly more efficient than an 80 efficiency furnace and should produce slightly cooler exhaust.