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Lots of Hot Air and Gases

By Tim Krof

As winter approaches and the thermostat plummets below zero, we will all be depending on our furnaces to keep us warm and toasty.  The monthly costs to run this appliance will vary depending on outside temperature, thermostat settings and the efficiency of the unit.

We have no control over the outside temperature, but we can control our thermostats more efficiently by using a programmable thermostat to regulate the interior temperature when we are not at home during the day.  We can also keep those monthly utility costs low by purchasing high-efficiency furnaces when it comes time to replacing ‘Old Bertha’.

Furnaces have AFUE (Annual Fuel Utilization Efficiency) ratings that measure the efficiency of the furnaces ability to convert the natural gas into space heat, and are generally reflected as a percentage, like 80%, 90% or 95%.   This percentage indicates the amount of gas used to create the space heat in relation to the amount of gas supplied to make it.  In other words,  an 80% efficient gas furnace indicates the furnace converts 80% of the natural gas supplied to the unit into usable heat dispersed through the ductwork,  the other 20% is lost in the combustion gases from the conversion process.  So the higher the efficiency, the less gas that will be used to heat the home.

There are 2 types of gas furnaces, conventional and condensing.  Both types use gas fuel burned in heat exchangers, but they differ in how they use the combustion gases.

A conventional furnace will exhaust the flue gases fast and hot, before they have had a chance to cool off and condense, thus making the heat exchange process less efficient.

Condensing furnaces actually capture heat even after the combustion gases have cooled and condensed. They do this by using two heat exchangers, one for primary heat exchange and the other to handle the corrosive condensed exhaust gases of water and carbon dioxide (which form carbonic acid). The exhaust gases are depleted of heat until the water condensate drips out of the furnace’s heat exchanger and the flue gases escape from a plastic PVC pipe instead of a metal flue or chimney.  This process allows the furnace to convert over 90% of the gas supplied to the unit.


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