When a furnace maintenance is performed a combustion analysis should be performed to identify any problems that couldn't be found by a visual inspection. You want to know what the Carbon monoxide level your furnace is producing as well as the oxygen/CO2 levels and stack temperate. The analyzer will also give you a calculated efficiency rating with the information the analyzer receives. You should have >100ppm of Carbon monoxide output during normal operation of your gas furnace. When you have higher numbers or CO numbers increasing during operation their is a problem with the furnace the technician should then find the cause of. This is just a brief explanation I'm trying to get across to those who get their furnace maintained and to those who don't! Check to see if anyone in the company has a NCI certification for carbon monoxide/combustion analysis.
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Finally, don't underestimate the thermostat as a potential reason why your furnace is not working. Everything on the heating unit itself may function perfectly, but your home still will not heat right if the console you use to set the temperature doesn't communicate your input. A faulty thermostat can either be due to the wall unit itself, or the wiring that connects it to your furnace. Average thermostat repair costs are between $108 and $282, including labor. Replacement costs, of course, depend on the thermostat you choose. Visit our thermostat repair cost guide to learn more about this type of issue.