You finally installed a brand new furnace and air conditioning system in your home and got rid of that old, energy-sucking system, congrats! Unfortunately, since it was installed, you’ve noticed that it has trouble making temperature and come to find that the furnace is overheating. How can this be? Let’s take a look at some reasons that this could happen.


If you installed the furnace by just matching the same BTU (British Thermal Units) as your old furnace, this may be your issue. This is because many low-efficiency furnaces were often oversized to give the contracting installer more revenue and because bigger is always better right? (Get your head out of the gutter) The correct answer is no. The correct size for your specific situation is always better. That means heat load calculations based on the size of the home, insulation ratings, window thickness, etc… Most cases will likely end up in replacing a furnace with a smaller, high-efficiency furnace. An oversized furnace can lead to overheating and inefficient heating. An oversized furnace will heat a home real quick and shut down, then have to start again rapidly over and over again. This will also cause premature failure of the unit. If the system is properly sized, then we need to move on to the next possible issue.


The ductwork that is improperly sized or installed can lead to airflow restrictions. The furnace above was installed by an inexperienced customer who didn’t consider airflow when designing this ductwork and plenum. The result was overheating almost immediately because the furnace couldn’t move the air fast enough through the supply and the unit would shut down on high limit. The same result can happen if your return air duct is undersized. In that case, the furnace would not be able to pull enough cold, fresh air through the unit, causing the heat exchanger to overheat and result in shutting down on high limit safety as well. If your ducts are unchanged and sized correctly, then we need to look at other options.


If you have any restrictions in your system, your new furnace will overheat. I recently had a client with a new system overheating and we found a restriction between the furnace and the supply. Once I got the evaporator coil door off, I spotted the reason right away. This particular coil was designed to work on a horizontal or vertical application and the horizontal drip pan was blocking airflow, causing a restriction and overheating (photo shown above). We removed the pan and the furnace regained normal operation.

Gas pressure

When a furnace is purchased from a manufacturer, it may not be tuned properly. If the gas valve pressure is too high, this can cause overheating. To adjust the pressure, you will need a manometer. This gauge measures inches of the water column and generally you will be looking for a 3.5″ water column. You can find your specific rating on the gas valve data sticker on the side of the gas valve. Do not try to adjust this pressure without a manometer as this can damage your new furnace.

Blower speed

Blower speeds are critical to the proper furnace and air conditioning operation. Incorrect blower speed could mean your furnace overheating and your air conditioner freezing up if not set according to the manufacturer’s spec. Each unit comes with a specified blower speed for heating and for cooling to get optimal airflow and performance from your system. You can find this information on the schematics for the furnace and in the furnace manual. You change blower speeds by changing the blower wire control board connected to the heat and cool terminal. Each color wire represents a different speed. If you have an ECM or variable-speed blower motor, this needs to be adjusted by programming the blower control.


Zoning is another issue that can cause overheating if not working correctly. Zoning dampers allow a home to be conditioned separately in different areas of the home from a single furnace and require a thermostat for each zone. The thermostat sends power to the actuator which then opens a damper and allows air to flow to that specific area of the home. When the temperature is satisfied, the thermostat kills power to the actuator and the damper closes. When the actuator fails to open when the thermostat calls for heat, this can cause the furnace to overheat because the hot air has nowhere to go.


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