The most important and urgent answer might not even cross your mind…

Most people that get regular maintenance on their furnace think that they are just making sure that the furnace lasts longer and gets them through the heating season, but there is a much more important aspect of regular maintenance that needs to be checked.

Furnaces generally like to have air flow.  Furnaces need more than just a clean filter, although that is important; a furnace can work much harder, can get dangerously hot, breach the heat exchanger and suffer a shorter life when it doesn’t get the airflow it needs.  This shorter life includes overheating the metal inside the furnace making it weak, prone to leaks of Carbon Monoxide and potentially fire from overheating.

The heat exchanger is a chamber in the furnace where the gas combustion occurs. The metal chamber conducts heat and when the fan is running it takes that heat and warms the air flowing over it.

Besides the air filter can you guess what can impede the air flow?

If you have ever closed off a register to force more airflow to other rooms it is one way of controlling the comfort in your home and it is one of the functions of the register to an extent.  If you close more than 20% of your registers you are potentially making it a dangerous high heat situation, damaging your heat exchanger and possibly over working your furnace.

Some homes have the grill to return the air to be conditioned (heated or cooled) up high near or on the ceiling and some have it at the floor level.  When located at the floor level, some of our furry friends like to lean next to this return grill. Sometimes this is a filter grill with a filter in it and sometimes it is just a grill.

Were you wondering?
I use the terms grill and register.  The grill does not have a lever on it to close it off and a register does have that lever to control air flow.

We have seen this many times where the fluffiest pets like to scratch themselves on this low return vent or maybe just leaning on it.  Unfortunately, whether there is a filter in this grill or not the blockage of the physical cat or dog leaning on the grill, the pet hair build-up on the grill or filter blocking air flow all affect the airflow to the furnace.

Blocked ductwork, or flue also affects the airflow. That needs to be checked with a special duct and/or flue inspection. To recap things that affect airflow are dirty filter(s), blocked or dirty return grills, more than 20% of the registers are closed off, blocked ductwork, blocked flue…

Pro tip:  NEVER EVER agree to have someone bypass a safety on your furnace, no matter how cold it is.  Staying the night in a hotel and if at freezing temps have a slight trickle out of the faucets to prevent them from freezing, until the furnace is fixed is much wiser.

Sometimes it is necessary to look inside the heat exchanger.

You might wonder what I’m referring to.  The chamber that helps heat your home can be accessed during the maintenance appointment.  The HVAC technician uses a camera to see inside the heat exchanger to assess the condition.  Not this inspection shows about 20% of the exchanger but an educated guess can be drawn if the heat exchanger is rusty, pitted or worse, cracked or have a hole in it.  These things bring the combustion gas into your home and it is potentially irritating to deadly.  If you have a carbon monoxide detector it can help, but most manufacturers request that the CO detectors are changed every 5 years. They work on cumulative exposure but should not be relied upon when there is known exposure.

When we looked inside this heat exchanger we saw rust, how bad the rust was we didn’t now until we removed it.  This is a dangerous furnace.

Please call a licensed HVAC contractor to check out your furnace. Your life may depend on it.

  • Cold Craft, Inc. License # 631837
  • Coldcraft.com
  • 408.374.7292
  • Silicon Valley Area
By Published On: December 19, 2022

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