Furnace Destroyer #3 – The Filter

Heat Exchanger Destroyer #3

By: Steve Holland


The dirty or clogged air filter.

End of blog. Jk


Alright, I’ll give you some helpful information. The dirty air filter is the one of the three main causes of no heat calls and no cooling calls across the United States. The filter is designed to keep dirt and debris from entering the mechanical components of your heating and cooling system.

On a normal cycle, your air conditioner will move 400 cubic feet of air per minute through the furnace. On a heating cycle this number is slightly less and will vary if you have a variable speed blower. Regardless, this is a whole lot of wind! All of this air contains dust that will eventually plug the surface of the air filter.

Once you start restricting air flow the furnaces heat exchanger will see higher temperatures resulting in failure. The blower in your furnace is designed to serve two purposes:

  1. Transfer the heated energy from the furnace to the home via the duct system
  2. Sub cool the heat exchanger so that the furnace does not overheat

Here is the simplest way to understand how a filter will destroy a furnace.

By design, the furnace or air handler will have a certain engineered air flow requirement. When you restrict the air flow, you also allow higher temperature and heat stress to develop in areas of the heat exchanger. This continued stress will destroy a furnace.

Dirty or plugged filters cause the following problems:

  1. Higher utility bill. Drive your car around with your foot on the brake and see if your fuel economy drops. The same is true with your blower motor and furnace. If the furnace can’t move the right amount of air, as a result of a restricted air filter, the unit will simply draw more gas and electricity.
  2. Comfort is reduced.
  3. Extra wear and tear and additional stress to your system
  4. Crack the heat exchanger or cause excess temperature stress
  5. It can cause a no heat or no cooling
  6. Your air conditioning coil could freeze and cause damage to the furnace or air handler
  7. Poor airflow to parts of the building




  1. Plan to check your filter monthly. Some filters will last up to 12 months. I suggest you always are safe than sorry and replace the filter on routine rather than wait for it to plug up.
  2. There is nothing wrong with a 1” filter, provided that the pressure drop across the coil is within range for that air handler or furnace.
  3. The 4” media air filter is a much better choice than a 1”. However, this will cost more at the time of installation.
  4. I suggest that you purchase a minimum of a MERV 10 filter. In most cases, I suggest a MERV 12 or higher. Be careful – some stores offer a filter that is manufactured by a leading brand that is extremely restrictive in air flow. I can’t say their name(s) or they will sue me.
  5. Avoid the cheap filters. They do nothing but keep beach balls from entering your furnace.
  6. Post filter contamination can be a problem. This is the area where you slide the filter into the slot. If there’s a gap – often times the filter will miss much of the debris. I suggest this gets sealed with a magnet or filter cover.

The bottom line is that if you neglect your furnaces’ air filter – you will pay for it. Chances are that you will need to replace your furnace or air conditioner sooner than normal or you will ruin major parts.

Get in the habit of changing your filter. Use your smart phone’s calendar or reminder system. With the technology available today there’s no excuse for neglecting the air filter.

In a few weeks I will share Heat Exchanger Destroyer #4 – The dirty air conditioning coil (evaporator coil)



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