Furnace Destroyer #2 – Poorly sized ducting

Heat Exchanger Destroyer #2  - The undersized duct system

Duck’s don’t whistle – they quack! Do your ducts whistle? The problem can be easily identified, corrected and resolved. All you need is an educated technician, an honest HVAC company and few good test instruments.

In my last article, I spoke of heat exchanger destroyer #1 – The Design Factors. In this article, I will share what I know about an undersized duct system.

Perhaps you have a furnace that has a cracked heat exchanger, or requires a pre-mature replacement. Let’s assume that you ruled out the manufacturers design. As an HVAC investigator, I want to know what is going on. The first place I look is at the size of the air conditioning system and Btu output of the furnace. Once I know these numbers, I can calculate what the required air flow requirement is for that system. Let’s assume that the furnace is sized correctly.

Let’s do the math.

In the air movement world, an air handler or furnace must provide 400 cfm (cubic feet air per minute) per ton of cooling. A ton of cooling is equal to 12,000 Btu’s (British thermal unit) of heat transfer or in simple terms 12,000 Btu’s of heat removal. Understand, in the cooling mode we remove heat and in the heating mode we transfer energy thus providing heat.

If you have a 3 ton cooling system that means that you should be moving a minimum of 1200 cfm across the evaporator coil. This also means that the duct system’s combined design capacity must also move 1200 cfm through the duct system and to all of your supply registers. Guess what? The return system must also provide enough return air so that the air handler or furnace can provide the correct air flow.

Think of this – how easy would it be to fill a swimming pool with a straw? How difficult is it for your heart to deliver oxygen and blood through a body with clogged arteries and veins? It’s no different with an undersized duct system. Bigger is not always better. Do not make the assumption that with giant ductwork the problem will be resolved. This is another issue that I may write about in the future. The duct system must be able to deliver the correct amount of air flow. Just like your heart must deliver the correct amount of blood and oxygen.

What do you suppose will happen if you are moving 1000 cfm on a system that requires 1200?

If you have too small of a duct system there’s a good chance that your registers will whistle and your system will be noisy. There’s also a guarantee that you will destroy a furnace or heat exchanger.

I know that I mentioned air conditioning and cooling, but what about the furnace and heat exchanger? The general rule is that if you have the proper airflow for the cooling mode, there’s a probable chance that you will also have enough airflow for the heating mode.

I recommend that duct systems be sized at .08 of static duct pressure with a 400 cfm air movement requirement. These are general numbers. In the commercial world, these numbers may not work. The best policy is to look at the manufacturers’ or engineers specifications and go with what they recommend.

Regardless, an undersized duct system will destroy a heat exchanger and it could result in one or more of the following problems.

  • Poor airflow to certain rooms in the home
  • Higher utility bills
  • Pre-mature failure of compressors
  • Pre-mature failure of blower motors and other furnace components
  • Loss of comfort
  • Whistling ducts – or, a noisy system
  • Freeze up a coil and cause water damage to the furnace or air handler
  • For those in the south – water leakage through the ceiling

The list goes on and on. Every technician must be privileged to properly trained on how to test, read and communicate static duct pressures. I also believe that each and every tech must learn how to calculate cfm requirements. All of this can be completed and addressed by training and educating technicians.

Watch for my next blog post on Heat Exchanger Destroyer #3 – The dirty or plugged filter.

Homeowners: Hire the right HVAC Company – many problems will be prevented.

Contractors: Invest in your people and hold them accountable – many problems will be prevented.


“Changing the HVAC industry, one technician at a time”

Steve Holland


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 Steve Holland is a national advocate for the skilled trades and also the owner of Mr. Holland's Heating and Air Conditioning in Wisconsin. If you would like to book Mr. Holland for an event please use the contact us section.





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