Duct Systems | Tri-County Mechanical Inc.
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Duct Systems – Complaints We Have Heard

Duct Systems – That component of your heating and air conditioning system that is out of sight and out of mind, IF IT WORKS PROPERLY.  But if it doesn’t we hear complains such as:

  • .I have 1 or more rooms that is hot in the summer and cold in the winter.
  • I hear a banging noise in my duct system when my system shuts off.
  • Why do I have to dust so frequently in my home?
  • When my system is running, the sound of air rushing through the duct system annoys me. Should it be that loud?
  • I just moved in and I am detecting a musty smell in my house.
  • I have multiple air filter returns in my house and I frequently replace only 1. The others don’t ever seem to get dirty!
  • I have called the company that installed my duct system about “You name the concern” and they seem to be unwilling to send someone out to check-out my concern.
  • I live in a home constructed by a “tract builder” builder. The company that is hired for air conditioning now is not the company that installed my system.
  • I don’t have the funds available to spend on a duct system. I have other needs to cover first.

And we hear another complaint TOO frequently!  This is the 2nd compressor replacement that has occurred with my system.  Most folks don’t realize that one of the most likely causes of a compressor failure is faulty air flow (duct system related).  Manufacturers maintain statistics that say compressors have close to a 95% reliability rate!  (You can go here and down load a lengthy PDF document regarding compressor failure.)

In the home re-sale market, the purchaser typically depends on the home appraiser to identify potential concerns. If a thorough duct testing procedure is not performed by a professional with the proper credentials, duct system inadequacies are likely unknown.

Construction of a properly performing duct system is somewhat of an art, and there is probably more than 1 way of getting there. (Please continue reading maybe you will think about what’s coming next and call us if you have duct system concerns.)The performance of the duct system is critical for the mechanical component(s) of your system to achieve the capacity and efficiency of its ratings. And this statement is as true for systems with minimal efficiency ratings AND significantly more critical to super-high efficiency systems. So, let’s begin with the method that quality heating and air conditioning companies use that result in systems with running at optimum conditions.

We don’t Take Shortcuts

  1. We don’t let our experience in installing duct systems provide shortcuts. We don’t assume. We use a calculation developed by energy specific mechanical engineers to determine the heating and cooling requirements for your home. And it is not the shortcut method that only utilizes the outside dimensions of your home as the base inputs of the calculation. For all rooms, we measure the dimensions of the floor, windows, walls and ceilings and note ceiling heights. (The duct system doesn’t just condition your entire home, it should be designed to condition each room in your home.) It’s called a room by room load calculation in lieu of a whole house calculation. With the room by room calculation, we obtain the results for the total requirements for you home and the requirements of each room.
  2. The sum of the requirements for each room provide the total capacity of the system needed for the structure. The capacity of a system dictates the volume of air flow to be utilized in the system.
  3. With the requirements for each room and our experience, we know the required size the “pipes” needed to supply the proper volume of air flow at the correct velocity for each room. When a portion of the air stream is diverted to supply air flow to the rooms, the main supply duct is reduced in size for the correct volume of air but also to maintain the proper speed for the air flow (velocity). And the main duct system will likely be reduced multiple times before the air flow gets to the end of the duct system. We know the recommended air flows for each size of pipe needed.
  4. In well-designed duct systems, sometimes it’s not the rooms connected to the end of the duct system that provide the least air flow. Sometimes it’s the rooms that are connected closest to the blower that receive the least air pressure. This should indicate that not only volume of air is important but velocity (air pressure within the duct system) is critical to the performance of a duct system.
  5. It appears to me, that the component absent in many duct systems are the lack of “baffles” installed in the duct system (where the air flows for each room are connected to the main supply trunk line). The most important rule of duct system construction is “Air is dumb, it doesn’t necessarily travel the way it is supposed to go, but follows the least path of resistance.” (Air sounds like a lot of people I know, including myself, sometimes.) Without the installation of simple baffles in the supply air trunks, there really isn’t a reason for testing unless you expect to tear out components that don’t work as expected. But with the proper air flow detection tools, the amount of air flow emitted from the vents in each room can be measured. If measurements prove unexpected results, the baffles are used to divert the air flows to where it is most needed. (Measurements / adjustments can be an iterative process.) If these measurements are “close” to the results of the Room X Room outputs of the load calculation, the duct system WILL PROVIDE COMFORT to each room, heating, or cooling. (One must remember that air flow measurements / calculations IS NOT an exact science but kind of like horseshoes and grenades; getting close is GREAT.) Since “air” is not very smart, how does one KNOW that a duct system will perform as expected WITHOUT TESTING?
  6. What if the air flow capacity of the blower is constrained and there is more air flow in the blower than the duct system has capacity to handle. In our industry this is referred to “high static”, simply translated, excessive air pressure. Sometimes detected by the excessive “rushing noise” emitted by poorly designed duct systems. Not only is this a waste of energy but can lead to premature compressor failure. Our load calculations prevent “over-sizing” and and the blower capacities are a critical component is our properly designed duct systems.
  7. Now let me discuss return air paths. Another one of my simple air flow guidelines is that “the amount of air supplied by the blower MUST EQUAL the amount of air returned to the blower”. If the return duct is not capable of supplying the amount of air supplied, one of two things will occur:(a)  Negative pressure (the lack of air flow) will be excessive and the blower will get air from wherever it can to relieve the negative pressure and supplement the air flow in the return duct. This is NOT GOOD. (If this happens, the “make-up” return air flow is coming, unfiltered, from the attic, garage, or crawlspace.) And the “make-up” air that is injected into the blower is NOT the same temperature or humidity as the air returning to the blower in the return duct. This is NOT GOOD. OR(b) The blower can’t generate enough “make-up” air and increased air pressure on the blower IS NOT RELIEVED, so that the blower can’t supply the air flow at the rates for which it is designed. Translated, the system is not able to utilize its rated capacity. (This is kind of like trying to take off in a boat without pulling up the anchor…You can’t get enough speed to plant the boat.)  This is NOT GOOD. And if it’s harder for the blower to perform work (produce proper air flows) than it should be, the blower requires more energy. This is NOT GOOD. This can ALSO even lead to premature compressor failure.  This is NOT GOOD.
  8. Lastly, let’s think about duct leaks (air leaks from inside to outside or from outside to inside the duct system to or from an unconditioned space). If a duct system has air leaks, the leaks could be the source of the reason for excess dust in your home. Secondly, duct leaks can cause air pressure differences (now I am talking about the air pressure that exists outside of the duct system and between the building envelope that separates conditioned air from non-conditioned air.) Duct system leaks can cause minute (small) air pressure differences that provide paths for heated air to escape in the winter and cool air to escape in the summer. (Another principle of air flow is that the path of air flow is from hot to cold.) What I am really trying to say here that duct leaks causes outside air (unconditioned air) to enter the inside air (the conditioned air) and significantly escalate energy consumption

In our opinion, the bottom-line consideration for duct system performance is this… In most cases, SUPPLY AIR FLOWS, on a room by room basis, impact the level of COMFORT that is received from your WHOLE system. RETURN AIR FLOWS impact the capability of utilizing the capacity and efficiency of the mechanical components of your comfort system. BOTH can contribute to poor air flows, reduce air quality, and cause premature compressor failures.  Both are critically important to achieve your expectations and ours. The performance of your duct system shouldn’t limit your comfort, contaminate indoor air, shorten the life of the compressor or constrain the capabilities of your heating and air conditioning system.

If you are considering a replacement of a heating and air conditioning system that will provide the comfort and efficiency that you would expect, how could you not be as concerned about the performance of your duct system? Neglecting the evaluation of your duct system could rob you of all the efficiency savings that you would expect to receive from the installation of a new system.

And if you are considering calling someone for an estimate for a replacement system and you are experiencing concerns like those referenced in the beginning of this document, doesn’t it make sense to call someone that takes a wholistic approach to comfort, in lieu of one that primarily emphasizes the efficiency ratings of the equipment?

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843.773.2442
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Pierre Rattini

Pierre Rattini

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Dean cain - January 9, 2017 Reply

I remember installing a duct system myself not knowing what I was doing and thought it was simple. Now I know why it didn’t work so well!

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