Service companies and their customers across the country have embraced the idea that unless the duct system performs well, the rated equipment energy efficiency means little. Let’s take a look at typical duct system upgrades necessary to deliver the efficiency promised on the yellow sticker.
The average residential HVAC system still only delivers an average of 57% of its rated equipment BTUs into the home. The typical government and utility programs in use today increase this statistic to a whopping 63%.
The good news is that service technicians who take an hour for a little measurement and diagnostics can increase the average sales price of their jobs by $3,200 and eliminate their competition. This is accomplished by delivering 85% or better system efficiency that your customers are happy to pay for.
Include the Duct System
Do your service agreements include the ducting portion of the system?
If you are like most companies the answer is no. A five minute static pressure reading can be a key diagnostic indicator if the duct system is in need of renovation. How have you been able to ignore this critical component of the systems that you install and service?
Unless measurements are taken of at least airflow and static pressure, your evaluation of the HVAC system's performance is only a guess. Rated SEER or AFUE only express the potential of the equipment in a laboratory environment. The air distribution system must be included in the formula to judge a system's performance.
This is still a new frontier in the HVAC industry. The best part of duct renovation work is that you find it, manufacturer the needed duct, install it, and keep 90% of the money in your company. Often less that 10% of the sales price of this work is paid to vendors. System renovation is a custom product.
Recurring Performance Issues
By considering the problems of the air side, installers will learn not to make these common mistakes as they build systems. Service technicians will pull their heads out of the 'box' and look at the entire system. Salespeople will find new opportunity for profitable work and a way to differentiate themselves from the competition.
Once again, this is a new opportunity that has been here all along. This is the 'air' in air conditioning.
When evaluating the systems that you install and service, you will find several duct conditions turn up again and again across the country.
We encourage you to review these practices and see which ones you could eliminate from your installation projects. More importantly, we encourage you to find these problems with your service customers and prescribe duct renovation work that includes the entire system and not just the boxes.
Deadheading at the Fan Discharge
The practice of installing a pan with a collar in it is a poor one. The restrictive nature of these fittings is from two causes. First, the required free area of a circle transitioning from a square has never made mathematical sense. In every instance the circle will be smaller, restricting airflow. Second, the immediate transition off a coil destroys air patterns and causes excessive system effect – reducing heat transfer and air delivery.
Better: A lengthy square to round fitting, or a "T" fitting with turning vanes.
Structural Restrictions
Most architects and framers have been exposed to HVAC contractors who will install ducting no matter what clearance is available. Most are somewhat surprised when they hear that the ducts won't fit in the space provided. A 10-in. flex-duct smashed to half its height carries less than 20% of the expected airflow. This is probably the most universal problem that we find.
Better: Review plans with architects and other trades before building. Inspect the accessible areas with your installers before starting construction and solve problems before beginning the work.
Loose Duct Liner
Usually hidden from view, loose duct liner falls into the air stream causing a restriction that may reduce airflow 80% or more. It can be found by taking static pressure readings every four feet or so, looking for a large pressure difference between two of the readings.
Better: Improved glue, mechanical fasteners and buttering the edges of raw duct liner when it is installed. External duct wrap is very effective. Why do attics in buildings have R-38 insulation and ducts have only R-4.2, R-6 or R-8? Good question.
Poor Quality Duct Suspension
Many HVAC contractors install flex duct suspension as an afterthought. It looks OK when installed, but flex duct droops and looks like waves in a very short period of time. This can increase the resistance through the ducting to five times what the system was engineered for.
Better: Install an 18-in. x 18-in. sheet of roofing felt between the strap and the flex duct. This small change will increase the life and performance of a flexible duct system.
Too Many Elbows
A review of ASHRAE standards and equivalent feet of duct is the best text to use in training technicians about elbows. With residential and light commercial low pressure systems, multiple elbows can render a duct run a failure.
Better: Find another route for the ducting, or have changes made in the structural framing, or find another location for that grille.
Remote Plenums and Duct Board Wyes
These contribute much to the ease of an installation, but rarely operate as intended. The reasons are the large pressure drop that they cause and the lack of direction for the airflow that enters and exits these boxes.
Better: Extended plenums, or lengthy transitions or wyes. As you can see, the cost to install a system properly does increase slightly. But the results far outweigh that cost.
Transitions at the Fan Discharge
Too many sharp transitions at the fan discharge leaves the air churning and rolling, if you could see it, it would be acting as though it were quite confused. At low pressures, air cannot perform as intended with poorly designed or installed fittings. Remember, required area for the duct to be installed is not an option, and ignoring this will increase future liability.
Better: Forget the low bidder mentality and find the courage to sell the type of duct system that the equipment will need to allow the system to operate properly.
Conclusion
When systems are installed and tested using standard air balancing practices, faults are revealed and can be corrected. If the systems you install and service are assumed to be operating properly, we challenge you to measure airflow and static pressure to confirm your beliefs. Far more likely than not, your testing will provide you with valuable information to help you improve your product and deliver a superior product.
Rob “Doc” Falke serves the industry as president of National Comfort Institute, an HVAC based training company and membership organization. If you're an HVAC contractor or technician interested in an NCI Duct Sizing Chart, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.
