Introduction
Build Equinox has designed our CERV-1000 smart ventilation and comfort conditioning system for positive pressure balancing when used for ducted air distribution systems. This article will describe positive flow balancing and discuss its performance and cost benefits.
Duct system designers often use negative pressure flow balancing, an inefficient and energy dissipative way to obtain desired air flows. Dampers, louvered grills, adjustable diffusers, and “friction” balanced duct design are examples of dissipative, negative pressure flow balancing. Today’s energy efficient, variable speed duct fans can be used to positive pressure balance duct airflows, improving energy efficiency and saving money.
Designing ducts to deliver varying levels of air flow to different regions of a building is complex. Build Equinox’s two reports on optimized duct design and duct distribution systems provide background information on duct sizing, duct installation cost and duct operation cost. Seemingly small differences in duct design can lead to significant economic and energy efficiency penalties.
A key to optimized duct design is to maintain air flow velocities around 300 to 400fpm (feet per minute), which over a broad range of duct installation and fan energy operation costs, results in minimized lifetime costs. This rule-of-thumb greatly simplifies duct design while saving a lot of money over a building’s lifetime. An added benefit is duct noise less than a quarter of the noise level in ducts with air velocities greater than 800fpm.
Balancing air flows in multiple, parallel duct branches is most often accomplished by selecting a fan that can pressurize duct air to an “external” static pressure as high as needed for the duct branch with the highest pressure drop. All other duct branches must adjust their air flows relative to that pressure, too. Usually, other duct branch air flows are controlled by a combination of designing duct branches to have similar pressure drops as the most restrictive (highest pressure) branch and/or adjustment of dampers and ventilation grills/diffusers.
Two problems of negative pressure balancing are increased noise and elevated fan energy usage. “Positive” air flow balancing with duct fans avoids these “negative” air flow balancing problems.
Comparison of Negative (Damper) and Positive (Duct Fan) Balancing
We present two examples to compare negative pressure balancing and positive pressure balancing of air flows in a duct network. Figures 1 and 2 show two duct systems with identical air flow requirements. In each figure, four duct branches are required to supply 250cfm to their respective zones. One zone has three times as much duct pressure drop (0.75”wg) as the other three branches with 0.25”wg for 250cfm airflow.
The situation in Figures 1 and 2 could occur with 6 inch ducting. The high pressure drop branch has a duct length equivalent of 150ft while the other three lower pressure drop branches would be 50ft equivalent, 6 inch diameter duct runs.
The fan system for Figure 1 supplies a total air flow of 1000cfm with 0.75”wg of external static pressure. Dampers are placed in the shorter ducts to reduce pressure to 0.25”wg. We only show three dampers, but having dampers to have flow control on all branches is common. In addition to dampers, other pressure dissipating balancing means are duct sizing (eg, reducing duct sizes on the low pressure branches to increase pressure drop), louvered grills, or diffusers.
Figure 2 is identical to Figure 1 except the system uses positive pressure flow balancing rather than dissipative damper balancing. The primary system fan for the positive pressure balancing system delivers 1000cfm airflow at 0.25”wg pressure to a duct manifold plenum. A duct fan in the high pressure drop branch adds an additional 0.5”wg for a total of 0.75”wg for its 250cfm air flow requirement while the other three lower pressure branches require no pressure adjustment.
Similar to Figure 1 comments, we only show one fan for the high pressure duct branch but note that one would tend to specify a duct fan for all four branches. Today’s high efficiency ecm (electrically commutated motor) fans are continuously adjustable with a single turn “pot” (potentiometer, a resistor that adjusts a 0 to 10VDC fan speed signal) often included with a duct fan purchase. We note that duct fans for this example cost about the same, and maybe less than a motor-controlled damper, or about $200, so capital costs are the same.
Table 1 shows results for Example 1 (damper dissipative) flow balancing and Example 2 (duct fan positive) flow balancing. Also included in Table 1 are assumptions for duct installation cost, utility cost, and fan efficiency. Background and details for these cost factors are included in Build Equinox duct optimization and duct distribution reports. Duct installation costs, for example, have factors related to duct length and duct diameter. While the costs assumed for Table 1 results may not be representative to your locale, these trends and conclusions will be the same, with even more justification for positive flow balancing in higher cost-of-living regions.
The results shown in Table 1 are typical for duct distribution systems. A duct velocity of 1274fpm (feet per minute), while much higher than Build Equinox recommends, is typical of the 1000 to 2000fpm range found in commercial duct systems (and is the reason why so many of you are rightfully annoyed at excessive ventilation noise in your classroom, house of worship, yoga studio, and workplace).
The internal fan in the primary ventilation and comfort conditioning unit (labeled CERV-1000 in Figure 1) would need to develop at least 0.75”wg of external static pressure with 1000cfm of total airflow. Dampers shown in Figure 1 create flow restrictions that drop pressure from 0.75”wg to 0.25”wg. A ventilation system that operates 24/7 would have 430Watts of fan power and consume 3764kWh per year. 100year lifetime energy cost (without assuming any inflation, energy escalation, or discount rates) would be $45,000. Installation cost for the duct system is estimated to be $7600. The large difference between lifetime energy cost and installation cost indicates spending more money on an improved system design might lower lifetime energy costs.
Table 1 also shows performance and economic factors for Example 2 with positive pressure flow balancing. Running the internal system fan at the lowest pressure required for the duct network, with duct fans elevating the pressure level to the amount required for higher pressure drop duct branches results in a lifetime energy cost of $22,500, or about half of the negative pressure, damper balanced system. Power is half at 215W and annual fan energy usage 1870kWh/year. The nearly 2000kWh/year of electricity savings is enough for 8000miles of EV (Electric Vehicle) driving per year!
Let’s Make it Better!
We previously mentioned that Build Equinox recommends keeping airflow velocities in the 300 to 400fpm range. This statement is backed by a detailed engineering economic optimization analyses over broad ranges of operating conditions, utility rates, duct system installation cost, fan efficiency, and other parameters described in our duct optimization report. Our examples, however, were based on duct systems with airflow velocity of 1200fpm, or 3 to 4 times our recommendation. The reason we formulated our examples on the much higher than we recommend duct velocity is because we wanted to use parameters typical of duct design we often find in the field.
In this section, we examine the same examples, but increase duct size to recast the examples to duct velocities in the range we recommend. Table 2 shows results for both examples (negative and positive flow balancing) with all other factors remaining the same.
10” diameter ducts reduce airflow velocity to 460fpm in comparison to 6” diameter ducts with 1270fpm velocity. Duct installation cost increases by $2000 for the larger ducting. Fan power and lifetime energy costs drop to a tenth of the more restrictive duct design. Noise generation potential drops to less than 15% (square of the ratio of duct velocities) of the more restrictive duct system. And, there is now room for ventilation flow enhancements for future needs such as implementing ASHRAE 241, infection risk reduction ventilation levels.
Table 2 shows that positive flow balancing with Build Equinox duct optimization recommendations saved $40,000 over the example building’s 100 year lifetime, or $400 per year from negative flow balancing with restrictive duct designs. The 3500kWh/y annual energy savings is enough for more than 12,000 miles of EV transportation, too!
Summary
Aesthetics are important, for sure, however, we have not come across any project in which some forethought, creativity, and recognition of the importance of an efficient ventilation system could not be accommodated. The only time larger ducting is a problem is when duct design is left as an afterthought.
The importance of fresh air ventilation on building occupants’ health and productivity is more complex than simply blowing air into and out of a building. Unfortunately, indoor air quality is too often relegated to checkmark status in many building projects. Moving ventilation system design to the highest level of project priority is important for health, productivity, and energy efficiency.
Positive pressure flow balancing can lead to significant energy savings, noise reduction, and robust ventilation system operation. In future articles, we will show how CERV smart ventilation systems can take advantage of positive pressure flow balancing. Stay tuned!
