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Positive Pressure: Flow Balancing of Duct Networks
Tech
Jul 25, 2024

jet turbines on a wall

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

CERV-1000 with duct manifold, including four branches.  on three of the branches are flow balancing dampers with a 0.5"wg drop.  Each branch has 250cfm
Figure 1: Example duct distribution system with dissipative (pressure drop) flow balancing using dampers, duct sizing and vent grills to adjust flows. The above flow conditions are similar to 6 inch diameter ducts with one 150 feet long duct for the higher 0.75”wg pressure drop and three 50 feet long ducts for the lower 0.25”wg pressure drop.

CERV-1000 with duct manifold, including four branches.  Three branches are left open, and the fourth has a flow-balancing fan to boost airflow.
Figure 2: Example duct distribution system with positive (pressure increase) flow balancing using high efficiency ECM duct fan. This system provides the same flows as in Figure 1, but with less noise potential, lower total fan power, lower operational cost, and often less capital cost because today’s high efficiency duct fans often cost less than a motorized damper!

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: Example 1 (Dissipative) and Example 2 (Positive) Flow Balancing Results

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.

Table 2 Build Equinox Optimized Duct Sizing Results for Examples 1 and 2.

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!

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Sep 20, 2019 Featured Article – Handling Humidity Pt.2 Climate Moisture Variations Tags: Comfort, Humidity, Ventilation, ZEROs 20190920
Sep 20, 2019 News – Brand New Release: Colorfil VOC Absorbing Filters! Tags: CERV, Filters, Health, Indoor Air Quality, Ventilation 20190920
Sep 20, 2019 Tech – From CERV to CERVEZA: a Quest for Smart Beer Tags: CERV, Heat Pump 20190920
Aug 27, 2019 Events – Earn Free CEUs with the Handling Humidity Webinar (Aug 29) Tags: Comfort, Humidity, Ventilation, ZEROs 20190827
Aug 27, 2019 Featured Article – Handling Humidity Report Series Tags: Comfort, Humidity, Ventilation, ZEROs 20190827
Aug 12, 2019 News – Listen to the Building HVAC Science Podcast, ft. Ty Newell! Tags: CERV, Comfort, Health, Indoor Air Quality, Webinar 20190812
Jul 29, 2019 Events – CERV2 Smart-er Ventilation Product Demo (Aug 27, 2019) Tags: CERV, Heat Pump, Ventilation 20190729
Jul 29, 2019 Featured Article – Poor Home Maintenance = Increased Health Risks Tags: Comfort, Ducting, Energy Efficiency, Health, Humidity, Ventilation 20190729
Jul 29, 2019 Tech – Say Hello to Our New Server! 20190729
Jun 19, 2019 Featured Article – Watch out for Cranky Heaters! Tags: Energy Efficiency, Heat Pump 20190619
Jun 19, 2019 Events – Free CEU Webinar: Smart Ventilation & Air Distribution (July 10) Tags: CERV, Ducting, Energy Efficiency, Indoor Air Quality, Ventilation 20190619
Jun 19, 2019 Spotlight – Marrakech Express! CERV2 Heading to Africa! Tags: CERV, Net Zero, Solar 20190619
May 13, 2019 Featured Article – Taylor Home CERV Testimonial Tags: CERV, Comfort, Indoor Air Quality, Ventilation 20190513
May 13, 2019 Events – 7 Steps for Designing an Economical Net Zero Home (Video + PDF) Tags: Energy Efficiency, Net Zero, Solar, Ventilation, Water, Webinar, ZEROs 20190513
May 13, 2019 Tech – AeroBarrier Demonstration Tags: Energy Efficiency, Ventilation 20190513
Apr 15, 2019 Events – Indoor Air Quality Metrics Free Webinar (Apr 24, 2019) Tags: Comfort, Health, Indoor Air Quality 20190415
Apr 15, 2019 Featured Article – CERV2 Geo–Boost Tags: CERV, Energy Efficiency 20190415
Apr 15, 2019 Review – 2019 National Home Performance Conference Tags: Indoor Air Quality 20190415
Apr 15, 2019 Tech – CERV2 Sketchup Model Now Available Tags: CERV 20190415
Mar 28, 2019 News – Visit us at the Chicago 2019 National Home Performance Conference Tags: CERV 20190328
Mar 28, 2019 Featured Article – Happy Equinox, St. Patrick’s Day, and Super Moon! Tags: Equinox House 20190328
Feb 27, 2019 Events – Free CEU Webinar: Smart Ventilation & Air Distribution (Mar 12) Tags: CERV, Ducting, Energy Efficiency, Indoor Air Quality, Ventilation, Webinar 20190227
Feb 27, 2019 Featured Article – Smart Ventilation and Smart Air Distribution Reports Tags: CERV, Ducting, Energy Efficiency, Indoor Air Quality, Ventilation 20190227
Feb 27, 2019 Spotlight – El Salvador NZEB Update Tags: CERV, Net Zero, Solar 20190227
Feb 25, 2019 Events – CERV2 Smart-er Ventilation Product Demo (Video + PDF) Tags: CERV, Heat Pump, Indoor Air Quality, Ventilation, Webinar 20190225
Jan 22, 2019 Events – Smart Ventilation & Smart Air Distribution Webinar (Video & PDF) Tags: CERV, Ducting, Energy Efficiency, Indoor Air Quality, Ventilation, Webinar 20190122
Dec 26, 2018 Spotlight – Progressive Canada Tags: CERV, Net Zero, Passive House, Solar 20181226
Dec 26, 2018 News – Happy Holidays from Build Equinox! 20181226
Dec 26, 2018 Review – CERV2 at Greenbuild 2018 Tags: CERV 20181226
Nov 26, 2018 News – CERV OEM Filter Store is OPEN!!! Tags: CERV, Indoor Air Quality 20181126
Nov 19, 2018 News – Check Out Our Social Media! 20181119
Oct 24, 2018 Events – Free CEU Webinar: Duct Design & Performance (PDF Download) Tags: Ducting, Energy Efficiency, Webinar 20181024
Oct 24, 2018 Featured Article – CERV2 is UL Approved! Tags: CERV 20181024
Oct 24, 2018 News – Stop by Our Booth at Greenbuild Chicago (Free Tickets!) Tags: CERV 20181024
Oct 24, 2018 Spotlight – University of Illinois Students Visit Build Equinox Tags: CERV 20181024
Jul 23, 2018 Featured Article – Happy 10th Birthday, CERV and Sunflower! Tags: CERV, Solar 20180723
Jul 23, 2018 Spotlight – Good News from El Salvador! Tags: CERV, Net Zero, Solar 20180723
Jun 22, 2018 Tech – Installing a Ductless Mini-split Tags: Heat Pump 20180622
Jun 22, 2018 Featured Article – Mini-split Mania! Tags: Heat Pump 20180622
Apr 30, 2018 News – Now offering on-demand webinars for CEUs! 20180430
Apr 30, 2018 Events – Free CEU Webinar! (May 2, 2018) 20180430
Apr 30, 2018 Featured Article – Ductology Part 2 Tags: Ducting, Report, Ventilation 20180430
Feb 19, 2018 Review – 2018 Better Buildings by Design Conference, Flu and Colds 20180219
Feb 19, 2018 Featured Article – Hot Water! Tags: Comfort, Water 20180219
Jan 22, 2018 Events – Visit us at BuildingEnergy Boston! (March 7-9, Boston, MA) 20180122
Jan 22, 2018 Events – Efficiency Vermont Better Buildings by Design Conference (Feb 7-8) 20180122
Jan 22, 2018 Featured Article – Ductology (Part 1) Tags: Ducting, Report, Ventilation 20180122
Nov 20, 2017 Featured Article – Heat Pump (Hybrid) Clothes Dryers are Coming! Tags: Heat Pump 20171120
Oct 31, 2017 Featured Article – CERV2 Measures IAQ at NAPHC & NAPHN 20171031
Sep 25, 2017 Featured Article – Introducing CERV2 Tags: CERV 20170925
Aug 21, 2017 Events – Free CEU Webinar! (Sep 27, 2017) 20170821
Aug 21, 2017 Featured Article – Quiz 20170821
Jul 27, 2017 Events – New IAQ Metrics Webinar (Video + PDF) Tags: Health, Indoor Air Quality, Ventilation 20170727
Jul 25, 2017 Featured Article – Endotoxins: Small But Very Significant Tags: Health, Indoor Air Quality, Report, Ventilation 20170725
May 22, 2017 Events – Economical Net Zero Design Webinar (Video+PDF) Tags: Energy Efficiency, Net Zero, Webinar, ZEROs 20170522
May 22, 2017 Featured Article – Styrax Japonicus 20170522
Apr 28, 2017 Events – 7 Steps for Designing an Economical Net Zero Home (Video + PDF) Tags: Energy Efficiency, Net Zero, Webinar, ZEROs 20170428
Apr 28, 2017 Featured Article – Engineering Net Zero Homes Tags: Net Zero, ZEROs 20170428
Mar 20, 2017 Featured Article – Build Equinox Zero Plus Facility Tags: Energy Efficiency, Net Zero, Solar 20170320
Feb 14, 2017 Featured Article – February Flu Tags: Health, Indoor Air Quality, Report, Ventilation 20170214
Jan 20, 2017 Events – HRV, ERV and Smart Vent Systems, Free CEU Webinar (Feb 15) Tags: CERV, Energy Efficiency, Ventilation, Webinar 20170120
Jan 20, 2017 Events – NESEA IAQ Metrics Presentation (Mar 9, Boston, MA) Tags: Health, Indoor Air Quality 20170120
Jan 19, 2017 Featured Article – The Perfect Dust Storm Tags: Health, Indoor Air Quality, Report, Ventilation 20170119
Jan 12, 2017 Events – Efficiency Vermont Better Buildings by Design Conference (Feb 1-2) Tags: CERV 20170112
Dec 26, 2016 Featured Article – Happy Holidays from Build Equinox! 20161226
Nov 29, 2016 Featured Article – Geo-Boost Tags: CERV, Energy Efficiency 20161129
Nov 29, 2016 Spotlight – This Old Passive House Tags: Energy Efficiency, Passive House, Ventilation 20161129
Nov 29, 2016 Review – House Music 20161129
Oct 28, 2016 Featured Article – Comparing ERV, HRV, and CERV Tags: CERV, Energy Efficiency, Heat Pump, Ventilation 20161028
Oct 28, 2016 Spotlight – Net Zero Eco-House (Monticello, IL) Tags: CERV, Energy Efficiency, Net Zero, Passive House 20161028
Oct 28, 2016 Spotlight – Forty Under 40 20161028
Sep 28, 2016 Events – Free CEU Webinar (Oct 5th): Why are new Indoor Air Quality metrics needed? Tags: Health, Indoor Air Quality, Ventilation 20160928
Sep 28, 2016 Review – 2016 North American Passive House Conference Tags: Passive House 20160928
Sep 28, 2016 Featured Article – New CERV-ICE IAQ Analytics Released! Tags: CERV, Health, Indoor Air Quality 20160928
Sep 28, 2016 Spotlight – CERVs in Passive Homes, pt. 2 Tags: CERV, Passive House 20160928
Aug 18, 2016 Featured Article – Understanding the House as a System Tags: Comfort, Energy Efficiency, Humidity, Indoor Air Quality, Net Zero, Ventilation 20160818
Aug 18, 2016 Spotlight – CERVs in Passive Homes Tags: CERV, Passive House 20160818
Aug 18, 2016 Review – “What is IAQ?”, P. Ole Fanger Tags: Indoor Air Quality 20160818
Aug 18, 2016 Tech – CERV CO2/VOC Library Tags: CERV, CO2, Health, Indoor Air Quality, Report, VOC 20160818
Jul 19, 2016 Events – September North American Passive House Conference Tags: Passive House 20160719
Jul 18, 2016 News – CERV Website Tags: CERV 20160718
Jul 18, 2016 Featured Article – VERMOD CERV Report Released Tags: CERV, Energy Efficiency, Indoor Air Quality, Report 20160718
Jul 18, 2016 Review – LBNL report: “Houses are Dumb Without Smart Ventilation” Tags: Indoor Air Quality, Report, Ventilation 20160718
Jul 18, 2016 Spotlight – Professor P. Ole Fanger (1934-2006); IAQ and Comfort Pioneer Tags: Comfort, Indoor Air Quality 20160718
Jul 18, 2016 Tech Note – Airflow Calculation for Ventilation Systems Tags: Ducting, Report, Ventilation 20160718

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