Energy Savings from Air Sealing Large Buildings
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Energy Savings from Air Sealing Large Buildings
- 1. ENERGY SAVINGS FROM AIR SEALING COMMERCIAL BUILDINGS Energy Design Conference & Expo, Duluth, MN Dave Bohac P.E. Director of Research February 23, 2016
- 2. Pg. 2 Continuing Education Credit Information • In accordance with the Department of Labor and Industry’s statute 326.0981, Subd. 11, “This educational offering is recognized by the Minnesota Department of Labor and Industry as satisfying 1.5 hours of credit toward Building Officials and Residential Contractors continuing education requirements.” For additional continuing education approvals, please see your credit tracking card.
- 3. Acknowledgements This project was supported in part by a grant from the Minnesota Department of Commerce, Division of Energy Resources through a Conservation Applied Research and Development (CARD) program
- 4. Pg. 4 What we do • Program Design and Delivery • Lending Center • Engineering Services • Innovation Exchange • Research • Education and Outreach • Public Policy
- 6. Pg. 6 Project Team • Center for Energy and Environment • Jim Fitzgerald • Martha Hewett • Andrew Lutz • Kirk Kholehma • Air Barrier Solutions • Larry Harmon • The Energy Conservatory • Gary Nelson • Paul Morin • Peter Burns Air Leakage Test Staff: CEE - Alex Haynor, Jerry Kimmen, Joel Lafontaine, Dan May, Erik Moe, Tom Prebich, and Isaac Smith Bruce Stahlberg of Affordable Energy Solutions
- 7. • Measure the air flow rate needed to pressurize & depressurize the building by 75Pa (0.3 in. wc.) • Divide by the building envelope area – typically the exterior walls + roof + floor (6 sides) • Results from 387 US C&I buildings o Average = 0.72 cfm/ft2 o Range 0.03 – 4.3 cfm/ft2 Large Building Tightness Specification
- 8. • US Army Corp Engineers = 0.25 cfm/ft2 o Tested over 300 buildings o Average = 0.16 cfm/ft2 • IECC 2012 (7 states) whole building compliance path = 0.40 cfm/ft2 • Washington State: Buildings over five stories require a whole building test, but do not have to pass a prescribed value. • City of Seattle : All buildings require a whole building test, but do not have to pass a prescribed value. Code Requirements
- 9. Why do we care about building air leakage? • HVAC systems pressurize buildings to eliminate infiltration – don’t they? • When HVAC is off => air infiltration • Pressurization not always effective or implemented correctly • NIST/Persily tracer gas results – infiltration can be significant
- 10. Air Handler Pressurization 10,500 cfm 2,075 cfm 4 Story 60,000sf Office Building: leakage = 27,000 cfm@75Pa, 0.5 cfm@75/ft2 =10,500 – 2,075cfm
- 11. Roof Top Unit Operation 10,500 cfm 2,075 cfm
- 12. • Supply and Return Fans turn on/off by schedule • Outside Air Damper has a minimum position setpoint for ventilation • Relief Damper controls air exhausted from the building Single-zone Constant Volume AHU Relief Air Damper 25% open Outside Air Damper 25% open Mixed Air Damper 75% open DAT SensorMAT Sensor Relief Air Outside Air To Space From Space Heating Coil Cooling Coil 10,500 cfm 2,075 cfm – Exhaust Fans Supply Fan Return Fan
- 13. Air Handler Pressurization 10,500 cfm 4 Story 60,000sf Office Building: leakage = 27,000 cfm@75Pa, 0.5 cfm@75/ft2 2,075 cfm
- 14. Air Handler Pressurization 10,500 cfm 4 Story 60,000sf Office Building: leakage = 27,000 cfm@75Pa, 0.5 cfm@75/ft2 2,075 cfm
- 15. Air Handler Pressurization 10,500 cfm 2,075 cfm Infiltration >> 4 Story 60,000sf Office Building: leakage = 27,000 cfm@75Pa, 0.5 cfm@75/ft2
- 16. Roof Top Unit Operation
- 17. • Economizer operation o Mild weather when building needs cooling o Open outdoor air dampers, exhaust dampers follow; OA – EA stays the same? Single-zone Constant Volume AHU Relief Air Damper 60% open Outside Air Damper 60% open Mixed Air Damper 40% open DAT SensorMAT Sensor Return Fan Supply Fan From Space To Space Relief Air Outside Air 24,600 cfm 16,175 cfm – Exhaust Fans
- 18. • Supply and Return Fans o Supply fan VFD modulates to meet Duct Static Pressure (DSP) Setpoint o Return fan lags supply fan to maintain positive pressure Variable Volume AHU with VAV Boxes Return Fan 77% speed Supply Fan 87% speed DSP Sensor (typically 2/3 down supply duct) V F D V F D
- 19. Model Infiltration: Range of Flow Imbalance 1 Story 60,560ft2 Elementary School: leakage = 44,670 cfm@75Pa (0.75cfm@75/ft2) Minimum outside air = 20,300cfm
- 20. Model Infiltration: Range of Flow Imbalance 1 Story 60,560ft2 Elementary School: leakage = 14,890 cfm@75Pa (0.25cfm@75/ft2) Minimum outside air = 20,300cfm
- 21. Model Infiltration: Range of Flow Imbalance Envelope Leakage= 0.75 cfm@75Pa/ft2 -3,450 0 3,450 6,900 17,250 Avg Infil. (cfm) 2,986 2,444 2,077 1,849 1,652 Avg Infil. (ach) 0.25 0.20 0.17 0.15 0.14 Heat Load (therms/yr) 7,264 6,114 5,260 4,732 4,308 % Space Heating 19% 16% 14% 12% 11% Cost ($) $4,213 $3,546 $3,051 $2,745 $2,499 HVAC Flow Imbalance, OA - EA (cfm)
- 22. Model Infiltration: Range of Flow Imbalance Envelope Leakage= 0.75 cfm@75Pa/ft2 Envelope Leakage= 0.25 cfm@75Pa/ft2 -3,450 0 3,450 6,900 17,250 Avg Infil. (cfm) 2,986 2,444 2,077 1,849 1,652 Avg Infil. (ach) 0.25 0.20 0.17 0.15 0.14 Heat Load (therms/yr) 7,264 6,114 5,260 4,732 4,308 % Space Heating 19% 16% 14% 12% 11% Cost ($) $4,213 $3,546 $3,051 $2,745 $2,499 HVAC Flow Imbalance, OA - EA (cfm) -3,450 0 3,450 6,900 17,250 Avg Infil. (cfm) 1,725 951 708 678 676 Avg Infil. (ach) 0.14 0.08 0.06 0.06 0.06 Heat Load (therms/yr) 4,004 2,439 1,875 1,813 1,809 % Space Heating 10% 6% 5% 5% 5% Cost ($) $2,322 $1,414 $1,087 $1,052 $1,049 HVAC Flow Imbalance, OA - EA (cfm)
- 23. • Why not run the exhaust air through an ERV to recovery some of that energy instead of forcing it out through the envelope? • Need a tighter envelope to accomplish ERVs with infiltration control What about Energy Recovery Ventilators?
- 24. Air Leakage Test Video This slide contains a 5 minute video that provides an overview of the whole building air leakage test process. The video can be found on CEE’s web site at: www.mncee.org/Innovation-Exchange/Projects/Current/Capturing-energy-Savings-from-Large-Building- Envel/
- 25. • Test results compiled by the National Institute of Standards and Technology (NIST) – Emmerich and Persily – over the past 15 years • 387 commercial and institutional buildings • NOT RANDOM: researchers, low-energy programs, private testing firms • Used to model air infiltration energy loads and help establish leakage standards How leaky or tight are US buildings?
- 26. NIST Results from US whole building tests Dataset Qty Mean Std Dev Min Max Efficiency Vermont 36 0.35 0.38 0.03 1.78 ASHRAE RP 1478 16 0.29 0.20 0.06 0.75 Washington 18 0.40 0.15 0.11 0.64 Other VT/NH 79 0.54 0.40 0.05 1.73 Other 10 0.30 0.23 0.09 0.75 All new data 159 0.36 0.30 0.03 1.78 All previous data 228 0.92 0.70 0.09 4.28 All Buildings 387 0.72 0.63 0.03 4.28 USACE & Navy 300 0.16 6-sided at 75Pa (cfm/ft2 ) USACE Std = 0.25 Emmerich and Persily 2013
- 27. NIST Results: Frequency Histogram Emmerich and Persily 2013USACE Std = 4.5 20-25% meet Std Multiply by 0.055 >> cfm/ft2
- 28. • Tighter – office, education, public assembly & long-term health care • Leakier – retail, restaurants, industrial • Leakier exterior walls – frame, masonry/metal, & frame/masonry NIST Results: Weak Trends
- 29. NIST Results: Effect of Building Size Emmerich and Persily 2013 Buildings > 54,000ft2 twice as tight 0.55 cfm/ft2
- 30. NIST Results: Effect of Climate Emmerich and Persily 2013 Heating degree days > 3,600 one third tighter 0.55 cfm/ft2
- 31. NIST Results: Effect of Age Emmerich and Persily 2013 138 buildings with no air barriers built since 1950 – no strong trend 0.55 cfm/ft2 Colder climate
- 32. • 23 LEED buildings; average = 0.29 cfm/ft2 • Significantly tighter than average of other 364 buildings • Slightly (5%) leakier than other 56 buildings with an air barrier NIST Results: LEED Buildings
- 33. Page 33 NIST Results: Effect of Air Barrier Emmerich and Persily 2013 USACE Std = 4.5, 0.25cfm/ft2 Buildings with air barrier are 70% tighter
- 34. Page 34 NIST Results: Effect of Air Barrier Emmerich and Persily 2013 USACE Std = 4.5, 0.25cfm/ft2 Compare no air barrier to tight construction 1.0 cfm/ft20.1 cfm/ft2
- 35. • Multizone infiltration and energy model • Compared air infiltration and energy use for: o “typical” - no air barrier reported leakage (4x USACE) o “target” – good practice (40% below USACE) • Five cities in different climate zones NIST Building Infiltration & Energy Models
- 36. Page 36 NIST Building Infiltration & Energy Models Emmerich and Persily 2013 Two-Story, 24,000ft2 Office Building One-Story, 12,000ft2 Retail Building
- 37. Model Infiltration: Range of Envelope Leakage 1 Story 60,560ft2 Elementary School: HVAC Imbalance = 3,450 cfm Minimum outside air = 20,300cfm
- 38. Model Infiltration: Range of Envelope Leakage 1 Story 60,560ft2 Elementary School: HVAC Imbalance = 3,450 cfm 0.05 0.1 0.15 0.25 0.4 0.75 1.25 2 Avg Infil. (cfm) 305 417 481 708 1,094 2,077 3,539 5,751 Avg Infil. (ach) 0.03 0.03 0.04 0.06 0.09 0.17 0.29 0.47 Heat Load (therms/yr) 855 1,139 1,305 1,875 2,832 5,260 8,867 14,322 % Space Heating 2% 3% 3% 5% 7% 14% 23% 37% Cost ($) $496 $661 $757 $1,087 $1,643 $3,051 $5,143 $8,306 Building Envelope Leakage (cfm@75/ft2 ) NIST office building model: 1.0 cfm/ft2 = 0.23 ach 0.1cfm/ft2 = 0.05 ach
- 39. • Goal: 24 to 36 existing mid- and high-rise buildings (16 Completed) • Non-residential • 4 stories or higher • Sustainability certification (14 of 16) • Built after the year 2000 • Climate zones 2-7 (All 6 Zones Represented) ASHRAE Research: selection criteria
- 40. • Average = 0.29 cfm/ft2 • Green building = 0.32 cfm/ft2; others = 0.22 cfm/ft2 • Air barrier specified and envelope expert = 0.13 cfm/ft2; others = 0.39 cfm/ft2 • Unsealing HVAC penetrations increased leakage by average of 27% with range of 2% to 51% ASHRAE Research Project: leakage results
- 41. • Roof/wall intersection • Soffits and overhangs • Mechanical rooms, garages, basements, loading docks • Roll-up and overhead doors ASHRAE Research Project: leakage sites
- 42. • Conduct investigations on 25 buildings: floor area of 25,000 to 500,000 ft2 • Air seal and pre/post leakage tests on 6 7 buildings • Continuous building pressure and HVAC operation data for 50 to 200 days • CONTAM pre/post air flow models that include mechanical system leakage and pressure effects • Compute infiltration/energy reductions Minnesota Leakage Study: work scope X
- 43. Floor # Constr Building ID Area (sf) Stories Year Wall Type Elem School TF 59,558 1 1951 Masonry & corrugated metal panel Middle School 138,887 3 1936 Cast concrete w/CMU infill Small Office 26,927 1 1998 EFIS tip up (3 walls) and CMU block Univ Library 246,365 3 1967 Cast concrete w/CMU infill & brick ext Elem School PS 60,968 1 1965 CMU w/brick exterior Library/Office 55,407 1 2007 Steel studs & brick or stone cladding Building Characteristics 3 elementary & middle schools: 1936 to 1965 with additions 60,000 – 139,000sf University Library 246,000sf Small Office 27,000sf Library/Office 55,000sf
- 44. Minnesota Leakage Study: leakage results All 7 buildings at least 25% tighter than the US Army Corp standard of 0.25 cfm/ft2 Envelope Floor Area (ft2 ) 6 Sides EqLA # Constr Building ID Area (ft2 ) 6 Sides2 (cfm) (cfm/ft2 ) (ft2 ) Stories Year Elem School TF 59,558 146,977 27,425 0.19 15.2 1 1951 Comm. College 95,000 164,844 28,881 0.18 17.2 2 1996 Middle School 138,887 208,733 32,818 0.16 16.6 3 1936 Small Office 26,927 65,267 9,177 0.14 4.6 1 1998 Univ Library 246,365 171,712 23,356 0.14 13.1 3 1967 Elem School PS 60,968 145,766 17,602 0.12 9.6 1 1965 Library/Office 55,407 139,965 12,321 0.09 6.9 1 2007 Minimum 26,927 65,267 9,177 0.09 4.6 Mean 97,587 149,038 21,654 0.14 11.9 Median 60,968 146,977 23,356 0.14 13.1 Maximum 246,365 208,733 32,818 0.19 17.2 Air Leakage at 75Pa
- 45. Comparison to US Buildings 6 buildings 7 building average is 85% less than the US average, slightly less than US Army Corp average
- 46. Tighter Buildings in Colder Climates? 7 building average is 85% less than the US average 6 buildings
- 47. Where Were the Leaks?
- 48. Where Were the Leaks?
- 49. Air Sealing Focused on Roof/wall Canopy leakage at exterior wall
- 50. Air Sealing Focused on Roof/wall Canopy leakage at exterior wall IR Before IR After
- 51. Where to look: IR view of rear CMU wall pre Same wall post
- 52. Look inside: 10 beam pockets Open above to parapet cap Open to inside Smoke shows airflow
- 53. Closed cell foam fill, don’t create fire hazard See ICC ES 3228 approvals. maintain exhaust on work space adj. to occupied office Sample MDI < 5ppb Manage exposure ¾ cu ft foam block max temp rise check for building official and owner before injection. Don’t start a fire
- 54. Beam Pockets IR Before IR After
- 55. Air Sealing Reduction “Tight” buildings tightened by 9% Leakier Tighter Air sealing work confirmed by visual, smoke puffer, and IR inspections 6 Sides Building ID (cfm/ft2 ) Pre Post (cfm) (%) Elem School TF 0.19 27,425 22,699 4,726 17% Comm. College 0.18 28,881 28,133 748 3% Middle School 0.16 32,818 28,872 3,947 12% Small Office 0.14 9,177 8,470 708 8% Univ Library 0.14 23,356 21,963 1,392 6% Elem School PS 0.12 17,602 15,837 1,765 10% Library/Office 0.09 12,321 11,369 953 8% Minimum 0.09 9,177 8,470 708 3% Mean 0.14 21,654 19,620 2,034 9% Median 0.14 23,356 21,963 1,392 8% Maximum 0.19 32,818 28,872 4,726 17% (cfm) Reduction Air Leakage at 75Pa Air Leakage at 75Pa
- 56. Air Sealing Reduction More expensive to seal tighter buildings? Leakier Tighter Cost per sq ft of sealing Building ID Total ($/CFM75) ($/ft2 ) Elem School TF 18,550$ 3.92$ 6,822$ Comm. College 17,845$ 23.86$ 17,273$ Middle School 23,700$ 6.00$ 8,434$ Small Office 4,768$ 6.73$ 10,058$ Univ Library 15,918$ 11.43$ 65,159$ Elem School PS 26,700$ 15.13$ 38,132$ Library/Office 1,152$ 1.21$ 1,297$ Median 17,845$ 6.73$ 10,058$ Air Sealing Cost
- 57. Air Sealing Reduction Contractor estimates better for leakier buildings? Leakier Tighter Building Leakage < Estimated sealing Building ID Pre Post (ft2 ) (%) Roof/Wall Total Meas/Est Elem School TF 15.2 12.5 2.7 18% 8.84 11.49 0.31 Comm. College 17.2 16.2 1.0 6% 5.47 5.47 0.19 Middle School 16.6 13.8 2.8 17% 11.73 14.98 0.24 Small Office 4.6 4.1 0.5 10% Univ Library 13.1 12.8 0.2 2% Elem School PS 9.6 8.9 0.7 7% 14.45 16.94 0.05 Library/Office 6.9 6.0 0.9 13% ReductionEqLA (ft2 ) Contractor Estimated Sealed Area (sf)Leakage Area
- 58. Air Sealing Energy Savings Modeled Infiltration and Energy Savings Building ID Total Infiltration Infil/Total Elem School TF 40,224 2,389 6% Comm. College 32,095 3,402 11% Middle School 44,469 7,779 17% Small Office 684 Univ Library 192 Elem School PS 26,563 2,387 9% Library/Office 18,108 2,829 16% Minimum 6% Mean 12% Median 11% Maximum 17% Space Heat Gas Use (Therms/yr)
- 59. Air Sealing Energy Savings Modeled Infiltration and Energy Savings Avg Leakage Building ID Total Infiltration Infil/Total (Therm/yr) ($/yr) Infil (cfm) Red. (%) Elem School TF 40,224 2,389 6% 549 $319 1,296 17% Comm. College 32,095 3,402 11% 174 $105 1,730 3% Middle School 44,469 7,779 17% 905 $525 4,330 12% Small Office 684 39 $24 964 8% Univ Library 192 11 $6 249 6% Elem School PS 26,563 2,387 9% 223 $129 1,453 10% Library/Office 18,108 2,829 16% 107 $68 1,477 8% Minimum 6% 11 $6 249 3% Mean 12% 287 $168 1,643 9% Median 11% 174 $105 1,453 8% Maximum 17% 905 $525 4,330 17% Space Heat Gas Use (Therms/yr) Gas Savings
- 60. Air Sealing Energy Savings Modeled Infiltration and Energy Savings Able to seal “tight” buildings, but work was not cost effective Total Leakage Payback Building ID (Therm/yr) ($/yr) (kWh/yr) ($/yr) ($/yr) Red. (%) Cost ($) (years) Elem School TF 549 $319 1,034 $101 $419 17% $18,550 44 Comm. College 174 $105 232 $23 $127 3% $17,845 140 Middle School 905 $525 2,523 $246 $771 12% $23,700 31 Small Office 39 $24 18 $2 $26 8% $4,768 182 Univ Library 11 $6 79 $0 $6 6% $15,918 2,872 Elem School PS 223 $129 487 $47 $177 10% $26,700 151 Library/Office 107 $68 -232 -$24 $44 8% $1,152 26 Minimum 11 $6 -232 -$24 $6 3% $1,152 26 Mean 287 $168 592 $56 $224 9% $15,519 492 Median 174 $105 232 $23 $127 8% $17,845 140 Maximum 905 $525 2,523 $246 $771 17% $26,700 2,872 Gas Savings Electric Savings
- 61. Building Pressure Measurements Average building pressure at ground level (Pa) Only 1 building operating greater than 12.5Pa at ground level 20F < outside temp <= 45F
- 62. Building Pressure Measurements Average building pressure at ground level (Pa)
- 63. Building Pressure Measurements Difference between occupied and unoccupied pressure (Pa) Pressure increase for almost all buildings 20F < outside temp <= 45F
- 64. Building Pressure Measurements Difference between occupied and unoccupied pressure (Pa) 20F < outside temp <= 45F
- 65. • Can we divide cfm50 by 20 to get savings? • It is not that simple for larger buildings • HVAC pressurization effects savings • Greater savings for taller buildings, open terrain, distance from neutral level, floor compartmentalization • Internal heat gain = cooling more important • Developing spreadsheets for savings calculations Computing Savings For Your Project
- 66. • Typical pressurization = 10% less 6Pa = 35% less 12.5Pa = 60% less • 1 story = 40% less; 5 story = 30% more; 10 story = 80% more • Urban wind shielding = 35% less Open wind shielding = 70% more Computing Savings For Your Project Three Story Commercial Building
- 67. Office Building Model: Heating & Cooling
- 68. Mechanical System Leakage Part of building envelope when not operating
- 69. Mechanical System Leakage Two most recently built (1998 and 2007) had low leakage Part of building envelope when not operating Mean 49% 0.06 cfm/ft2 (6 sides) Range 17% to 103% 0.02 to 0.12 cfm/ft2
- 70. • Tight buildings: 85% tighter than U.S. average & at least 25% below Army Corp standard – due to cold climate location? • Sealing = 9% reduction, more reduction and less expensive for leakier buildings • Contractor over-estimated sealing area • Long paybacks for air sealing work • Including mechanical systems increased leakage by 17 to 103% (0.02 to 0.12 cfm/ft2) • HVAC systems tend to pressurize buildings. Not as great as typical design practice Summary
- 71. • You can see out the envelope gaps & leak is accessible • Taller (5+ stories) in open terrain • Reported problem that is likely to be caused by air leakage • You live in portion of US that hasn’t had to worry about infiltration Other Opportunities • Older/leaky dampers (cost?) • Building pressure control When Is Air Sealing Worthwhile?
- 72. Thank you!
Notes de l'éditeur
- I know that I’m in the MF track, but I’m NOT going to be talking about air sealing MF buildings. This will be about air leakage testing and air sealing commercial and institutional buildings. So if you were expecting me to talk about MF sealing, now is your chance to change rooms.
- There is continuing education credit for this presentation. For additional continuing education approvals, please see your credit tracking card
- We would like to acknowledge and thank the MN Division of Energy Resources and the Conservation Applied Research & Development (CARD) support that funded this research.
- CEE is involved in a variety of energy efficiency services including program design & delivery; loans; engineering services; research; education & outreach; and public policy. For example, the One-Stop Efficiency Shop program has worked with over 10k businesses resulting in a reduction of 97 MW of demand and $98 million in annual savings for participants We have serviced over 25,000 residents through our programs CEE has originated more than 25,000 loans resulting in over $190 million in community rehabilitation We have published over 100 technical papers on building and mechanical system performance This is one of many state funded research projects. There is another presentation here on an assessment of window films/panels and we are submitting an assessment of water pump water heaters. We also have projects on MF ventilation, MF/commercial condensing boilers, commercial ERVs, indoor pool RCx, and using aerosols to seal apartment buildings. The Innovation Exchange is the research and dissemination arm of the Center for Energy and Environment, creating opportunities to communicate research and knowledge to energy and building experts through… webinars, forums, blogs posts, and social media as well as data visualizations and other tools that help make information more applicable.
- We updated our web site this past year, making it easier to download past reports, presentation, and webinars. The final report for this project will be posted on our web site and we have posted an abbreviated version of this presentation on the site. If you come to our booth, you can get your name on our distribution list and we’ll let you know when the final report is available.
- Just wanted to recognize the many people who contributed to this project. We had a number of staff working into the night and sometimes the next morning to complete air leakage tests. The Energy Conservatory donated staff and equipment for the testing and Larry Harmon of Air Barrier Solutions conducted many of the air leakage investigations.
- I’m going to start out by going over some nomenclature. If you are like me a few years ago you are used to thinking about the cfm50 or ach50 of a house. Larger buildings are commonly tested for their air leakage at a pressure difference of 75Pa (higher than 50 used for residential). Typically tested under both pressurization and depressurization. Most common specification in C&I is that air flow divided by the envelope area – including the roof, above & below grade exterior walls
- For the past few years the Army Corp has required their new buildings to pass an air leakage test. The standard is no more than 0.25 cfm of leakage per sq ft of leakage at 75Pa. 7 states now include an air leakage test for code compliance. Washington State and City of seattle require some of their commercial buildings to test.
- Commercial buildings that operate on 6 day work schedule typically have HVAC running about 50% of time. So the rest of the time there is no pressurization. Even when the building isn’t occupied, uncontrolled infiltration contributes to the space conditioning load and can cause problems such as frozen pipes near infiltrating air. For example, one of our RCx projects involved a commercial high rise building that had a water main pipe freeze and burst on a cold night. The pipe was near the location where the canopy attached and there was significant air leakage. As a result, they ran their air handlers through the night when ever the temperature was much below 30F. We air sealed the canopy attachment and at the top of the building – now they can keep their air handlers off at night all through the winter.
- The figure on the left shows a 4 story building that has an envelope leakage of 27,000 cfm@75Pa (20,745 cfm50 or 1.7 ACH50). With the inside temp = outside, no wind and the Rooftop unit is off. There are no driving forces, so no air infiltration. On the right shows the same building with the rooftop unit operating. The system has been designed to pressurize the building to 0.05 in water or 12.5Pa. It does that by bringing in 10,500cfm of outside air into the rooftop unit – which is the amount of outside ventilation air required for the building. And it is exhausting 2,075cfm through a combination of exhaust fans and other pressure relief. The difference is 8,425cfm. With more outside entering than leaving through the HVAC system the building is pressurized and the 8,425cfm of air is leaving or exfiltrating through the building envelope. The arrows on the right show the pressure profile on the building – the entire envelope is pressurized to 12.5Pa.
- This is a typical roof top unit that are used in all types of C&I buildings. There are often multiple units for larger buildings. The diagram on the right show typical operation for minimum outdoor air conditions. The fan draws in return air from the space and outdoor air. The mixed air is heated or cooled and supplied to the space. Pressure relief can be either active or passive. Sometimes the exhaust fan flow balances the outdoor air. There may be passive relief or gravity dampers that open when the building pressure is too high. Or sometimes relief dampers are operated by a building pressure control. {do not discuss } It is even more important to have pressure control when the roof top unit has an economizer. That type of operation is shown in the lower figure. When the outside air temperature is lower and the building requires cooling, the unit brings in more or sometimes 100% outside air. Active or passive pressure control is necessary to keep the building inside to outside pressure at a reasonable level. Often passive relief dampers are built into the unit.
- This is a simple single-zone constant volume air handler. It has separate supply and return fans that operate at constant speeds and turn on/off according to the building schedule. They may operate during unoccupied hours as necessary to heat or cool the building. The outside air damper has a minimum position setpoint necessary to bring in the minimum required amount of outside air for ventilation. In this case 10,500cfm. There is also a relief air duct that exhaust air out of the building. The amount of exhaust air is controlled by the relief air damper position – these are usually set to be equal or somewhat less than the OA position.
- If the rooftop or constant volume air handler system operates properly, it will provide a constant amount of air that helps pressurize the building. These diagrams show the same building and rooftop unit with an outside temperature of 20F. On the left the system is not operating and the stack effect is causing air to enter at the bottom of the building and leave the top. The arrows show the pressures on the building. There is more leakage at the top of the building, so the neutral level where the inside pressure = outside is closer to the top of the building. The infiltration rate is 2,350cfm which is over 20% of the outside air requirement when the building is occupied. That requires about 30 therms/day to heat. The diagram on the right shows the same weather conditions with the rooftop unit running to pressurize the building. With the thermal stack effect you don’t get an even pressure distribution. It is lower at the bottom than top of the building. So you don’t get as much protection against wind pressures at the bottom of the building.
- In fact, when the temperature drops to 0F (which happens for about 350 hours in a typical year in Minneapolis) the building is no longer pressurized at the bottom and there is some air infiltration. The diagram on the left shows that when the building is pressurized and the outside temperature drops to 0F, there is some infiltration at the bottom of the building. So our intent was to pressurize the building to avoid infiltration – but at colder conditions we lose that pressurization at the bottom of the building.
- This final diagram shows the same building at 20F outside air temperature and a 15mph wind coming from left to right. The wind causes a negative pressure of about 12Pa at the bottom of the building towards the wind and results in an air infiltration rate of about 400cfm. For higher winds and colder outside temperatures you’ll get more infiltration. The infiltration will also go up if the HVAC system pressurization is reduced. Our experience is that happens quite often during economizer operation.
- Here is our rooftop unit again. The diagram on top shows the same operation with minimum outside air for ventilation. Pressure relief can be either active or passive. Sometimes the exhaust fan flow balances the outdoor air. There may be passive relief or gravity dampers that open when the building pressure is too high. Or sometimes relief dampers are operated by a building pressure control. It is even more important to have pressure control when the roof top unit has an economizer. That type of operation is shown in the lower figure. When the outside air temperature is lower and the building requires cooling, the unit brings in more or sometimes 100% outside air. Active or passive pressure control is necessary to keep the building inside to outside pressure at a reasonable level. Often passive relief dampers are built into the unit.
- In Minnesota air handlers are often setup for economizer operation. When the outside air temperature is below the inside temperature and the space needs to be cooled, the outside air dampers will open beyond the minimum position to provide some of that cooling. The OA damper will often be controlled to achieve a mixed air temperature setpoint. Typically, the relief air damper will track the OA damper position – it may sometimes be set to lag the OA position. Some systems attempt to automatically adjust to achieve a building in/outside pressure difference. A sensor is used to monitor the building in/outside pressure difference and the return fan speed or relief air dampers are adjusted to achieve the setpoint pressure. This can be a great approach for keeping a the desired building pressure for different economizer settings and changes in outside temperature. However, building pressure control often doesn’t work well. We’ve seen outdoor pressure terminations installed upside down so that they fill with water, terminations installed on the roof when they were intended to control the ground level pressure, and overly responsive controls that cause relief dampers to be either all the way open or closed on windy days. Buildings often economize down to 20F.
- This can get even more complicated with variable air volume air handlers and variable air volume boxes. I won’t discuss this any further for this webinar, but suffice to say that VAV systems can be challenging to setup for proper heating and cooling without considering building pressure control. There may be a control algorithm for the return fan speed based on the supply fan speed. Or sometimes the building in/outside pressure difference is used to control the return fan speed or exhaust damper position. Some buildings use the in/out pressure at each floor to control the return dampers for that floor.
- We can model that building with a multizone air flow model called Contam. This graph shows the variation in building infiltration with different levels of pressurization. The blue line shows no pressurization. The top line shows depressurization. You can see that even when the building is pressurized to 17Pa, there is still infiltration. That is partly because the system doesn’t run 24/7 and partly because even that high of pressure isn’t enough to overcome wind in colder weather.
- Here is what happens when we tighten up that building and provide the same levels of pressurization. You can see that the infiltration (many during unoccupied times) drops.
- Using $0.58/therm
- Using $0.58/therm
- They have been collecting this data since about 1998. Includes information on year built, building type, floor area, # stories, location, & wall type
- TRNSYS Model includes weather and HVAC pressure effects. Conducted for five US cities in different climate zones. Office building 0.23 ach = about 1,100cfm average. Retail building 0.22 ach = about 440 cfm average.
- Cfm50 is about 77% of the cfm75
- 55,000sf library/office building. Started at 12,300 cfm75 or 0.09 cfm75/sf or EqLA of 6.9 sf. Reduced cfm75 by 8% to 11,370 and sealed about a square ft of leakage using two-part foam to seal wall/roof joint. Cost of about $1,200.
- 55,000sf library/office building. Started at 12,300 cfm75 or 0.09 cfm75/sf or EqLA of 6.9 sf. Reduced cfm75 by 8% to 11,370 and sealed about a square ft of leakage using two-part foam to seal wall/roof joint. Cost of about $1,200.
- Front and sides have Dryvit EIFS at roof wall joint, FSK tape at corrugated deck joint, no CFM75 reduction likely from upgrade for strength or durability– improvements are judged on reduction in cfm @ 75 pa.
- 44% tighter than the Army Corps Standard before sealing: 9,180 cfm75, 4.6 sf EqLA, 0.14 cfm75/sf After sealing: 8,470 cfm75, 4.1 EqLA Reduced leakage by about 0.5 sq ft (8%) Normalized leakage now 0.13 cfm/sq ft
- 55,000sf library/office building. Started at 12,300 cfm75 or 0.09 cfm75/sf or EqLA of 6.9 sf. Reduced cfm75 by 8% to 11,370 and sealed about a square ft of leakage using two-part foam to seal wall/roof joint. Cost of about $1,200.
- Whole building air leakage tests are typically conducted with temporary seals on the mechanical dampers or penetrations. For this project we also conducted a single point test with the seals removed. We found that the mechanical system leakage increased the enclosure leakage by 17% to 103%. There was a trend for higher mechanical leakage in older buildings. For the buildings built before 1985 the average mechanical leakage as a percentage of envelope leakage was 64%, where it was only 25% for the buildings built after 1985. Moreover, the three buildings with the greatest mechanical leakage were built before 1968. Brennan et al. (2013) conducted similar tests for 14 buildings and found that the mechanical leakage ranged from 2% to 51% of the envelope leakage. The average mechanical leakage of 27% for these buildings built since 2000 is consistent with the average of 25% for the four newer buildings in this study. These results suggest that improving damper tightness offers a significant energy efficiency opportunity to existing buildings
- Whole building air leakage tests are typically conducted with temporary seals on the mechanical dampers or penetrations. For this project we also conducted a single point test with the seals removed. We found that the mechanical system leakage increased the enclosure leakage by 17% to 103%. There was a trend for higher mechanical leakage in older buildings. For the buildings built before 1985 the average mechanical leakage as a percentage of envelope leakage was 64%, where it was only 25% for the buildings built after 1985. Moreover, the three buildings with the greatest mechanical leakage were built before 1968. Brennan et al. (2013) conducted similar tests for 14 buildings and found that the mechanical leakage ranged from 2% to 51% of the envelope leakage. The average mechanical leakage of 27% for these buildings built since 2000 is consistent with the average of 25% for the four newer buildings in this study. These results suggest that improving damper tightness offers a significant energy efficiency opportunity to existing buildings