HVAC Equipment Sizing Strategies: Taking Advantage of High-Performance Buildings
EEBA Excellence in Building Conference
October 24, 2001 Building Science Corporation www.buildingscience.com
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 1 of 17 Other Resources
•Proctor Engineering Group (San Rafael, CA) www.proctoreng.com
Article: “Bigger is Not Better: Sizing Air Conditioners Properly”
•Florida Solar Energy Center (FSEC) www.ucf.fsec.edu
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 2 of 17 Benefits of ‘Right-sizing’ Equipment
•Reduces short cycling (lower efficiency) •More moisture removal (latent load) •Reduces electrical peak load •Smaller and simpler HVAC system—easier to fit inside conditioned space •Comfort & noise—“blast of cold air” effect & mixing •Lower equipment cost—recoup costs of energy- related upgrades
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 3 of 17 Reasons for Equipment Oversizing
•Rule of thumb sizing strategies (e.g., 400 sf/ton) •Tradition—always done this way •Avoiding callbacks (covers up problems with underperforming equipment) •Room for expansion or for unforeseen loads
•Oversizing from rule of thumb is worse in buildings with high-performance envelopes
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 4 of 17 So how is equipment sized correctly?
•Load calculation: ACCA Manual J •Computes heat flow in/out of the building at design conditions (1% design temperature) •Manual J has safety factors built in—fudging above that load is unnecessary
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 5 of 17 Components of a load calculation
•Regular heat conduction—walls, roofs, floors •Windows—add radiation (sunlight) •Air movement—infiltration & ventilation
•Latent load—moisture/humidity to be removed by cooling system
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 6 of 17 Heat conduction
•U x A x ÄT = heat flow through wall/roof/etc
•U = 1 / R-value (e.g., R-30 ˜ U=0.033) •A = area •ÄT = temperature difference
•Be sure that upgraded building components (e.g., 2x6 walls, insulating sheathing) are accounted for
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 7 of 17 Windows •1/3 to 2/3 of cooling load typically from windows •U-value (insulation) •SHGC (solar heat gain coefficient) or SC (shading coefficient) •Shading—external and internal •House orientation—if known, use it.
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 8 of 17 Air movement
•Infiltration—unintentional air movement. Measured in ACH (air changes per hour) •Ventilation—intentional air movement. Measured in CFM (cubic feet per minute)
•Well-sealed buildings have lower infiltration—0.1 ACH measured in Building America houses
Therefore lower infiltration loads.
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 9 of 17 Other Items
•Latent load—don’t use “30%” rule of thumb •Duct losses—vary with location and insulation level. Are zero with ducts inside conditioned space. •“Swing multiplier”—used to account for equipment capacity loss at high outdoor temperatures. Sometimes used incorrectly as general fudge factor.
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 10 of 17 Equipment Resizing Example (Northern CA) Modifications included: 5.5 5.0 4.5 •Spectrally-selective 4.0 windows 3.5 3.0 2.5 •Unvented roof/ducts 2.0 inside conditioned space 1.5 1.0 •Tighter building envelope 0.5 0.0 Plan 1 2459 sf (1 story) Plan 3 3054 sf (2 story) •Thicker wall insulation
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 11 of 17 Sizing equipment below Manual J Loads: Las Vegas, NV (Arbor View)
36.0 30.0
24.0 Latent 18.0 Sensible
kBtu/hr 12.0
6.0 - 1787 1787 Calc. 2260 2260 Calc. Equipment Load Equipment Load
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 12 of 17 Arbor View Plan 2260 On-Time Frequency
Frequency of cooling on-time
200 100
Lot 6 80 150 Arbor View
60 100 40
50 20 Percent of 0.5 h intervals Number of 0.5 h intervals 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 On-time fraction
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 13 of 17 Arbor View Plan 2260 Runtime vs. ÄT
Cooling On-time fraction vs. Outside to Inside Temp. Diff. 1 Lot 6 Arbor View 0.8
0.6
0.4
0.2
Cooling On-time fration 0 -5 0 5 10 15 20 25 30 35 Temperature Difference (F)
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 14 of 17 Problems Seen When Resizing Equipment
•Customer perceptions—“Why do I have 3 tons when my neighbor has 5 tons?” •Customer complaint—“Why is my equipment running so long? Its never done that before.” •Greater vulnerability to poorly installed systems— duct leakage, improper refrigerant charge, or low airflow. •Higher recovery times from deep setbacks— instruct customers to “set & forget” thermostats.
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 15 of 17 Window Effects on Loads & Efficiency
Glazing Ratios vs. Energy Star
89.0 88.5 88.0 87.5 87.0 86.5 86.0 85.5 12.5% 15.0% 17.5% 20.0% 22.5% 25.0%
Floor Glazing Rest Wall Glazing Rest Wall Glazing
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 16 of 17 Window Effects on Loads & Efficiency
Cooling Load vs. Window Area
80.0
70.0
60.0
50.0 Design Load (kBtu/hr) 40.0 400 500 600 700 800 900 Window area (sf rough opening)
Building Science Corporation
PR-0110: HVAC Equipment Sizing Strategies—Taking Advantage of High-Performance Buildings Page 17 of 17