seem updates: infiltration and ventilation
DESCRIPTION
Michael Logsdon, Ben Larson, David Baylon 13 December 2011. SEEM Updates: Infiltration and Ventilation. [email protected] 4056 9 th Avenue NE, Seattle, WA 98105 (206) 322- 3753. Intro. SEEM: Simple Energy and Enthalpy Model - PowerPoint PPT PresentationTRANSCRIPT
SEEM Updates:Infiltration and Ventilation
Michael Logsdon, Ben Larson, David Baylon13 December 2011
[email protected] 9th Avenue NE, Seattle, WA 98105(206) 322-3753
2
Intro
SEEM: Simple Energy and Enthalpy Model Used at the RTF and throughout the region
to model energy use of residential buildings The simulation currently has an energy
balance and air moisture balance Under a NEEA project, Ecotope has
developed of an infiltration and ventilation module, an air mass balance, for SEEM
3
Outline
Changes to SEEM Natural infiltration sources Calculating infiltration due to natural
& mechanical sources SEEM specific modeling assumptions Example infiltration model output
Comparison to other infiltration models Comparison to field measurements Discussion
4
Change Overview
Current SEEM uses a fixed value for the outside air infiltration to the house. Input in ACHn (natural air changes per hour) & is constant every
hour of year Input value includes sources of outside air: infiltration &
mechanical ventilation Duct leakage impacts are calculated separately
Updated SEEM calculates a different outside air infiltration amount for every hour of the year based on mass balance: stack effect, wind, mechanically inducted airflows, and both
balanced and unbalanced duct leakage Key new inputs:
▪ CFM50Pa: the blower door test result of air leakage at 50 Pascals pressure difference
▪ Stack height: average height of a column of warm indoor air above grade
▪ Fan flows and schedules Additional updates: hourly schedules for internal gains and
thermostat settings
5
New Inputs & Outputs to SEEM Inputs:
CFM50 Stack Height Fan Type (Exhaust, Supply, HRV) Fan CFM Fan Efficiency or HRV Efficiency Schedules: Fan, T-Stat, Internal Gains▪ Schedules are hourly & include 7 individual days per week
Outputs: Average Annual ACH Ventilation Fan Energy (kWh/yr to run ventilation system) Balance Point
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Definitions
Natural Infiltration: airflow caused by pressure differences across cracks and leaks
Total infiltration: airflow caused by the cumulative effects of natural infiltration and mechanical ventilation.
The model does not account for occupant effects such as opening doors or windows.
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Step 1: Compute Pressure Differences Due to Stack and Wind
Driving Force: ΔT Driving Force: Wind Speed
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Step 2: Compute Flow from Pressure
Empirical Power Law Flow: Q=CΔPn
Q – Flow rate, typically Cubic Feet per Minute (CFM)
C – Constant with units CFM/(Pascals^n) ΔP – Pressure difference n – Dimensionless flow exponent
Rewrite equation to define a “leakage area”
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Step 3: Find ΔP Satisfying Continuity
Inflows are positive, outflows are negative, and all flows must sum to zero: Mass is neither created nor destroyed in this process.
Flow through the floor, walls, and ceiling depend on pressure difference as found from stack effect and wind.
Mechanical flow is the net, unbalanced flow rate due to mechanical sources, such as exhaust fans, unbalanced duct leakage, etc…
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SEEM Specific Assumptions: Flow
Flow Exponent n=0.65
Leakage Area DistributionsCrawlspace
Floor: 25%Walls: 50%Ceiling: 25%
Slab/Heated BasementFloor: 0%Walls: 67%Ceiling: 33%
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Average value of flow exponent from Modeled & Measured Infiltration Papers is 0.658 (sample size = 10)
Blower Door User Manual suggests using n=0.65 as a typical flow exponent for large sample sets
In progress RBSA dataset
SEEM Specific Assumptions: Flow
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SEEM Specific Assumptions: Wind
Houses are Square Wind acts only on the walls Wind approaches either orthogonal to a face or at
a 45° angle Leakage area is distributed uniformly along the
walls
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SEEM Specific Assumptions: Wind
Velocity is found according to the AIM-2 method
Meteorological wind speed is corrected to site wind speed
Vsite is further reduced for local shelter to Veffective
Assume Shelter Class 3 “Heavy shielding, many large obstructions within two house heights with Sw=0.7.
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Example Results: Figures
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Natural Infiltration Compared
Annual average ACHMethod
Climate SEEM AIM-2 LBL 62.2
Seattle, WA 0.26 0.24 0.35 0.40
Portland, OR 0.25 0.23 0.34 0.35
Boise, ID 0.26 0.25 0.36 0.41
Spokane, WA 0.30 0.29 0.41 0.40
Kalispell, MT 0.30 0.28 0.38 0.37
Sample calculations for a house with CFM50=2182 (7ach50), stack height=16 ft, floor area=2200ft2, volume=18,700ft3, flow exponent=0.65
Std 62.2 calcs from spreadsheet for whole house ventilation requirements – natural infiltration only
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Example Results: Figures
Sample calculations for a house with CFM50=2000, stack height=16 ft in a Seattle climate. Exhaust fan flow is continuous.
Solid lines calculated combined infiltration and exhaust flows using full model
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Example Results: Figures
House Characteristics:▪ Floor area 2200 ft2
▪ 16 ft stack height▪ Leakage of 2182
cfm at 50Pa (7ach50)
▪ Volume 18,700 ft3
▪ Duct leakage: ▪ 12% supply ▪ 10% return
PortlandSeattleSpokane Boise Kalispell0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
Average Annual ACH
Ducted Furnace w/ AC & 100cfm ex-haust 4hrs/day
Ducted Furnace w/ AC & 50cfm exhaust 4hrs/day
Ducted Furnace w/ AC 70-64F Setpoint
Zonal Heat 70-64F Setpoint
Zonal Heat 66F Setpoint
Air
Ch
an
ges p
er
Hou
r
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Example SEEM Output: Tables
gfacsf1344, Seattle WashingtonFan CFM ACH Ave Heating Load (kWh)
0 0.26 533810 0.27 540220 0.28 546830 0.29 553540 0.29 560550 0.30 567760 0.31 574970 0.33 582480 0.34 590490 0.35 5988
100 0.37 6079
Fan runs 8 hours per day, simulated in a house with 7 ACH50.
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Field Data Comparison
Comprehensive measurements of infiltration in houses using a multi-tracer measurement system (MTMS) Tracer gases injected in a controlled way to each zone. Gas concentrations
were sampled every 12 minutes to measure infiltration on small time steps. Measurement period typically lasted 2-5 days depending on the site Data presented in report provides average values of temperature, wind
speed, and measured infiltration over measurement period Logged, interval data provides the best basis for comparison - currently
have this data for one site, Site #9
Third in a series of reports which covered all 10 houses in the entire project.
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Field Data Comparison
2 3 4 5 6 8a 8b 9 100
20
40
60
80
100
120
140
160
Simulated Natural Infiltration @ Average Con-ditions vs Average Measured Natural Inilftra-
tion Across All Conditions
Predicted from Average Condi-tions
Average Measured
Site/Test#
CFM
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Field Data Comparison
House characteristics 2-story site-built house
over a daylight basement 1930s era construction Seattle 1500ft2 20ft stack height Electric furnace & supply
ducts in basement 13ACH50 Duct leakage unmeasured
Measured data sampled every 12 minutes (5x/hr)
Model comparisons made for natural infiltration only – excluding duct leakage effects
March 27
Air handler on
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Field Data Comparison
April 1
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SEEM Infiltration w/ Air Handler On
Sample SEEM simulation output showing varying infiltration and effects of duct leakage & air handler
House characteristics: 2200ft2 Seattle TMY3 climate 16 ft stack height 7ach50 Duct leakage 15% supply,
12% return
2700 2724 2748 2772 2796 2820 28440
50
100
150
200
250
300
SEEM Simulation Ouput w/ Air Handler Flow
Hour of Year
Infi
ltra
tion (
cfm
)
Air handler on
April 22
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New Capabilities w/ Updated Model
Houses with ventilation systems which operate on an hourly level can be modeled
Infiltration more accurately modeled over the entire year More infiltration under strong heating and cooling
conditions and less in the shoulder seasons Energy impacts of ventilation codes/stds, such
as ASHRAE 62.2 can be modeled Interior installations of heat pump water heaters
combining a ventilation and internal gains schedule can model both vented and unvented scenarios
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Implications
What do we mean when we say a house has 0.35ach? (effective annual average outside air changes) “divide by 20” rule of thumb for converting BD tests to ach
natural was largely derived from datasets for total infiltration in the heating season
Without mechanical sources, the natural infiltration implied by a 7ach50 test, gives 0.22-0.31 effective annual ach depending on building type and climate.
To get to 0.35ach, if the blower door test is 7ach50, the annual effective air change will also include mechanical sources
New infiltration model allows (requires) us to understand (assign) separate sources of outside air: ▪ stack, wind, ducts, mechanical ventilation
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Discussion
Infiltration calculations make SEEM more physically grounded Leads to better understanding of house leakage
and ventilation systems Hourly schedules add more flexibility and
complexity
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Open Issues
Input value calibration exercises for site-built and manufacture houses Given existing priorities in the RTF work plan,
recalibration of existing single family, site-built house simulations and measures to be conducted at a later date ▪ Potentially not until the measures sunset
Manufactured house calibrations presented later today
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Decision
Motion: Adopt the updated version of SEEM, with
its new infiltration calculations, for use in modeling site-built houses, manufactured houses, and small-scale multi-family buildings.