SEEM Updates:Infiltration and Ventilation

Michael Logsdon, Ben Larson, David Baylon13 December 2011

ben@ecotope.com4056 9th Avenue NE, Seattle, WA 98105(206) 322-3753

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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

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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

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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

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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.