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Michael Logsdon, Ben Larson, David Baylon 13 December 2011. SEEM Updates: Infiltration and Ventilation. ben@ecotope.com 4056 9 th Avenue NE, Seattle, WA 98105 (206) 322- 3753. Intro. SEEM: Simple Energy and Enthalpy Model

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Michael logsdon ben larson david baylon 13 december 2011

Michael Logsdon, Ben Larson, David Baylon

13 December 2011

SEEM Updates:Infiltration and Ventilation


4056 9th Avenue NE, Seattle, WA 98105

(206) 322-3753


  • 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


  • 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

Change overview
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

New inputs outputs to seem
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


  • 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.

Step 1 compute pressure differences due to stack and wind
Step 1: Compute Pressure Differences Due to Stack and Wind

Driving Force: Wind Speed

Driving Force: ΔT

Step 2 compute flow from pressure
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”

Step 3 find p satisfying continuity
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…

Seem specific assumptions flow
SEEM Specific Assumptions: Flow

  • Flow Exponent n=0.65

Leakage Area Distributions


Floor: 25%

Walls: 50%

Ceiling: 25%

Slab/Heated Basement

Floor: 0%

Walls: 67%

Ceiling: 33%

Seem specific assumptions flow1
SEEM Specific Assumptions: Flow

  • 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 wind
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

Seem specific assumptions wind1
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.

Natural infiltration compared
Natural Infiltration Compared

  • 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

Example results figures1
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

Example results figures2
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

Example seem output tables
Example SEEM Output: Tables

Fan runs 8 hours per day, simulated in a house with 7 ACH50.

Field data comparison
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.

Field data comparison2
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

Air handler on

March 27

Seem infiltration w air handler on
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

Air handler on

April 22

New capabilities w updated model
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


  • 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


  • Infiltration calculations make SEEM more physically grounded

    • Leads to better understanding of house leakage and ventilation systems

  • Hourly schedules add more flexibility and complexity

Open issues
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


  • 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.