SEEM Updates: Infiltration and Ventilation

1. Michael Logsdon, Ben Larson, David Baylon 13 December 2011 SEEM Updates:Infiltration and Ventilation ben@ecotope.com 4056 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, to 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

6. 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.

7. Step 1: Compute Pressure Differences Due to Stack and Wind Driving Force: Wind Speed Driving Force: ΔT

8. 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”

9. 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…

10. SEEM Specific Assumptions: Flow • Flow Exponent n=0.65 Leakage Area Distributions Crawlspace Floor: 25% Walls: 50% Ceiling: 25% Slab/Heated Basement Floor: 0% Walls: 67% Ceiling: 33%

11. 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

12. 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

13. 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.

14. Example Results: Figures

15. 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

16. 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

17. 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

18. Example SEEM Output: Tables Fan runs 8 hours per day, simulated in a house with 7 ACH50.

19. 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 can be sampled at short intervals 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 • We’d rather have hourly data to compare to the hourly infiltration model but…  Third in a series of reports which covered all 10 houses in the entire project.

20. Field Data Comparison

21. 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

22. 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

23. 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

24. Open Issues • Ecotope still exploring comparison of hourly MTMS-based infiltration measurements to simulation predictions (requires recovering 20-year old archived data) • 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

25. 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.

26. End