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SEEM Updates: Infiltration and Ventilation

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### SEEM Updates:Infiltration and Ventilation

Michael Logsdon, Ben Larson, David Baylon

13 December 2011

4056 9th Avenue NE, Seattle, WA 98105

(206) 322-3753

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

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

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

- 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

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.

Step 1: Compute Pressure Differences Due to Stack and Wind

Driving Force: Wind Speed

Driving Force: ΔT

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 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

- Flow Exponent n=0.65

Leakage Area Distributions

Crawlspace

Floor: 25%

Walls: 50%

Ceiling: 25%

Slab/Heated Basement

Floor: 0%

Walls: 67%

Ceiling: 33%

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

- 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: 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

- 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: 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: 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

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

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 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

Field Data Comparison

April 1

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

- 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

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

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

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

- 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

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.

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