SEEM Updates Preview: Infiltration and Ventilation

1. Ben Larson 1 November 2011 SEEM Updates Preview: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 which predicts the annual heating and cooling energy requirements of a building • NEEA has funded development of an infiltration and ventilation module to SEEM

3. 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 covers sources of outside air including infiltration and mechanical ventilation • Duct leakage impacts are calculated separately • Updated SEEM will calculate a different outside air infiltration amount for every hour of the year based on mass balance: • stack effect, wind, unbalanced duct leakage, and mechanically induced air flows. • New input will be CFM50Pa, the blower door test result of air leakage at 50 Pascals pressure difference • The user may input a fan schedule and airflow to interact with natural effects, • Additionally, internal gains can now be scheduled on an hourly basis (previously a user-set constant value)

4. Alternative Predictive Models • LBL (1980): Sherman and Grimsrud • Approach currently used in ASHRAE Stds • AIM-2 (1990): Walker and Wilson • Fundamentals the same across all models • Airflow through house described in set of non-linear equations • Implementations diverge • Computationally simple • LBL and AIM-2 solve eqns with analytical approximations but with differing assumptions/implementations • Computationally intensive • Ecotope model solves eqns numerically  leads to differing outcomes

5. Comparison to Other Methods Hourly Infiltration Estimates for Stack and Wind Effects • Building Parameters • Climate: Seattle • Stack height: 8.5’ • CFM50: 1800 • R: 0.5, X: 0 • ¼ of leaks in floors, • ¼ in ceilings, • ½ in walls • Flow exponent: 0.65

7. 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 Dashed lines combined infiltration and exhaust flows using Palmiter/Bond ½ rule.

8. New Capabilities with Updated Model • Houses with ventilation systems which operate on an hourly level can now be modeled • Infiltration now 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

9. Implications • What do we mean when we say a house has 0.35ach? (effective annual average outside air changes) • To get to 0.35ach, if the blower door test is 7ach50, the annual effective air change will also include mechanical sources • 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. • “divide by ___” rule of thumb for converting BD test to ach natural • New infiltration model allows (requires) us to understand (assign) separate sources of outside air: • stack, wind, ducts, mechanical ventilation • Potential implications for weatherization and sealing measures