SEEM Updates Preview:Infiltration and Ventilation

Ben Larson1 November 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 which predicts the annual heating and cooling energy requirements of a building

NEEA has funded development of an infiltration and ventilation module to SEEM

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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 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)

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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

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Comparison to Other Methods

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

Hourly Infiltration Estimates for Stack and Wind Effects

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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.

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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

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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