Objectives

• Finish air cleaner problem

• Measure ventilation rates• Focus on smaller buildings

Measuring air exchange rates

• Two general strategies to get λ• Direct measurement

• Tracer gas (constant injection or decay)

• Apply well-mixed reactor model

• Indirect measurement • Blower door

• Apply infiltration model

Tracer Gas Testing

• Release gas and measure concentration

• Use model to estimate ventilation rate

• Properties of a tracer• Nontoxic• Non-reactive (chemically inert)• Not present at significant concentrations in typical

environments• Easy to measure over wide concentration range

Common tracers

• SF6

• ppm with IR absorption• ppb with GC/ECD

• Tracers of chance (eg. CO2)

Tracer Gas Decay Test

• Seed tracer gas

• Mix well

• Aim for 10 samples over ~1 time constant

• Use reactor model to predict concentrations

Δt

CC

eCC

C

iΔti

t

dtdC

lnln0

Decay Test

• Advantages• Don’t need to release precise amount• Don’t need to measure volume (if you just want air

exchange rate)

• Disadvantages• Need to keep building well-mixed• Recontamination from buffer spaces• House needs to stay in one condition for entire test

Building Air Exchange Rates

0

0.5

1

1.5

2

2.5

3

0 20 40 60 80 100 120

% Less Then

Air

Exc

han

ge

Rat

e (1

/hr)

Residential Commercial

Murray and Burmaster (1995) Risk Analysis

Constant Injection

• Continuously release known amount of tracer gas into space

• Continuously mix air

• Use model to determine air exchange rate

Q

EC

eQ

EtC

ss

tVQ

1)(

Constant Injection

• Advantages• Can determine time-dependence of air exchange

rates

• Disadvantages• Need to keep building well-mixed• Recontamination from buffer spaces• Need to have mass flow controller• Typically uses more tracer gas• Need to measure volume

Common problems

• Contamination from buffer spaces

• HW #2 Problem 3 (last year)

Summary

• Tracer gas can be used to measure ventilation rate• Assumes reactor model• Constant injection or decay

• Measures ventilation rate at time of test• Varies with time, ΔT ,wind

Fan Pressurization – The Early Years

• In 1970s, smoke evacuation fans used to find air leakage

1. Install blower door

2. Use fan to create artificial pressure difference between inside and outside

3. Use smoke stick (or cigarette, etc.) to visualize flow patterns.

4. Seal leaks

Ref: Home Energy Magazine, 1995

Limitations

• Flow rate (can use multiple fans…)

• Not good for complex leakage paths• Multizone buildings

• Does not give you air exchange rate• Independent of wind, ΔT, time• Useful for comparison of different buildings

Blower Door (quantitative)

house averagean for 65.0,5.01

leaks of Size

1 Refor constant, 1 Refor

2

5.0

nn

c

PcQ

Pcc

PAcQ

ndd

d

Procedure for Blower door test

1. Install blower door

2. Use fan to create artificial pressure difference between inside and outside

3. Measure flow at several inside-outside pressure differences

4. Find n and C

Ref: Sherman and Dickerhoff (1994, 1998)

Residential Component Leakage

Component Range Average

Walls 18-50% 35%

Ceilings 3-30 13

Windows/Doors 6-22 15

Fireplaces 0-30 12

HVAC 3-28 18

Other Vents 2-12 5

Reporting blower door data

• Flow (or air exchange rate) at a given pressure• Why is air exchange rate preferable?

• Leakage Area

• What units are c in?

)4(2

)4(

Pa

PacELA

n

Where does that leave us?

• ELA is at specific reference pressure.

• So, use LBL model and improvements to get infiltration and ventilation rates• ASHRAE 2001 Handbook of Fundamentals 26.21-26.24

• Requires L, ∆T, wind and shelter information• More advanced forms require distribution of leakage

LBL Model

22swws QQQ Ref: Sherman (1992) Indoor Air

2VCtCAQ wsL

Q = Ventilation air flow (CFM)

AL = ELA from blower door test (in2)

Cs = stack coefficient f(height) (CFM2/(in4 °F))

Δt = temperature difference (°F)

Cw = wind coefficient f(height, shielding) (CFM2/(in4 mph))

Ref: ASHRAE Fundamentals (2001) ch. 26

Summary

• In order to measure air exchange rates• Need to use direct tracer gas measurements to get a

snapshot of λ• Use reactor model

• Need to use a blower door to compare to other buildings

• Use LBL (or other) model to get λ