Humidity Implications for Meeting Residential Ventilation

Requirements

Max H. Sherman LBNL

Myths and Ghost-stories

• Oversized A/C is bad – E.g. short-cycling hurts humidity control

• ERVs can dehumidify • Moisture is a contaminant • Humidity can be controlled by a thermostat • Ventilation standards are for moisture

Question: Could complying with ASHRAE 62.2 lead to moisture

problems? Answer: Typically NO

Why not? – In humid climates, high (>70% RH) indoor humidity occurs when

ambient temperatures are low (less than 65F), air conditioning is off – shoulder seasons – ventilation air is not high humidity for these outdoor conditions

– In summer AC operation leads to humidity controlled to 50% to 60% RH maximums

ASHRAE 62.2

• 62.2 requires same mechanical ventilationindependent of climate

• 62.2 increases ventilation rates for homes with tighter envelopes (compared to just infiltration)

• Typically 50 to 75 cfm for a new home • Additional kitchen and bath fans for source

control How does this affect moisture? Lets

“do the math”

Overview

• Combined ventilation, heat and moisture calculations

• Minute-by-minute for a year • Auto heat/cool • Six climates: Houston, Phoenix, Charlotte,

Minneapolis, Seattle and Kansas City • Output: Energy use & indoor conditions

Ventilation Mass balance Multi zone: house,

attic, supply &return ducts

Local and distributed envelope, attic andduct leaks

Mechanical ventilation Forced air system

operation Many air flow paths

one unknown (indoor pressure)

determined iteratively

Heat Transfer Energy balance combining conduction,

radiation, air flows, heating andcooling equipment, ventilation fan motors in air stream, internal and solar gains

Multi path: house, attic, supply & return ducts

• Duct insulation, roof materials, envelope insulation

Equipment models: • Furnace – constant heat • A/C – capacity and SHR depend on

indoor and outdoor temperature andhumidity (charge and air flow), drycoil/wet coil

16 unknown temperatures & 16 heat balance equations solvedsimultaneously

Moisture

– Mass balance for indoor air, attic air, supply air and return air.

– Duct leakage – Storage/exchange with indoor materials – Track coil moisture

• Dry coil takes time (about 3 minutes) to wet and remove moisture

• End of cycle/central fan operation w/o A/C re-evaporation

–Internal Sources

Internal Moisture Sources

• Occupant related only – no foundation drying or hydroponics… • Cooking and washing moisture removed at source (as required by

62.2) – 8.8 lb./day (NIST) – CBD – 17 lb./day + 2 lb. On a washday – NRCan – 2.75 lb./day/person + 5.3 lb./day (16.3 lb./day) – CBD 231 – 9.5 lb./day/person (38 lb./day) – ASHRAE Humidity design guide – 5.25 lb./day/person (21 lb./day) – ORNL – 12 lb./day/family + 13.7 lb./day (25.7 lb/day) – ASTM Manual 18: 14 to 15 lb.day – FSEC – 0.4 lb./hour + 0.4 lb./hour in evening (10.8 lb/day) – ASHRAE Std 160 – 30.4 lb./day/family

• Use ASHRAE Std160 values and subtract cooking and washing and assume 2/3 occupancy: 14.4 lb./day for four people

• Constant generation

Dehumidification Laboratory and manufacturers data relates Sensible Heat

Ratio (SHR) to humidity ratio (hr):

005.0501 hrSHR

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60 80 100

Indoor RH

SHR

76 F 78 F 80 F

Coil Model • Track mass of moisture on coil • Linear increase in latent capacity over first three

minutes of operation • Mass condensed on coil:

1 SHRQtotalmcond qlatent

Qtotal= total system capacity qlatent= latent heat of condensation

Coil Model

• No moisture removal until coil saturated

• Coil capacity mlimit = 0.66 lb./rated ton • After coil saturation moisture transported

to coil leaves system • Fan only – coil moisture re-evaporated

(takes 30 minutes to dry):

m ratedtonslim itmevap 1800s

Internal Moisture Storage

• Storage capacity = 12.3 lb./ft2 floor area • Mass transport coefficient = 0.0006 lb./ft2

• Gives same variation as field data (BSC/Henderson) ~ 10% RH diurnal swing

• Damping in indoor air variability similar to EPAs Indoor Humidity Assessment tool that assumes 5 to 10% moisture to air and 90% to 95% to contents

TMY Weather

House Info • 1000 ft2 1 story 1bath, 2000ft2 two story two bath and 4000 ft2 two

story three bath • ICC 2005 envelope • Windows 18% of floor area evenly NSEW • Envelope has NL=0.3 about 5.8 ACH50 • 3% supply & 2% return duct leakage for attic ducts (Houston,

Phoenix, Charlotte and Seattle) • 1.5% supply and 1% return duct leakage for basement ducts in

Kansas City and Minneapolis • EER 11/SEER 13 AC • 78% AFUE gas furnace • Heating: 70F 8 a.m. to 11 p.m., 68F rest of the time • Cooling: 78F 8 a.m. to 4 p.m., 76F rest of the time (76 constant at

weekends) • 1F deadband for thermostat

Ventilation Systems • No mechanical ventilation • Leaky Envelope – leaky enough to meet 62.2 • Continuous exhaust power from HVI directory • H/ERV power from HVI directory

– Three times 62.2 rate for 1/3 time • Source control:

– Each bathroom 50 cfm (55W) 7:30 to 8 a.m. – Kitchen 100cfm (99 W) 5 to 6 p.m.

• Central fan operation for mixing/distribution minimum 10 minutes per hour

Annual Average Ventilation (ACH) No Mech. Vent

Leaky House

Cont. Ex.

H/ERV

Houston 0.18 0.28 0.29 0.38 Phoenix 0.18 0.31 0.30 0.39 Charlotte 0.20 0.34 0.29 0.39 KC 0.24 0.39 0.32 0.43 Seattle 0.24 0.38 0.30 0.43 Minneapolis 0.29 0.42 0.36 0.47

Continuous Exhaust adds about 0.1 ACH H/ERV adds about 0.2 ACH

House Size Effect on Ventilation

• Continuous exhaust • Large house = 10% less ACH • Small House = 25% more ACH

Humidity Results

• RH binned every 1% RH • Mean humidity not interesting unless

mean is very high (over 60%) • Frequency of high humidity extremes • Extremes: number of hours above 60% &

70% • Duration of extreme events

0

100

200

300

400

500

600

0 10 20 30 40 50 60 70 80 90 100

Relative Humidity

Num

ber o

f Hou

rs

Standard House Leaky House Continuous Exhaust HRV

Phoenix 62.2 decreases mean RH Winter effect

0

100

200

300

400

500

600

0 10 20 30 40 50 60 70 80 90 100

Relative Humidity

Num

ber o

f Hou

rs

Standard House Leaky House Continuous Exhaust HRV

Minneapolis 62.2 no effect

Wide range of weather leads to wide RH range

0

100

200

300

400

500

600

0 10 20 30 40 50 60 70 80 90 100

Relative Humidity

Num

ber o

f Hou

rs

Standard House Leaky House Continuous Exhaust ERV

Houston 62.2 vent increases mean RH

62.2 small effect On elevated RH

ERV has no significant effect

0

100

200

300

400

500

600

0 10 20 30 40 50 60 70 80 90 100

Relative Humidity

Num

ber o

f Hou

rs

Standard House Leaky House Continuous Exhaust HRV

Seattle

Low outdoor humidity: 62.2 reduces RH Seattle is dry

0

100

200

300

400

500

600

0 10 20 30 40 50 60 70 80 90 100

Relative Humidity

Num

ber o

f Hou

rs

Standard House Leaky House Continuous Exhaust ERV

Charlotte 62.2 no effect at higher RH 62.2 lower average humidities

0

100

200

300

400

500

600

0 10 20 30 40 50 60 70 80 90 100

Relative Humidity

Num

ber o

f Hou

rs

Standard House Leaky House Continuous Exhaust HRV

Kansas City

WINTER SUMMER

Humid climate – higher RH ERV not effective

Dry climate – lower RH with increased ventilation

Standard House

Leaky House

Continuous Exhaust

ERV/HRV

Above 60% RH Houston 2659 2542 3266 3076 Phoenix 128 40 53 27 Charlotte 1496 1561 1619 1592 Kansas City 1499 1506 1653 1626 Seattle 1532 728 915 529 Minneapolis 1138 1120 1167 988

Above 70% RH Houston 797 624 758 614 Phoenix 3 0 0 0 Charlotte 338 287 312 262 Kansas City 222 224 240 168 Seattle 195 57 68 15 Minneapolis 258 226 219 155

Houston – Worst Case

• High humidity events are short duration: – Longest over 70% RH: 46 hours – Longest over 80% RH: 14 hours

• Summer humidity 50% to 60% • High humidity in shoulder seasons when

no AC operation: – Hours over 70% - outdoor temperature was

16.5C (62F)

0

10

20

30

40

50

60

70

80

90

100

1 21 42 63 84 105 126 146 167 188 209 230 251 271 292 313 334 355

Day of Year

Indo

or R

H (%

)

-10

10

30

50

70

90

110

Tem

pera

ture

, C

Indoor Temp

Outdoor Temp

Indoor RH

Houston – Continuous Ex.

Effect of Blower Operation for Mixing/Distribution

• Median humidity increased by 2% • No significant change to high humidity

events – High humidity NOT during AC operation so no

moisture to re-evaporate to contribute to high humidity events

Effect of House Size

• Occupant density – Larger house lower density – Lower density lower moisture generation per unit

volume lower humidity • Large house in Houston:

– 62.2 leads to increased hours above 70% RH (from267 to 405 hours) because large house has low indoor humidity

• Small house in Houston: – 62.2 leads to reduced hours above 70% (from 1937

hours to 1404 hours) because small house has higher indoor humidity such that it is more humidity than outdoors

Conclusions • In most climates 62.2 has small effects

– generally reducing high humidity events • In a humid climate 62.2 increased median humidity by 5 to 10%

BUT decreased high humidity (above 70% RH) events • ERV does not change indoor humidity in hot, humid • Operation of forced air system for mixing/distribution did not

lead to increased high humidity events • House size/occupant density matters • Caveats:

1. Indoor generation is key – unfortunately the thing we know best is that the range is wide and generally unknown for a specific house

2. 62.2 ventilation can increase (for larger houses/lower densities) or decrease (for smaller houses/higher density) humidity

Looking Forward

• Lower Sensible Loads, but not Latent – Humidity Control/design will be needed – New technologies?

• Figure out internal latent loads – Need measurements to resolve

• Evaluate/develop strategies – Simulations then field tests – Change to protocols/standards.

Myths and Ghost-stories

• In hot, humid climates….. – Oversized A/C is bad

• E.g. short-cyling hurts humidity control – ERVs can dehumidify – Moisture is a contaminant – Humidity can be controlled by a thermostat – Exhaust is bad