objectives finish talking about ways to minimize ventilation losses discuss role of ducts in...
TRANSCRIPT
Objectives
• Finish talking about ways to minimize ventilation losses
• Discuss role of ducts in building energy use
• Describe coheating test methodology
• Interpret duct retrofit results
• Measure duct leaks
Path Forward
• Read Chapter 1 of MCH for 3/2 Basics• Read Chapter 2 for 3/7 Transport• Read Chapter 3 for 3/21 Design• Get errata from:
http://www.buildingscience.com/updates/moisture_control_handbook_update.pdf
• MCH reading assignments – online version• http://www.ornl.gov/roofs+walls/facts/index.html• Incomplete (no Chapter 7, other differences)
Economizer
• Some buildings need cooling – even in winter
• Can get “free” cooling if outside air is below thermostat set-point
• Economizer = damper that lets in outside air• Many commercial buildings (Seay Psychology
building), some residential buildings
Economizer Issues
• Outdoor conditions• Particularly humidity levels
• Control logic failures• ~50- 60 % of commercial buildings have control
problems
• Real world concerns• Dampers stuck open/closed
Nighttime Flush
• Electricity is less-expensive at night• Time-of-use pricing• Lower electrical demand
• Fans use much less electricity than compressors
• Strategy:• High ventilation rate overnight• Large thermal mass accessible to air in building
Nighttime Flush
• Thermal Mass• High density• E = A∫q(t)dt• Release energy next day• Trombe wall/ rock wall for heating
• http://www.squ1.com/index.php?http://www.squ1.com/passive/trombe-wall.html
Challenges for Nighttime Flush
• Radiation (comfort)
• Condensation
• Losses
Ducts
• Last major issue in ventilation
• Largely a US problem• Swedish standard is 10,000 times stronger• Largely a small building problem
Duct losses
• We (in the US) locate ducts outside of the conditioned space• Duct losses account for 5-40% of the
heating/cooling bill for the average American home
• Interaction between duct losses and indoor air quality?• Some ducts are under negative pressure
Duct locations
• Attic
• Crawlspace
• Garage
• Interior soffits
• Basements
• Floor joists
Duct terminology
• Supply and return ducts
• Operating pressure
• Leakage to inside and outside
• Air handler flow
Duct efficiency
• System efficiency• Penalty associated with duct system
• Conduction losses (β)• Leakage losses (α)• Infiltration interaction
• Heat delivery efficiency• HDE = energy out of registers/energy in at furnace
• ASHRAE Standard 152• Calculation and measurement protocol
http://oikos.com/esb/28/duct_losses.html
How do we measure duct efficiency?
• Short term coheat tests
• Heat house normally with furnace + ducts
• Maintain same temperature distribution with space heaters
• Monitor total power consumed by house
• ηsys = Pheaters / Pfurnace
020
4060
8010
0
Duc
t Effi
cien
cy, %
HEATAG MAP HUD GAS ASH152
Heat Delivery Efficiency Distribution Efficiency
Coheat challenges
• Only one group has done them on a large scale and successfully• Very expensive and labor-intensive
• Doesn’t work for cooling (why not?)• Attempted in Florida in 1999-2000
Because of challenges with coheating,
• Most conservation programs focus on duct leakage• Sealing addresses leakage and infiltration losses• Duct efficiency leakage losses• Need an accurate way of measuring leakage
Duct leakage
• Subtraction
• Duct pressurization
• Nulling test
• Delta-Q
Blower door subtraction
• Can use blower door• Measure building
leakage with registers sealed and unsealed
• Difference is duct leakage
• Problems?
Duct pressurization
• Seal registers and pressurize ducts (like blower door testing)
• Add blower door to eliminate pressure difference between ducts and building
• Leakage to outside• Problems?
Duct leakage
Site
Fan Flow(CFM)
PS1 PS2 MV3 SA4 SA5 TX6* LV7 LV8 LV9 LV10 LV11
Lea
kag
e F
ract
ion
(%
)
0
5
10
15
20
25
1861 1962 1504 1361 1260 1415 1381 1944 1755 1455 1061
SupplyReturn
Ref: Siegel et al. (2003)
Air handler flow
• Duct efficiency scales with fraction of flow that is leaked to the outside (α)
• Typically, we measure duct leakage flow, so, in order to get α• Need air handler flow
• Several approaches• Replace air handler fan with calibrated fan and match
pressure distribution• Hot-wire anemometer traverse• Temperature rise method q = mCΔT
• Measure q, ΔT and calculate m
Nulling test
• Use blower door or duct blaster• Zero out pressure difference caused by unbalanced
leakage• Seal off return side of system measure supply
leakage directly• Directly measures leakage flows at operating
pressures
Ref: Francisco and Palmiter (2000) ASHRAE Transactions
Unbalanced Leakage
Qs-Qr
Delta-Q test
• Measure the pressure/flow response of house• With air handler on• With air handler off
• Use software and power-law relationship to get duct leakage to outside• Requires complicated model
• Accuracy still being debated
Walker et al. (2001)
Other ideas
• Flow plate w/ flow hood
How do you seal leaks?
http://www.wisconsinpublicservice.com/business/art/EA39_1.jpg
Duct mastic
Why no duct tape?
http://www.tence.net/people/images/duct-tape.jpg
http://www.homeenergy.org/898ductape.title.html
http://ducts.lbl.gov
020
4060
8010
0
Duc
t Effi
cien
cy, %
HEATAG HUD GAS
Pre-retrofit Post-retrofit
System Efficiency
Percentage reduction in space heating energy requirements
The morals of the story
• Duct leakage is important
• Measuring it accurately is difficult
• Several approaches are available
• Active area of research