Objectives Finish talking about ways to minimize ventilation losses Discuss role of ducts in...

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