Building Systems and Performance: an Introduction to Building Operator

Certification –Lesson 6

The Building Boundary and

Thermal Comfort

CUNY Institute for Urban Systems Building Performance Lab

Lesson 5 Review

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Lesson 6Section 1 (60 min)

Section 2 (60 min)

Section 3 (60 min)

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Lesson 6 Objectives

• Understand building thermal loads, how they are calculated, and what it means for your boiler plant operations

• Understand part-load operation (both heating and cooling) and how such operation impacts your facility’s energy efficiency

• Understand your building’s thermal behavior, what variables matter to thermal comfort, and such variables affect your control strategies and complaint responses

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Section 1• Introduction to Building Loads

• Building Dynamics and Building Boundary

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Building Loads: Two TypesHeating Loads and Cooling Loads:

• Heating Load - the heat lost from a building, this must be replaced by the heating system

• Cooling Load - the heat gained into a building – this heat must be removed by the cooling system; (include humidity control where necessary)

• Heating or cooling must be supplied to match the losses or gains in order to maintain steady temperature within the building

Heat normally flows “downhill”, from higher to lower temperature

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Fundamental Energy Unit

BTU = British Thermal UnitThe amount of heat required to raise one pound of water by one degree Fahrenheit.

1 LBWater

Raised 1 degree Fahrenheit

or

1 match

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Three Forms of Heat Transfer - How buildings lose or gain heat

• Conduction• Movement of heat through materials• Trapped air has low heat conductivity

• Convection• Movement of air carries heat • Infiltration and Ventilation

• Radiation• Transfer of heat from body to body• Through space – no medium• Direct line of sight

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Heat Loss through Conduction

Conduction: the direct transfer of heat through materials

Conduction Slab Loss

Conduction Roof Loss

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• Transfer of heat via air movement• Surface effects• Wind pressure

• Air leakage

• Stack effect(s)

• Ventilation systems

Low Pressure

High Pressure

Wind

Low

Heat Loss through Convection10

conductionand radiation

sensibleheat gain

Heat Gain

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heat gains

conduction

infiltration

Building heat-up loads

Building Dynamics

Loads are not steady over the course of a day or in all areas of a building• Morning start-up issues • Thermal momentum • Varying solar gains and activities

radiation

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Building Dynamics: Solar Gains

• When does the greatest solar gain occur?

• Is solar gain occurring equally on all facades at any given time?• tables for gain by time of day by orientation by season.• Different types of glazing, shading, and reflectance (SHGC)

coefficients.

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The Building Boundary• The “boundary” - the enclosure of the building keeps

elements in and/or out. • Made up of:

• Thermal boundary – heat• Pressure boundary – air• Moisture boundary – rain, elements

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Building Envelope Components & Characteristics

• Roofs• Walls• Insulation• Doors• Windows• Tightness of Building• Shading devices

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Section 2• Insulation and Envelope Performance, Insulation Values

and Types, and Building Envelope and Reduced Performance

• Load Calculations

• Equipment Sizing

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More Energy TermsR-value and U-value:

R-value is the resistance a material has to heat flow.

U-value is a measure of a material’s conductivity of heat.

R-value = 1/ U-valueHow they relate:

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Insulation valuesR-Value is the Resistance of heat flow through a materialU-Value is the Conductance of heat through a material

R = 1/ U U = 1/ R Values for many materials and constructions via standardized lab tests and

found in handbooks such as ASHRAE Fundamentals

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• Batts• Boards / Rigid Insulation• Blown-In• Spray Foam• Pipe Sleeves

Insulation Types20

• Air is a poor conductor of heat, especially if it can’t move. This makes it a good insulator

• Insulation is effective because it contains tiny pockets of air, which slow heat flow

• Minimizing heat flow reduces the energy demands on equipment, operating costs, and environmental impact

Building Envelope21

Envelope Performance is Reduced by:

Insulation may not perform as expected • “Effective” R-Value – less than rated R-Value• Thermal “Bridges” – studs and columns in walls• Air by-passes - allow heat to be carried around insulation• Moisture in Walls - condensation and migration

Construction may not result in a Tight Building• Air infiltration at joints between different materials• Seals deteriorate, caulks dry out, weather-stripping fails• No good testing procedure for larger buildings

(“Blower Door” used for testing homes)

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Factors that can Decrease Envelope Performance

Issues with Insulation:May not perform as expected (“effective” R-value less than rated R-

value)• Thermal “Bridges” - studs and columns in walls• Air bypasses - allows heat to be carried around insulation• Moisture in walls - condensation and migration

Issues with Construction:May not provide a tight envelope• Air infiltration at joints between different materials• Seals deteriorate, caulks dry out, weather-stripping fails• No good testing procedure for larger buildings (“Blower Door”

used for testing homes)

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Load Calculations

Engineers use methods built on these concepts for calculating heating and cooling loads in buildings - the loads used to size equipment

Equipment is sized to maintain temperature (db= dry bulb, wb= wet bulb) in each building zone on design day

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How Loads are Calculated Heating Load = Conduction + Convection = Btu/Hour

Convection- movement of air from infiltration or ventilation

Conduction = U x A x dTA = building surface areas

U = conductive value of the surfaces

Infiltration/Ventilation = CFM x 60 min/hr x .018 x dTCFM = volume of air moving through the building

.018 = a constant for the heat capacity of air (per cubic foot per dF)

dT = “delta” Temperature = the difference between the Indoor Temperature and the Outdoor temperature

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Design to Peak Conditions

• What is the “Design dT” ?• “delta T” = difference between two temperatures, in this

case outside and inside

• So what’s the implication of “deltaT”?

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How is Equipment Sized?

Design Load Calculation: Determine the maximum heating load on coldest day in winter.The “Design Day” in New York City is 10 degrees outdoor temp.

What is dT for the heating design condition?

dT = 70 degrees indoor - 10 degree outdoor for NYC = 60 dT

Q = Conduction + Infiltration / Ventilation = Heating Load

= [ (U x A) + (CFM x 60 x .018) ] x dT

What is the common dT for heating your building?A common outdoor temperature is 40 degrees. What’s the significance of that T?

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How is Equipment Sized?• Further Boiler Plant sizing rules

• Sizing of Boilers: the Public School Example

• Most schools have 100% redundancy (Older practice, say before 1975)

• Current SCA rules:

• If 2 boilers, each 75% of design load = 150%

• If 3 boilers, each 50% of design load = 150%

• If 4 boilers, each 30% of design load = 120%

• Safety margins - addition of 10 - 20% capacity

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How much is the heating plant (equipment) over-sized?

• Depends on the outdoor temperature

• If plant was designed to a 10 degree out door temp day, then the design dT = 70 - 10 = 60 degrees

• If the outdoor temperature is 40 degrees, then the heating load dT at that time = 70 - 40 = 30 degrees

• 60 / 30 = 2, so the boiler plant is oversized by a factor of 2 (100% extra capacity) for that heating condition

• But wait, if 2 Boilers are designed for 150% of design load, it’s even worse. Two Boilers are 300% of design load when dT is 30 degrees on a 40 degree day.

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Practice the CalculationFraction of Design Load

= Actual LoadDesign Load

= Actual dT Design dT

= 70F – 40F70F – 10F

= 30F = 160F 2

If one boiler is sized at 100% of design load, it is 200% of actual load

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What is the effect of boiler plant over-sizing?

• Burner Fractional “On Time” plotted against boiler efficiency

• Steep fall-off at 30% of“Burner On Time”

• IMPLICATION: BOILER CAPACITY CONTROL IS VERY IMPORTANT

• AVOID SHORT-CYCLING HOW?

Source: Brookhaven Nat’l Lab 1978

Boiler Part-Load Curves

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Section 3• Building Dynamics and Part Load

• Building Dynamics and Comfort

• Cooling Loads

• Conditioning for Comfort, Class Review

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Building Dynamics Loads are not steady over the course of a day or in all areas of a building

• Morning start-up issues • How long does it take your building to come up to

temperature?

• Thermal momentum • The difference of “heavy” vs “light” construction• How do you deal with “control overshoot”• How early could you shut-down?

• Varying solar gains and activities

• How do you experience these kinds of variations in your facilities?

• Some good data logger projects here!

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Part-Load Control StrategiesMost of operation at Part-Load : What happens to

equipment efficiencies at part-loads?

Strategies better or worse for systems operating at part-load

Load Matching• Modulation - Control of the capacity of the equipment over a range.

Example 25% to 100%• Lead-lag – Control of the capacity on-line by

controlling the number of boilers on-line • Temperature Reset – To change the set point of the working fluid of

a system based on outdoor temperature. Applies well to hot waterbut not to steam

• Variable speed/frequency drives for pumps and fans

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Indoor Environmental Quality

Building Operations as a Quality Assurance process• Inputs (equipment, energy, labor)

• Outputs (environmental conditions, comfort, productivity, health)

Quality Assurance perspective introduces concepts such as Root Causes, data and statistical analysis

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Building Envelope & The Indoor Environment

Envelope as boundary

• Outdoor and Indoor Conditions

• Thermal, air, moisture, light, sound

Dimensions of Indoor Environment & IEQ

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Building Dynamics & Comfort

ASHRAE Standard 55 - Thermal Environmental Conditions for Human Occupancy

• This standard specifies the combinations of indoor spaceenvironment and personal factors that will producethermal environmental conditions acceptable to 80% ormore of the occupants within a space

• The environmental factors addressed are: temperature, thermal radiation, humidity, and air speed

• The personal factors are activity and clothing.

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Building Dynamics & Comfort • COMFORT FACTORS

• Air temperature and air mixing (steady temp)• Air movement• Relative Humidity• Thermal Radiation • Activity Levels and Clothing

• These all contribute to the sensation and perceptionof the temperature

• People are acutely sensitive to change in these conditions

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IEQ: Comfort & HealthRelative Humidity

• High humidity• Discomfort• microbiological problems• structural damage

• Low humidity• Discomfort - respiratory effects

• Static electricity

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IEQ: Thermal Comfort Clothing and Activity

• Winter: 71º F • heavy slacks, long-sleeve

shirt & sweater (10% dissatisfaction range 68-75º F)

• Summer: 76º F• light slacks, short-sleeve shirt• 10% dissatisfaction range 73-

79º F• Summer: 81º F

• minimal clothing (10% dissatisfaction range 79-84º F)

What People Say and DoWhat people say and do to

indicate system performance issues:• Complaints• Sweaters• Added Space Heaters• Taped over registers

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Cooling Loads

People, lights and office equipment all give off heat into the conditioned space. We look these up on tables.

Solar gain through glass is instantaneous and can be a dominant heating load. We look this up on tables.

Getting rid of excess humidity can be a major load. We use the Psychometric Chart to calculate this load.

Cooling loads and the Pyschrometric Chart are the subject of a separate BOC level 2 module.

Cooling loads are more complex to calculate – need to take into account internal gains, solar gains, and latent heat (humidity) of ventilation air.

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Psychrometric Processes

What happens in various HVAC modes

Increasing enthalpy (total energy content)

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Outside Air2,000 cfm96 db, 78 wb

Return Air8,000 cfm80 db, 67 wbSupply Air

10,000 cfm55 db, 99 Rh

Sensible Heat Ratio, dictates Supply air conditionSHR = Sensible Heat Gain / Total Heat Gain (sensible + latent)

Supply Air Reheated10,000 cfm65 db, 70 Rh

Mixed Air10,000 cfm83 db, 69 wb

Sensible

Latent

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Conditioning for Comfort: What else besides Psychrometrics?

• Changes in temperature • Radiative effects – hot and cold • Air movement (envelope & stack effects) • Air distribution • Zones • System Balance

Building DynamicsBuildings aren’t uniform across space or time!

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Building DynamicsThermal requirements are a function of building materials, orientation,

external and internal conditions• How do we take care of varying conditions in the building at any

given time?

Thermal “momentum” and control

• Light vs. Massive construction and building response

• How do we calibrate for control overshoot?

• When should systems be shut-down for night set-back and when should they be started-up?

Time of Day Impacts• Varying solar gains and activities• Increase/decrease in the number of occupants

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Is it Rocket Science?

•Moving target

•Energy inputs

•Constraining forces

•Figuring trajectories

•Hit or miss?

A number of factors at play – heat loss and gain sources

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Review for Energy and Comfort Optimization

• Most of your plant operation is at Part Load.What happens to equipment efficiency at part-loads?

• Thermal conditions are changing throughout the building. What can you anticipate about how the building or specific areas will behave thermally?

Are you utilizing strategies that

• Reduce overheating and heat imbalances to get the best thermal comfort possible?

• Are better for operating a heating system at part-load?

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Review and Reading for Class #7

• What were the main learning points in this class?

• Readings for next class:

• FEMP section 9.2 (BOILERS) and 9.3 (STEAM TRAPS) and

• BOC 1001 Handbook pages 72-119

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