Three Types of
Heat Transfer
Heat/Energy Transfer


Movement of Thermal Energy: Heat Flow
Three methods



Conduction
Convection
Radiant
*Most heat transfer has some combination of all three
occurring at the same time
Definitions

Conduction
A method by which heat is transferred from a
warmer substance to a cooler substance by
molecular collisions. Direct contact.

Convection
The transfer of thermal energy from a fluid flowing
over a solid object- [Air is a fluid!]

Radiation
A method by which heat can be transferred through
objects and empty space. Electromagnetic.
CONDUCTION
Conduction Examples
 Liquid
- Liquid - Pouring cold cream into coffee
 Liquid - Gas - Ocean and Atmosphere
 Gas - Gas – Cold and warm weather systems mixing
 Solid - Solid – Touch a hot pot on a stove
Photo © Kevin Kennefick 2001
Conduction Rate Factors
 Contact Area
 Type of
Material i.e. Cast Iron vs. Stainless
Steel
 Temperature Difference
 Distance
heat must travel
CONVECTION
Convection
The transfer of thermal energy from a fluid
flowing over a solid object- [Air is a fluid!]
 But, air is a relatively poor conductor of heat



A solid object = dense arrangement of molecules
Liquid = less dense arrangement
Gas = least dense arrangement of molecules
 Transferring

heat using a gas is inefficient
Must pass a lot of molecules over an object to equal
the carrying capacity of a denser material.
Convection Examples
 In a closed
room cool air will settle to the
bottom while warm air will rise
 Warm air rising through a heat register
Convection Examples
 Bowl
of soup – Hot liquid in the center moves
to the cooler outside where it drops and is
reheated at the center and the cycle continues.
CONVECTION
What is required for convection to occur?
Air Flow = Pressure Difference + Path (Hole)
The Stack Effect
Convection
Temperature
Wind
Exhaust/Mechanicals
Temperature is typically the dominant effect
RADIATION
Radiation
A method by which heat can be
transferred through objects and empty
space. Electromagnetic.
The transfer of thermal energy or heat that
is in direct line of sight of the object
being heated.
Radiation Examples
 The sun’s
A
heat
bonfire
 Warm soil
on a cool night
Radiation Rate Factors
 Surface area
 Temperature difference
 Type of
material
Emittance
Reflectivity
Radiation Terms
 Emittance (or emissivity), refers to the ability of
a material’s surface to give off radiant energy.
All materials have emissivities ranging from
zero to one. The lower the emittance of a
material, the lower the heat radiated from its
surface.
Radiation Terms
 Reflectance
(or reflectivity) refers to the
fraction of incoming radiant energy that is
reflected from the surface. Reflectivity and
emissivity are related and effect each other
inversely.
 For example, aluminum with a reflectance of
0.97 has an emittance of 0.03
Emissivity or Emittance
Material Surface
Emittance
Asphalt
0.90 - 0.98
Aluminum foil
0.03 – 0.05
Brick
0.93
Fiberglass
0.80 – 0.90+
Glass
0.95
Steel
0.12
Wood
0.90
HUMAN
COMFORT
And
Energy
Efficiency
Human Comfort Zone
As humans we try to maintain a body temperature of 98.6° F
Three Mechanisms
Heat generated within the body
Heat gained
from surroundings
Heat lost to surroundings
Human Comfort Zone
We shiver to
generate heat
Human Comfort Zone
We sweat to
Give off heat
Human Comfort Zone
We get goose bumps
Human Comfort Zone
Blood Flow
 Decreases to hands and feet in winter
 Increase in summer to encourage heat
loss
Thermal Neutrality
To be comfortable humans must loose heat at the
same rate as it is produced or gained.
Factors Affecting Human Comfort
 Air temperature
 Air Speed
 Humidity
 Mean radiant temperature
Each has a direct influence on heat loss or gain to
the human body
Factors Affecting Human Comfort
 Air Temperature
- This affects temperature
differences between the body and the
surroundings, consequently affecting the rate
of heat loss or gain by convection.
Factors Affecting Human Comfort
Air Speed - This affects the rate at which
the body loses heat by convection.


An air temperature of 35°F and a wind speed of
20 miles/hour combine to give a wind chill
temperature of 11.2°F.
Air speed is also very important during summer
when the body is trying to lose heat to maintain
comfort.
Factors Affecting Human Comfort
Humidity - Affects the rate at which the
body loses heat by evaporation. During hot
weather, high humidity increases discomfort
by making it more difficult to evaporate
perspiration into the air.
*Most humans find 40% to 60% humidity comfortable
The “Chill Factor”

The Chill Factor can be a direct cause of discomfort

Heating the air in a room does a relatively poor job
of heating solid objects
Those objects in the room at a temperature lower than
ones’ body, act to rob the body of heat (through
radiation), requiring higher room temperatures to offset
that effect
Factors Affecting Human Comfort
Mean Radiant Temperature (MRT) - MRT is the
average surface temperature of the surroundings
with which the body can exchange heat by
radiant transfer.
Radiant heat transfer to and from the body is quite
apparent when sitting near a fireplace (high
MRT) or large cold window area (low MRT).
Mean Radiant Temperature
72°F
70°F
68°F
59°F
64°F
Mean Radiant Temperature: = 67 °F
Cold Surfaces
72°F
59°F
59°F
64°F
Mean Radiant Temperature: = 63 °F
Cold Surfaces
66°F
65°F
65°F
69°F
Mean Radiant Temperature: = 66 °F
Comfort
 Comfort is achieved
by either increasing the
ambient temperature or by raising the mean
radiant temperature of an environment.
A
higher radiant temperature means that
people become comfortable with a lower
ambient temperature and the reverse is also
true.
Mean Radiant Temperature
In general for every 1 degree F that MRT drops, the air
temperature must be raised about 1.4 degrees F to
achieve comfort conditions.
How can you raise the MRT?
 Close blinds and curtains
 Solar Film on windows
 Seal heat leaks
 Low-E Windows
 Insulated exterior doors
Bioclimate Chart
Example 1
 Dry Bulb 73°
 Relative Humidity 50%
In the zone
Example 2
 Dry Bulb Temp.
78°
 Relative Humidity 70%
Example 2
 Dry
Bulb Temp. 78°
 Relative Humidity 70%
 Requires a wind
of 250 FPM
(250*60)/5280
MPH = 2.84
speed
Example 3
 Dry Bulb Temp.
= 50°F
 Relative Humidity 55%
Example 4
 Dry Bulb Temp.
= 64°F
 Relative Humidity 40%
What Does All
This Mean?
Seasonal Enemies
Cooling:
CONDUCTION
Heating:
CONDUCTION
CONVECTION
RADIATION
CONVECTION
RADIATION
British Thermal Units
 The basic
measure of heat
 The amount of heat needed to raise one pound
of water one degree Fahrenheit
BTU =
A kitchen match contains about 1 BTU of Heat Energy
Heat Content of Fuels
1
Kilowatt-hour electricity = 3,413 Btu
 1 cubic foot of natural gas = 1,025 Btu
 1 gallon fuel oil = 138,700 Btu
 1 gallon kerosene = 135,000 Btu
 1 ton of coal = 27,000,000 BTU
 1 gallon LPG = 91,000 Btu
 1 pound LPG = 21,500 Btu
Energy and Power
 Power is the INSTANTANEOUS
energy



Think of it as POTENTIAL use, whether it is running
or not (engine, light bulb)
Btu/h
Watts, Kilowatts (Watts = Volts x Amps)

the amount of voltage across a circuit x the
current through the circuit
 Energy is
energy)


use of
USE of power over TIME (heat
Btu/h x hours = Btu
Watts x hours = Watt hour (Kilowatt x h = kWh)
FUELS
 Natural
Gas - Piped in under pressure
 Liquid Propane (LP) Stored in home
tank
 Fuel Oil
 Electricity
 Coal
 Wood
Forced Air Furnaces
Gas

AFUE - annual fuel-utilization-efficiency rating
measured as a percentage

The higher the percentage, the more heat the furnace can
ring from each therm of gas—and the lower the
environmental impact of its emissions.

The lowest allowed is 78%

The most efficient models have an AFUE of about 97
percent—or near-total efficiency.
Energy Star Qualified
 Minimum AFUE of
85% to 90%
Up Flow Furnace
Down Flow Furnace
HEPA Filter
Radiant Floor Heat
Three types
 Radiant Air Floors
 Electric Radiant
Floors
 Hot Water (Hydronic)
Radiant Floor Heat
Types of installation
Wet Installations


Large thermal mass of a concrete slab floor
lightweight concrete over a wooden subfloor
Dry Installations
Where the installer "sandwiches" the radiant floor
tubing between two layers of plywood or attaches
the tubing under the finished floor or subfloor.
Radiant Floor Heat
Air Heated Radiant Floors Not recommended
for residential applications
Electric Radiant Floors -
Electric Radiant Heat - Wet
Installation
Wet Installation
Wet Installation
Dry Installation
Dry Installation
Hydronic Radiant Heat
Wet Installation
 PEX piping
in Concrete (thick slab)
Wet Installation
 Thin
Slab Application Gypcrete over plywd
Toe Kick Electric Heat
Hydronic Heat
Btu/hr. Range: 4,247 / 6,188
Heat Pumps – Combined Heating
& Cooling
Extracts heat from a space of low
temperature and
discharges it to another space at higher
temperature.
Unlike a furnace, a heat pump doesn't burn
fuel to make heat. It simply uses electricity
to move heat from one place to another
Two Measures of Efficiency
 HSPF
– Heating
 SEER - Cooling
Seasonal Energy Efficiency Ratio
SEER





Power output/Power input The higher the number
the more efficient
All Air Conditioning units sold after January 2006
must have a SEER rating of at least 13.
Energy Star central air conditioning must have a
SEER rating of 14
Window air conditioners are exempt from this
rating
Updating from a 9 SEER rated system to a 13 can
save 30% on your energy consumption
Heating Seasonal
Performance Factor
HSPF
 BTU’s produced/watt-hours
 Heat
Pumps manufactured after 2005 must
have a HSPF of at least 7.7
 The Most efficient have a HSFP of
10
Pros and Cons of Heat Pumps
 Electrically
Powered
 Can
be used in conjunction with a forced air
furnace
 Not well
 Noise
suited in extremely cold climate
Heat Pump and Air Handler
Thermostat
Air Handler
Heat Pump
Air Cleaner
Heat Pump and Furnace
Indoor
Cooling
Coil
Thermostat
Furnace
Heat Pump
Air
Cleaner
Cooling
Cooling
Air Conditioner
and
Furnace
Thermostat
Indoor Cooling
Coil
Air Conditioner
Air Cleaner
Furnace
Air Conditioners and Air
Handlers
Thermostat
Air Handler
Air Conditioner
Air Cleaner
Cooling
Whole House Ventilation
Natural Ventilation
Natural Ventilation
Ceiling Fans
Sizing Ceiling Fans
Largest dimension of
room
12 feet or less
12 -16 feet
16 – 17.5 feet
17.5 -18.5 feet
18.5 feet
Minimum Fan Diameter
36-inches
48-inches
52-inches
56-inches
2 fans
http://www.matthewsfanco.com
Heating Degree Days
 Heating
Degree Days are a measure of
how cold your climate is.
 If the average outside temperature for a
day is 1 degree less than the inside
temperature, then you would
accumulate 1 degree day on that day.
 http://www.ncdc.noaa.gov/oa/climate/o
nline/ccd/nrmhdd.txt
UA Calculation
R
= (A * Δ T) / (3.413 * W)
W
= Watts
 3.413 = Constant (BTU/Watt)
 ΔT = Change in temp
 A = Area in Square Feet
R = (A * Δ T) / (3.413 * W)
 Outside
Temperature = 35°
 Heat source = 100 Watts
 Total Area = 520 SF
 Temperature inside = 55°
 Calculate
the R Value for the given area
R30
R = (A * Δ T) / (3.413 * W)
 Outside
Temperature = 20°
 Insulation = R30
 Total Area = 3520 SF Home 30x40x8
 Temperature inside = 68°
 Calculate
wattage of the heat source for
the given area
1650.16Watts