Load CalculationsCommon Oversights
2013 Midwest Residential Energy Conference
William E. Murphy, PhD, PE
University of Kentucky
Why We Do Load Calculations?
• Design load calculations are the basis for
sizing a building’s heating and cooling systems
• The calculations should represent
– Suitably severe weather conditions
– Representative occupancy patterns
– Reasonable indoor design conditions
– The actual building characteristics
What are Suitably Severe,
Representative, Reasonable, Actual?
• The selected size of an HVAC system is a
compromise between:
– Ability to maintain comfort conditions 24/7/365
– First cost of the HVAC equipment and systems
– Operating cost of the HVAC systems
– Other factors, such as noise, unit location, duct
size, aesthetics, time/cost to determine loads
Suitably Severe
• Few people would design their A/C system to
handle a 109⁰F day (like we had last year)
– It would cost too much
– It would not operate efficiently (during most hours
of operation, it would be running at less than 30%
of capacity)
– It would probably be too noisy
– It likely would not dehumidify properly during the
host humid parts of the cooling season.
Representative Occupancy
• At design cooling conditions, we usually don’t:
– Have cocktail parties
– Cook all day
– Take 18 showers
– Have every electrical appliance on all day long
Reasonable Indoor Conditions
• Indoor design conditions should be the
warmest acceptable “comfortable ” conditions
in summer, and the coolest acceptable
“comfortable” conditions in winter.
• Homeowners can set their thermostat to
whatever they want, but the warmest/coolest
conditions are most economical and should be
what we design the system for.
Actual Building Characteristics
• This information often taken from blueprints
• Can be determined from on-site inspections
for existing structures
• Some parameters are easily measureable
• As-built is never the same as blueprints
• Some parameters can only be estimated, at
best
What We Will Do Today
• Address load characteristics that impact the
total design load and which are sometimes
overlooked in the data gathering process
– Building envelope conduction
– Infiltration impacts
– Solar radiation
– Indoor loads
– Load diversity
Envelope Insulation Areas
Courtesy of Owens-Corning
Missing Insulation
Look for missing insulation. A prime location
is the band joist.
Continuous Insulation
An uninsulated band joist in a 2400 square foot
house is equivalent to no insulation in 35 lineal feet
of wall (the whole end wall). It will show up as a
cold floor and/or ceiling.
Batt insulation or
sprayed in foam
or cellulose
Band Joist Insulation
• Band joist insulation
must be cut to fit, so
it presents more
opportunities for
irregular installation
than for walls and
attics that use precut pieces.
How to Handle Band Joists
• If software does not provide for separate band
joist specification, you may need to force it to
use some other type of conducting surface.
• You may need to specify a 9 foot high wall
with two types of wall insulation
• Hand calculations can account for this easily.
• De-rating of applied insulation may be
appropriate (refer to wall insulation that
follows).
Wall Cavity Insulation Techniques
Insulation Effectiveness
• Insulation works by providing:
– many re-radiating surfaces
– significant resistance to convective air movement
• Fibrous insulation R-value is rated at a given
“loft”. Its R-value is reduced if compressed to
less than its rated loft.
• Air gaps in the cavity allow bulk air movement,
negating the insulation effect.
Compressed Insulation R-Values
2x12
11 1/4"
37
38
30
2x10
9 1/4"
32
35
30
30
25
2x8
7 1/4"
27
30
25
27
24
22
21
19
2x6
5 1/2"
21
22
20
19
21
18
2x4
3 1/2"
14
15
13
2x3
2 1/2"
2x2
1 1/2"
2x1
3/4"
15
13
11
11
10
8.9
6.6
6.2
Product RValues
R-38 R-38C R-30 R-30C R-25 R-22 R-21 R-19 R-15 R-13 R-11
Standard
Thickness
12" 10 1/4" 9 1/2" 8 1/4" 8" 6 3/4"5 1/2"6 1/4"3 1/2"3 1/2"3 1/2"
Taken from Owens-Corning Web site – www.owenscorning.com
What is Effective R-Value?
• From the previous table, a 3-1/2 inch
fiberglass batt compressed by 1 inch would go
from an R-15 to R-11 or from R-13 to R-10
• Neglecting the convection space and the
impact of air circulating within the cavity, a
side stapled batt of insulation may have a
reduction in R-value of 2 to 3, resulting in a
degradation of performance approaching 1520%.
Fibrous Insulation R-value
• In most cases, insulation is not applied exactly
as the product rating procedure specifies.
• Some de-rating is usually appropriate,
depending on the application and the
knowledge and conscientiousness of the
contractor
• A de-rating of 10% of the nominal value for
batt insulation would likely be appropriate for
most applications.
Other Cavity Insulations
• Every field installed (blown in, sprayed, etc.)
cavity insulation will vary depending on the
skill of the installer.
• Given the ratings are likely determined for
near optimal product application, some derating would also be appropriate for most
other field installed insulations as well.
• A de-rating of 5-10% may be appropriate in
most cases, depending on your knowledge of
the workmanship of the contractor.
Attic Insulation
• Attic insulation may be deficient due to:
– Coverage area
– Uniform application
– Openings
– Compression near eaves
Attic Coverage
• Blown in insulation
often suffers from
poor coverage at the
extreme edges.
• Depending on the
height of the roof
near the eaves,
coverage may be
significantly less
than in the center.
Uniform Application
• Inspections of most attics will reveal
– Gaps between batts
– Varying depths for blown in products
– Poor fits around truss cords and other
obstructions
– Compressed batts
– Settling with time for blown-in cellulose
– Evidence of rodents or other pests
Attic Openings
• There are usually a number of openings
through the ceilings of most houses:
– Can-light fixtures
– Electrical boxes for suspended fixtures
– Attic hatches
– Ductwork penetrations
– Chimneys
Light Fixtures
• Most can-light
fixtures require
the insulation to
be 3 inches away
to prevent
overheating
• Every light leaves
about 1 square
foot of ceiling
area that is
uninsulated.
Electrical Boxes
• Many electrical boxes that will be covered
with lighting fixtures may not be well sealed.
• The hydrostatic pressure of the warm air in
the house in winter produces a constant
upward draft of air through the box.
• Although the air movement effect on load will
be addressed later under “infiltration”, the air
movement also reduces the insulating
effectiveness of porous insulations.
Attic Hatches
• Drop down stairs or
attic hatches can be
sizeable areas that are
left uninsulated.
• Stair covers tend to be
expensive, so are not
always used.
• Insulation of attic
hatches involves custom
work, so is also not
always done.
Ductwork Penetrations
• A ceiling supply register poses several
problems for attic insulation effectiveness:
– Poorly sealed, so allows air movement in winter
– Requires special cut-to-fit application, often
resulting in gaps or compressed insulation
– Ducts are insulated much less than the attic, so
even with no air leakage, the duct loses heat like a
poorly insulated (R-3 vs R-38) part of the attic
when the heating system is not operating.
Chimney Penetrations
• Chimney can’t have
insulation for at least
3 inches around to
prevent excessive
temperature
buildup.
• When not heating,
produces at least 1
square foot of
uninsulated area.
Sample Calculation
• Net effect of an uninsulated 1 square foot of
attic floor
– Combined R-values of drywall and air films add up
to about 1.5
– Heat loss is 25 times that of R-38 insulated areas.
– Four can-lights will lose about as much heat (by
conduction alone) as the rest of the insulated
ceiling in a room (not counting air infiltration
effects).
Ceiling Load Calculation Adjustments
• For every can-light and chimney, add 1 square
foot of ceiling area with an R-value of 2.0
• For every uninsulated attic hatch, add 2
square feet of area with an R-value of 2.0
• For every uninsulated drop down ladder
access, add 6 square feet of ceiling area with
an R-value of 2.0
Software Adjustments
• Since software probably doesn’t allow you to
account for can-lights, etc. you could make an
adjustment to the overall attic R-value by the
following approximation:
R Adj


ATot (2.0)
 R Nom x 

A
(
2
.
0
)

A
R
Unins Nom 
 Tot
• Where RNom is the R-value of the insulated attic,
ATot is the total attic area, and AUnins is the area of
can-lights, etc.
Example Attic Calculation
• Consider a 2400 square foot attic, 10 can-light
fixtures, 1 chimney, and two attic hatches that
are 2.5 square feet each.
R Adj


2400R (2.0)
 38x 
 33.7

 2400 (2.0)  16 (38) 
• This represents an insulation de-rating of
about 11% assuming the insulation itself is
installed for optimum performance.
Other Attic Adjustments
• Ceiling electrical fixtures and ductwork
penetrations can probably best be accounted
for by their impact on air infiltration, since
that effect would dominate compared to
conduction effects.
Framing Effects
• There are three areas that framing can
influence heat losses
– Uninsulated outside corners
– Uninsulated interior wall intersections with
outside walls
– Uninsulated headers over doors and windows
Outside Corners
Drywall Clip
The 2-stud corner with the drywall clip reduces the stud
short circuit from 2 studs to only one. A simple rectangular
house with four outside corners would reduce the number
of wall studs by only 4, out of perhaps 140, or about 3%.
The total heat loss from the corner would be somewhat
greater than two stud short circuits if uninsulated.
Wall Intersection
Using a backer board
instead of a stud reduces
1 stud per wall, or about
15 studs per house.
Door/Window Header
Top Plate
Open Cavity
2 x 4 construction has a ½”
cavity, while 2 x 6
construction has a 2-1/2”
cavity. If no insulation is
placed in the cavities, every
14” length of 2 x 12 header is
equivalent to one stud. A
36” door header would be
equivalent to almost 3 studs.
Filling with insulation
reduces both to about 1 stud
equivalent.
Framing Effect on Loads
• If software uses an adjustable percentage
factor for framing area, this percentage can be
made smaller for energy efficient framing
techniques.
• If the framing percentage is calculated, an
adjustment to the nominal insulation R-value
may be required.
Example
• Consider a 32’ x 75’ rectangular house with 2
exterior doors, 16 intersecting interior walls, a
6’ patio door and 40 linear feet of windows.
• With conventional 2 x 4 construction, there are
about 212 studs or mostly full length jack
studs. Combined with the double top plate,
the bottom plate, and the headers, these
represent about 326 square feet of framing.
• Out of 1712 gross square feet of wall, framing
represents 19% (close to the 20% usually used).
Example – cont’d
• For energy efficient framing, the equivalent of
4 studs (corners), 16 studs (intersecting walls),
and 26 studs (insulated headers) would be
reduced. The framing would be reduced by
about 44 square feet, or by about 2.5% of the
gross wall area.
• Instead of using a 20% framing area factor, we
could use a 17.5% factor and all the same Ufactors as before. This construction
represents an approximate 3% reduction in
wall U-value.
How to Incorporate into Software
• If the framing area percentage is adjustable, it
can be reduced from 20% to 17.5%.
• If framing is not adjustable and it is computed
based on conventional construction methods,
the wall R-value can alternatively be increased
by 2% when using the energy efficient framing
techniques described earlier.
Infiltration Considerations
• Infiltration is the migration of unconditioned
outdoor air into the structure, resulting in an
equal volume of conditioned indoor air being
forced out of the structure.
• Infiltration can be produced by:
– natural wind effects
– thermally induced buoyancy effects
– an unintentional side effect of mechanical
ventilation through leaky duct systems.
Where Does the Air Get In?
Where Air Leaks In
• Every house will be different, and you cannot
see where the air leaks into a house
• The conventional approach to sealing a house
by caulking windows and weatherstripping
doors may affect only 10-20% of air leakage.
• In general, you must intentionally build a tight
house by doing all the little things right at the
various stages of construction.
Wind Driven Air Leakage
Temperature Driven Air Leakage
Powered Ventilation Air Leakage
Induced Air Leakage
• While air distribution ductwork is intended to
transfer air from the HVAC equipment to the
various zones in the building, all air ducts will
leak somewhat
• Leaks on the supply duct side will pressurize
the spaces that the ducts are in
• Leaks on the return duct side reduce the
pressure in the spaces those ducts are in.
Leaks with HVAC Equipment Located
Outside the Conditioned Space
Leakage When Ducts Are Located
Inside the Conditioned Space
Estimating Air Leakage
• Estimating air leakage for a particular house at
design conditions is both an art and a science.
• Air leakage will depend on:
1) Building leakage characteristics (cracks,
openings)
2) Natural or mechanical ventilation
3) Building shape and local terrain (shielding)
4) Occupant activities
5) Outdoor temperature and wind speed
Measuring Air Leakage
• A blower door can be useful for quantifying
item (1) and partly item (3).
• Items (2), (4), and (5) can almost never be
accounted for in any sort of measurement,
especially as they relate to design conditions.
• Items (2), (4), and (5) may be the most
important drivers of air leakage into a home
General Guidelines
• General guidelines for how to characterize the
air leakage of a house for a blower door test at
50 Pa pressure follow something like:
•
•
•
•
Tight
Moderately Tight
Typical
Leaky
– 2 ACH at 50 Pa
- 5 ACH at 50 Pa
- 10 ACH at 50 Pa
- 20 ACH at 50 Pa
Winter Load Calculation Numbers
• For the categories shown, the air changes per
hour (ACH) for winter design conditions would
look something like:
•
•
•
•
Tight
Moderately Tight
Typical
Leaky
– 0.2 ACH
- 0.5 ACH
- 1.0 ACH
- 2.0 ACH
Summer Load Calculation Numbers
• For the categories shown, the air changes per
hour (ACH) for summer design conditions
would look something like:
•
•
•
•
Tight
Moderately Tight
Typical
Leaky
– 0.2 ACH
- 0.35 ACH
- 0.5 ACH
- 1.0 ACH
What Would Cause a Difference?
•
•
•
•
•
•
•
Fireplace with the damper open (+)
Continuous ventilation of any type (+)
House completely surrounded by trees (-)
Extraordinary effort to make house tight (-)
Air ducts located within conditioned space (-)
An unshielded location (e.g. top of a hill) (+)
Unusual construction (attached greenhouse,
operable clerestory windows, etc.) (+)
Will Retrofit Change Results?
• While leaky ducts can be sealed and windows
can be caulked, it would be very difficult to
make a typical house into a tight house
because so many of the leaks are unseen and
built into the structure (wall plates, ceiling
fixtures and wall receptacles, wall
penetrations, vents, etc.)
• Use same “suitably severe” principle for
estimation of infiltration estimation.
How Important is Occupancy?
• Consider a 2400 ft2 house. A 50 cfm bath fan
produces 0.16 ACH, while a 100 cfm range
hood will add 0.32 ACH. Both operating at the
same time makes for almost 0.50 ACH, the
entire summer design leakage for a typical
house.
• At winter design conditions (10⁰F), the 150
cfm causes almost a 10,000 Btu/h heating
load, fully 1/3 of a 2-1/2 ton heat pump rated
capacity and probably 50% of its actual
capacity at 10⁰F.
So What Goes in the Software?
• I would suggest using the typical numbers for
the four categories, then adding or subtracting
0.05 ACH for each (+) or (-) change that you
feel would be significant to infiltration.
• Obviously a blower door test helps you
characterize the leakiness of the structure, but
your understanding of the rest of the situation
lets you estimate the impacts of other things
that also influence air leakage.
Solar Radiation
• Solar radiation is the largest contributor to
cooling load for most houses.
• While air infiltration cannot be seen, solar
radiation can be visualized.
• However, its calculation can be quite
complicated due to orientation, time of day,
surface radiation properties, fenestration
properties, etc.
Solar Spectrum
• Much of the sun’s
energy is absorbed
or scattered by the
gases in the
atmosphere
• A significant part,
but not the
majority, of the
sun’s energy is in
the visible part of
the spectrum.
• Kentucky has
only modest
solar
availability,
which explains
our moderate
climate.
Solar Radiation - Glazing
• Radiation entering
the glazing of a
house is the sum of
direct beam
radiation, scattered
radiation from the
sky, and reflected
radiation from the
ground and
surrounding
surfaces.
Solar Positions in the Sky
• The sun crosses the
sky with its arc
peaking on June 21
at about 76⁰ above
the horizon.
• In June and early
July, north walls will
actually be directly
illuminated for a
few hours in early
morning and
evening.
Taking Advantage of the Sun’s Angle
• On June 21, the sun
gets to about 76⁰,
while on Dec. 21 it is
only about 40⁰ above
the horizon.
• Roof overhangs for
south walls can shade
out the summer sun
in summer while
allowing in the winter
sun.
External Solar Radiation
• Dark or metallic surfaces absorb more of the
incident solar radiation, and so get much
hotter than surfaces that are lighter in color.
• Roof decks can reach 160⁰F while air
temperatures inside attics can reach 140 ⁰F.
• A significant part of the heat conducted
through sunlit west walls comes from the
solar radiation on the outside of the wall.
Solar Radiation Through Glazing
• The incident solar radiation intensity
perpendicular to the sun’s rays is on the order of
300 Btu/h per square foot.
• For tilted surfaces, you would multiply this
number by COS(Θ) where Θ is the incident angle
at the surface (perpendicular is Θ=0⁰)
• A vertical south window that is fully illuminated
at solar noon would receive only about 72 Btu/h
of direct solar radiation due to the acute angle.
West Glazing
• Because the sun’s angle decreases as it sets in
the afternoon, west facing windows are
particularly vulnerable to high solar loads
because overhangs cannot shield them from
the low sun angle.
• A west facing window may receive 250 Btu/h
per square foot in late afternoon. A west
facing patio door will receive about 10,000
Btu/h just from solar radiation.
Solar Load Calculations
• Calculating solar angles, shade lines, and the
transient nature of solar energy being
conducted through the wall or roof require
complex algorithms that most residential load
software does not provide.
• There are several situations that you may wish
to override or modify the solar calculations in
your load software.
Solar Radiation Situations
1) Semi-permanent shading (such as trees, deck
coverings, etc.) (-)
2) Reflections from ground or water (+)
3) Human intervention (draperies) (-)
4) Roof color (+ or -)
5) Attic radiant barriers (-)
Semi-permanent Shading
• A shade tree can be blown down or cut down the
day after the A/C unit is installed, hence the
axiom is always to ignore them as a shade
provider.
• However, healthy deciduous trees properly
located (south to west sides) can provide very
significant cooling load reduction benefits.
• Discuss this with the homeowners to gauge their
interest in keeping the tree(s) in place.
• The same may apply to deck coverings/overhangs
that are not part of the primary house structure.
Reflections From Ground or Water
• A home located on the east side of a large body
of water may have expansive glass areas on its
west side for the views of the water.
• Depending on the water’s proximity to the house
and the wall orientation, there may be significant
reflections of direct radiation in late afternoon.
• Few solid surfaces would produce similar effects.
Even light colored concrete would have a
reflectivity of 20% or less.
• This is not a problem with snow cover since such
heat gains are welcomed in the winter.
Human Intervention (Draperies)
• Manual control of draperies is usually not
considered in commercial buildings since the
occupant’s actions can never be anticipated.
• While most homeowners want their windows
open during the day, very few would endure the
glare and internal heating from afternoon direct
sunlight coming through a large west window.
• While drapery control should probably not be
considered for N, E, and S windows, the particular
oppression caused with west windows makes
them a candidate for presumed manual control.
Roof Color
• Roof color can produce a significant difference in
attic temperatures, and hence cooling load.
• The impact on annual energy use may be the
opposite of that produced on the cooling load.
• A light colored roof in a rural or suburban
location might be a candidate for a modest attic
load reduction.
• This would be a modest effect, as even light gray
shingles would likely have an absorptivity of 0.7
or more, versus over 0.90 for black shingles or
dark metal roofs.
Attic Radiant Barriers
• Radiant barriers rarely pay for themselves in
Kentucky, but if present, they would provide
some cooling load reduction in summer.
• Residential load software may not be able to
handle radiant barriers, so they can be
approximated by an increase in attic R-value.
• Just as with the roof radiation, radiant barriers
will actually somewhat increase heating
requirements in many climates.
• An increase in attic R-value by 2 would be a
reasonable estimate for a radiant barrier film.
Indoor Loads
• Air conditioning has helped keep kids inside
rather than spending most of the summer
outside playing sports or other activities.
• While they are indoors, they are often using
electronics (TV, video games, computers, etc.)
• For A/C, consider two occupants for the
master bedroom and one occupant for each
bedroom. For a somewhat more conservative
estimate, you could add 1 or two guests to the
total (larger family or frequent stayovers).
Plug Loads
• While there are many more electronics in
homes than in recent years, the trend is
toward them being much more energy
efficient.
• Flat panel TVs typically use less energy than
CRTs, and current Energy Star sleep mode
energy consumption for most electronics is
much less than just 10 to 15 years ago.
Lighting Loads
• The prevalence of compact fluorescent bulbs has
greatly reduced cooling loads from indoor lighting.
• A 60 Watt equivalent bulb uses only about 13
watts, with an additional savings of 4 to 5 Watthours in reduced A/C consumption.
• While every light in the house would not be on at
the peak cooling load, even considering 5 CFLs in
use would reduce internal loads by over 200 Watts
(800 Btu/h).
Other Appliances
• Nearly every appliance has undergone energy
reductions since the time when most of the
appliance energy usage studies were
performed.
• Along with the trend toward more microwave
oven usage for food preparation rather than a
range or conventional oven, the traditional
average 1200 Btu/h allowance for cooking
may even be reduced in most situations.
Latent Loads
• Latent loads are attributed to infiltration,
occupants, and various activities (cooking,
bathing, etc.)
• While infiltration trends have reduced that
contribution to latent loads, the presence of
house plants, fish tanks, indoor pets, and
other moisture sources have increased.
• Such moisture sources should be looked for in
any existing home load survey.
Sample Latent Load Values
Moisture Load
• A careful assessment
of latent load is
needed to ensure
that the latent
capacity of the A/C
will be adequate.
• Most house load
reductions have
reduced the sensible
load on the
equipment, not the
latent load.
Load Diversity
• Most residential software assumes that the air in
a house freely circulates between each room
(open doors, single air distribution system).
• The building load diversity is largely ignored by
“smearing” much of the instantaneous cooling
load throughout the house.
• In practice there are some cases where loads
should not be uniformly spread between the
various parts of the house.
Load Segregation
• Individual room load characteristics may need to
be separated out when:
–
–
–
–
A single zone is solar load dominated
Separate upstairs/downstairs zones exist
Conditioned basements with no outside openings
Different attic/ceiling insulation
• For these situations, simply dividing the total
building load by the floor area and assigning it to
rooms proportionally will result in significant
oversizing or undersizing in certain zones.
Questions??