Load Calculations-Common Oversights

2013 Midwest Residential Energy Conference

William E. Murphy, PhD, PEUniversity 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 InsulationLook for missing insulation. A prime location is the band joist.

Continuous InsulationAn 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 pre-cut 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-Values2x12 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 15 13 11

2x3 2 1/2"                 11 10 8.9

2x2 1 1/2"                   6.6 6.2

2x1 3/4"                      

Product R-Values   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 15-20%.

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 de-rating 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:

• 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.

NomUninsTot

TotNomAdj RA)0.2(A

)0.2(AxRR

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.

• This represents an insulation de-rating of about 11% assuming the insulation itself is installed for optimum performance.

R

7.33)38(16)0.2(2400

)0.2(2400x38RAdj

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 U-factors 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 ventilation3) Building shape and local terrain (shielding)4) Occupant activities5) 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 – 2 ACH at 50 Pa• Moderately Tight - 5 ACH at 50 Pa • Typical - 10 ACH at 50 Pa• Leaky - 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 – 0.2 ACH • Moderately Tight - 0.5 ACH • Typical - 1.0 ACH• Leaky - 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 – 0.2 ACH • Moderately Tight - 0.35 ACH • Typical - 0.5 ACH• Leaky - 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 Watt-hours 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??