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Load Calculations- Common Oversights

Load Calculations- Common 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

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Load Calculations- Common Oversights

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  1. Load Calculations-Common Oversights 2013 Midwest Residential Energy Conference William E. Murphy, PhD, PE University of Kentucky

  2. 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

  3. 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

  4. 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.

  5. 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

  6. 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.

  7. 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

  8. 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

  9. Envelope Insulation Areas Courtesy of Owens-Corning

  10. Missing Insulation Look for missing insulation. A prime location is the band joist.

  11. 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

  12. 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.

  13. 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).

  14. Wall Cavity Insulation Techniques

  15. 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.

  16. Compressed Insulation R-Values Taken from Owens-Corning Web site – www.owenscorning.com

  17. 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%.

  18. 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.

  19. 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.

  20. Attic Insulation • Attic insulation may be deficient due to: • Coverage area • Uniform application • Openings • Compression near eaves

  21. 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.

  22. 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

  23. 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

  24. 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.

  25. 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.

  26. 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.

  27. 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.

  28. 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.

  29. 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).

  30. 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

  31. 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.

  32. 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.

  33. 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.

  34. 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

  35. 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.

  36. Wall Intersection Using a backer board instead of a stud reduces 1 stud per wall, or about 15 studs per house.

  37. Door/Window Header 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. Top Plate Open Cavity

  38. 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.

  39. 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).

  40. 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.

  41. 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.

  42. 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.

  43. Where Does the Air Get In?

  44. 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.

  45. Wind Driven Air Leakage

  46. Temperature Driven Air Leakage

  47. Powered Ventilation Air Leakage

  48. 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.

  49. Leaks with HVAC Equipment Located Outside the Conditioned Space

  50. Leakage When Ducts Are Located Inside the Conditioned Space

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