As mentioned above, heating loads may be conveniently calculated with specialized computer software. One such program, HvacLoadExplorer, is included on the web- site. While primarily aimed at performing 24-hour dynamic cooling load calculations, the program is quite capable of calculating heating loads also. While a user manual may be found on the website, it may be useful to discuss general considerations for calculating heating loads with HvacLoadExplorer. Most of these will also apply when calculating heating loads with either a cooling load calculation program or building
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energy analysis program. Since a steady-state heating load with no solar input or inter- nal heat gains is usually desired, the following actions should be taken:
• Choose “Heating Load Calculation” in the building dialog box. This causes the analysis to use the “Winter Conditions” weather data.
• Select the weather data. Usually, the peak temperature will be set as the 99.6 percent or 99 percent outdoor design temperature. The daily range will be set to zero, which will make the outdoor air temperature constant for the entire 24- hour analysis period. The solar radiation must also be set to zero—in Hvac- LoadExplorer and many other programs, this may be achieved by setting the clearness number to zero.
• Describe walls with studs or other two-dimensional elements. In Chapter 5, a procedure for calculating the U-factor when the wall has parallel heat-flow paths was described. In programs such as HvacLoadExplorer, it is common to describe the wall in a layer-by-layer fashion. In this case, the layer that con- tains the parallel paths (e.g., studs and insulation) should be replaced with an equivalent layer. This equivalent layer should have a conductivity such that its resistance, when added to resistances of the other layers, gives the correct total resistance for the whole wall, as would be calculated with Eq. 5-18.
• Describe unconditioned spaces. For situations where an attic, crawlspace, or garage is adjacent to conditioned space, the user can set up HvacLoadExplorer to estimate the temperature similar to the procedure described in Example 5-4.
In order to do this, the attic or crawlspace should be placed in a “Free Floating Zone.” This allows the zone temperature to be calculated without any system input. Surfaces that transfer heat between the unconditioned space and the con- ditioned space should be specified to have an external boundary condition of type “TIZ.” In the conditioned space, the “other side temperatures” can be taken from one of the unconditioned rooms. In the unconditioned space, the “other side temperatures” can be specified to be at the conditioned space temperature.
• Set internal heat gains. For cooling load calculations it is necessary to account for internal heat gains such as people, lights, and equipment. For heating load calculations, these should be set to zero. In HvacLoadExplorer, in each internal heat gain dialog box, there is a check box (labeled “Include in Heating”) that may be left unchecked to zero out the heat gain in a heating load calculation.
• Specify interior design conditions. Interior design temperatures are set at the zone level. For a steady-state heating load, they should be specified to be the same for every hour. “Pick-up” loads may be estimated by scheduling the design temperatures.
• Design air flow. At the zone level, a system supply air temperature for heating may be set. The required air-flow rates will be determined based on the sensi- ble loads.
Further information on the methodology employed for HvacLoadExplorer may be found in Chapter 8.
REFERENCES
1.ASHRAE Cooling and Heating Load Calculation Manual, 2nd ed., American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, GA, 1992.
2.ASHRAE Handbook, Fundamentals Volume, American Society of Heating, Refrigerating and Air- Conditioning Engineers, Inc., Atlanta, GA, 2001.
178 Chapter 6 Space Heating Load
3. L. G. Harriman III, D. G. Colliver, and K. Q. Hart, “New Weather Data for Energy Calculations,”
ASHRAE Journal, Vol. 41, No. 3, March 1999.
4. P. E. Janssen et al., “Calculating Infiltration: An Examination of Handbook Models,”ASHRAE Trans- actions, Vol. 86, Pt. 2, 1980.
PROBLEMS
6-1. Select normal heating design conditions for the cities listed below. List the dry bulb tempera- ture, the mean wind speed and direction, and a suitable humidity ratio.
(a)Pendleton, OR (d)Norfolk, VA (b)Milwaukee, WI (e)Albuquerque, NM (c)Anchorage, AK (f)Charleston, SC
6-2. Select an indoor design relative humidity for structures located in the cities given below.
Assume an indoor design dry bulb temperature of 72 F. Windows in the building are double glass, aluminum frame with thermal break. Other external surfaces are well insulated.
(a)Caribou, ME (e)San Francisco, CA (b)Birmingham, AL (f)Bismarck, ND (c)Cleveland, OH (g)Boise, ID (d)Denver, CO
6-3. A large single-story business office is fitted with nine loose-fitting, double-hung wood sash windows 3 ft wide by 5 ft high. If the outside wind is 15 mph at a temperature of 0 F, what is the percent reduction in sensible heat loss if the windows are weather stripped? Assume an inside temperature of 70 F. Base your solution on a quartering wind.
6-4. Using the crack method, compute the infiltration for a swinging door that is used occasionally, assuming it is (a)tight-fitting,(b)average-fitting, and (c)loose-fitting. The door has dimen- sions of 0.9 ×2.0 m and is on the windward side of a house exposed to a 13 m/s wind. Neglect internal pressurization and stack effect. If the door is on a bank in Rapid City, SD, what is the resulting heating load due to the door for each of the fitting classifications?
6-5. A room in a single-story building has three 2.5 ×4 ft double-hung wood windows of average fit that are not weather-stripped. The wind is 23 mph and normal to the wall with negligible pressurization of the room. Find the infiltration rate, assuming that the entire crack is admit- ting air.
6-6. Refer to Example 6-1. (a)Estimate the total pressure difference for each wall for the third and ninth floors.(b)Using design conditions for Billings, MT, estimate the heat load due to infil- tration for the third and ninth floors.
6-7. Refer to Examples 6-1 and 6-2. (a)Estimate the infiltration rates for the windward and side doors for a low traffic rate. (b)Estimate the curtain wall infiltration for the first floor. (c)Com- pute the heating load due to infiltration for the first floor if the building is located in Charleston, WV.
6-8. A 20-story office building has plan dimensions of 100 ×60 ft and is oriented at 45 degrees to a 20 mph wind. All windows are fixed in place. There are double vestibule-type swinging doors on the 60-ft walls. The walls are tight-fitting curtain wall construction, and the doors have about in. cracks. (a)Compute the pressure differences for each wall due to wind and stack effect for the first, fifth, fifteenth, and twentieth floors. Assume ti−to=40 F. (b)Plot pressure dif- ference versus height for each wall, and estimate which surfaces have infiltration and exfiltra- tion.(c)Compute the total infiltration rate for the first floor, assuming 400 people per hour per door. (d) Compute the infiltration rate for the fifteenth floor. (e)Compute the infiltration rate for the twentieth floor. Neglect any leakage through the roof.
6-9. Refer to Problem 6-8.(a) Compute the heat gain due to infiltration for the first floor with the building located in Minneapolis, MN. (b)Compute the heat gain due to infiltration for the fif- teenth floor. (c)What is the heat gain due to infiltration for the twentieth floor?
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6-10. Compute the transmission heat loss for the structure described below. Use design conditions recommended by ASHRAE Standards.
Location: Des Moines, IA
Walls: Table 5-4a, construction 2
Floor: Concrete slab with 2 in. vertical edge insulation
Windows: Double-insulating glass; in. air space; =0.6 on surface 2, 3 ×4 ft, double-hung, reinforced vinyl frame; three on each side
Doors: Wood, in. with wood storm doors, three each, 3 ×6 ft Roof–ceiling: Same as Example 5-3, height of 8 ft
House plan: Single story, 36 ×64 ft
6-11. Compute the design infiltration rate and heat loss for the house described in Problem 6-10, assuming an orientation normal to a 15 mph wind. The windows and doors are tight fitting.
6-12. Rework Problem 6-10 for Halifax, Nova Scotia. Include infiltration in the analysis.
6-13. An exposed wall in a building in Memphis, TN, has dimensions of 10 ×40 ft (3 ×12 m) with six 3 ×3 ft (0.9 ×0.9 m) windows of regular double glass, in. air space in an aluminum frame without a thermal break. The wall is made of 4 in. (10 cm) lightweight concrete block and face brick. The block is painted on the inside. There is a in. (2 cm) air space between the block and brick. Estimate the heat loss for the wall and glass combination.
6-14. Consider Problem 6-13 with the wall located in Concord, NH. The air space between the block and the brick is filled with in. (2 cm) of glass fiber insulation. Estimate the heat loss for the wall and glass.
6-15. Compute the heating load for the structure described by the plans and specifications furnished by the instructor.
6-16. A small commercial building has a computed heating load of 250,000 Btu/hr sensible and 30,000 Btu/hr latent. Assuming a 45 F temperature rise for the heating unit, compute the quan- tity of air to be supplied by the unit using the following methods:(a) Use a psychrometric chart with room conditions of 70 F and 30 percent relative humidity. (b)Calculate the air quantity based on the sensible heat transfer.
6-17. Suppose a space has a sensible heat loss of 100,000 Btu/hr (29 kW) but has a latent heat gain of 133,000 Btu/hr (39 kW). Air to ventilate the space is heated from 55 F (13 C), 35 percent relative humidity to the required state for supply to the space. The space is to be maintained at 75 F (24 C) and 50 percent relative humidity. How much air must be supplied to satisfy the load condition, in cfm (m3/s)?
3 4
3 4
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3
134 4
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180 Chapter 6 Space Heating Load
Chapter 7
Solar Radiation
Solar radiation has important effects on both the heat gain and the heat loss of a build- ing. These effects depend to a great extent on both the location of the sun in the sky and the clearness of the atmosphere as well as on the nature and orientation of the building. It is useful at this point to discuss ways of predicting the variation of the sun’s location in the sky during the day and with the seasons for various locations on the earth’s surface. It is also useful to know how to predict, for specified weather con- ditions, the solar irradiation of a surface at any given time and location on the earth.
In making energy studies and in the design of solar passive homes and solar col- lectors, the total radiation striking a surface over a specified period of time is required.
The designer should always be careful to distinguish between the maximum radiation that might strike a surface at some specified time (needed for load calculations) and the average values that might strike a surface (needed for energy calculations and for solar- collector and passive design). Solar collectors are not discussed in this text, but Ben- nett (1) has given methods for identifying cost-effective solar thermal technologies.