Residential HVAC System Sizing

Transcription

1 Residential HVAC System Sizing William P. Goss University of Massachusetts, Amherst, Massachusetts, USA Corresponding SUMMARY Heating, ventilating and air-conditioning (HVAC) system sizing for existing single family residents in hot and humid and temperate climates present different problems. In hot and humid climates, the proper sizing of residential air-conditioning systems is an important issue, since if the system is over-sized the resulting mold problems can cause significant problems with occupants who are susceptible to airborne spores like mold that can severely affect their asthmatic illnesses. In more temperate climates, HVAC system sizing for existing single family residential unit is primarily concerned with heating systems. However, there is often a need to retrofit the residential unit with an air-conditioning system. The technique for properly sizing residential air-conditioning and heating systems is based on methods described in the ASHRAE Handbook of Fundamentals. Heating and cooling spreadsheet programs that duplicate the methodologies were developed and have been used to evaluate the design cooling load for a number of single family residential units (condominium and homes) in Florida and in New York and Massachusetts. Two examples, a condominium in subtropical Tampa, Florida and a condominium in temperate Albany, New York are presented and discussed. Conclusions and recommendations are made on the sizing criteria for residences in these two diverse climates. INTRODUCTION In hot and humid climates, the proper sizing of residential air-conditioning systems is an important issue, since if this is not done correctly the resulting mold problems can cause significant problems with occupants who are susceptible to airborne spores like mold that can severely affect their asthmatic illnesses. The author has examined several hundred residences (single family homes and condominiums) that were constructed in the last 10 to 30 years in the State of Florida where both new and replacement air-conditioning systems were significantly oversized. In a recent seminar presentation by Cochell [1] indicated that a study of 1600 homes in Florida by the University of Florida showed that 78% of the homes had oversized air-conditioning systems by more than ½ Ton (6,000 Btu/hr or 1759 W). As a result, the occupants were exposed to a cooled, damp indoor climate where the oversized airconditioning system runs for short periods of time to bring the indoor air temperature down to the set-point temperature but does not run long enough time to significantly reduce the humidity levels in the air. The mold accumulates in the air-conditioning ducts that were installed at the time of the original air-conditioning system. In several of the worse over-sized air-conditioning systems that the author has seen, the occupants became quite ill from the resulting mold growth and their entire living quarters had to be completely renovated.

2 In more temperate climates, many middle-aged (30 to 60 years old) and older (greater that 60 years old) residences have old inefficient heating systems where approximately 50% of the heating fuel is wasted. In most situations, the heating and cooling system sizing exercise is more complex since design drawings are usually not available and many residences have undergone a number of renovations. This makes it necessary to spend a lot of time in developing the building envelope plans and ascertaining the construction materials. In addition, most of these residences did not have air-conditioning systems installed at the time of construction. One recent development in air-conditioning duct systems is the use of properly designed smaller diameter higher velocity air ducts that can be installed in a manner similar to the way smaller diameter electrical wiring can be snaked into the walls, ceilings and floors without disturbing the inside and outside surfaces. Here, in addition to the more difficult air-conditioning sizing exercise, the design of the high velocity air duct retrofit also has to be carried out. The technique for properly sizing a residential air-conditioning and heating system is based on methods described in Chapter 28 of ASHRAE [2]. Spreadsheet programs that duplicate these heating and cooling load methodology have been developed and used to evaluate the design heating and cooling loads for the residences in both hot and humid and more temperate climates using the specified outside design temperatures given in Chapter 27 of ASHRAE [2]. For cooling load calculations, the estimate of the latent cooling load is the most critical element in the selection of the size and type of retrofit air-conditioning system. For heating load calculations, the estimate of the air infiltration/ventilation rate contribution is the most critical element in the selection of the size and type of heating systems. The sizing results for two residences, one for cooling in a sub-tropical climate and one for heating in a temperate climate are presented in the next section of the paper. Conclusions and recommendations are made on the sizing criteria for residences in these two diverse climates. CLIMATIC DATA TABLES Two single family residences in a temperate location in Albany, New York (a second floor condominium) and a subtropical location in Tampa, Florida (a first floor condominium Unit) were studied. Tables [1] and [2] present a small portion of the data available for these two locations from Chapter 27-Climatic Design Information in ASHRAE [2] which presents selected climatic data for the United States, Canada and World locations. More detailed climatic data can be found in ASHRAE [3]. In Table [1], for winter design conditions, the second column gives the latitude, longitude and elevation for the two locations given in the first column. The third column gives the design (99.6% and 99% percentile) winter heating dry bulb temperatures that represent temperatures that are expected to only be exceeded 0.4% and 1.0% of the winter heating season. The fourth column represents design (5%, 2.5%, and 1% percentile) extreme wind speeds that are expected to only be exceeded 5%. 2.5% and 1% of the winter heating season. In Table [2], for summer design conditions, the second column gives the latitude, longitude and elevation for the two locations given in the first column. The third column gives the design (0.4%, 1% and 2% percentile) summer cooling dry bulb and mean coincident wet bulb temperatures that represent temperatures that are expected to only be exceeded 0.4%, 1% and 2% of the summer cooling season. The forth column gives the mean daily range (difference between the daily maximum and minimum temperatures during the hottest month) of the dry bulb temperature.

3 The data in Tables [1] and [2] can be used in the determination of the design heating and cooling loads that are presented in the next section. It should be stressed here that the emphasis is on equipment sizing, not annual energy use. Table 1 Winter Heating Design Conditions LOCATION Albany, New York Latitude Longitude Elevation (m) 42.75º 70.38º 89 Tampa, Florida 30.38º 84.37º 21 Design Heating Dry Bulb ºC (99.6%) (99%) 2.2 (99.6%) 4.4 (99%) Extreme Wind Speed km/h 30.6 (5%) 35.4 (2.5%) 38.6 (1%) 24.9 (5%) 27.4 (2.5%) 30.6 (1%) Table 2 Summer Cooling Design Conditions LOCATION Albany, New York Latitude Longitude Elevation (m) 42.75º 70.38º 89 Tampa, Florida 30.38º 84.37º 21 RESULTS Cooling Dry Bulb/Mean Coincident Wet- Bulb ºC 32.2/21.7 (0.4%) 30/21.1 (1%) 28.9/20.6 (2%) 33.3/25 (0.4%) 32.8/25 (1%) 32.2/25 (2%) Range of Dry Bulb ºC The following Chapters in the ASHRAE Handbook [2] were used in the calculation of the heating and cooling loads presented in the load calculation results tables: Chapter 25: Thermal and Water Vapor Transmission Data Chapter 26: Ventilation and Infiltration Chapter 27: Climatic Design Information Chapter 28: Residential Cooling and Heating Load Chapter 30: Fenestration Heating Load Calculation Results Table [3] presents a summary table that summarizes the detailed room-by-room spreadsheet results for determining the heating load of a second floor condominium unit located in an approximately 100 year old three story building that was being converted to a two unit condominium association. The second floor unit also includes an unheated attic above it and a first floor unit below it. The ground floor is a partially above ground basement that contains the individual heating systems for the two condominium units. The second floor unit heating system uses oil as the heating fuel and the first floor unit uses natural gas as the heating fuel.

4 The two Unit Owners own the interiors of their individual units and share ownership of the exterior surfaces of the building and the surrounding land. Table [3] gives results that follow the procedures presented in Table 12 (Summary of Loads, Equations, and References for Calculating Design Heating Loads) of Chapter 28: in ASHRAE [2]. The wall and floor areas and the ceiling height were obtained using a laser measuring device. The various U-values were obtained from the data and calculation methods presented in Chapters 25 and 30 in ASHRAE [2] for the specified building components. The determination of the construction of the wall, floor and ceiling required the temporary removal of a number of electrical fixtures (wall outlets and switches, ceiling lights) to determine the type and thickness of the various building materials used in the original construction. The data for the Albany, New York location from the Climatic Design Information given in Table 1B of Chapter 27 in ASHRAE [2] is listed in the top portion of Table [3]. The inside to outside temperature difference of 42.7ºC comes from the indoor design temperature of 21.1ºC and the 99.6% winter design dry bulb temperature of -21.6ºC. The temperature differences for the unheated attic were calculated from methods given in Chapter 25 in ASHRAE [2]. The critical heating load element, air infiltration, requires the use of Chapter 26 in ASHRAE [2] to estimate the sensible heating load due to air infiltration for the second floor condominium unit. The 5% design exterior wind speed of 30.6 km/h (19 miles/h) was used in applying two calculation models to obtain estimates of the infiltration flow rates. The basic model in Chapter 26 in ASHRAE [2] gave an infiltration rate of 0.62 air changes per hour (ACH) and the enhanced model gave an infiltration rate of 0.80 ACH. To be conservative in sizing the heating equipment for the not very well sealed 100 year old building, a value of 1.0 ACH was used. Multiplying the floor (or ceiling in this case) area of 118 m 2 by the ceiling height of 2.6 m gives a volume of 307 m 3. With one ACH, the air infiltration volume flow rate is 307 m 3 /h as shown in Table 3. Figure 3-RESIDENTIAL HEATING LOAD CALCULATION Location: Albany, New York Weather Data Location: Albany: Indoor Design Temperature: 21.1ºC Winter Design Dry Bulb 99.6%: -21.7; 99%: -16.7ºC Extreme Wind Speeds: 5%: 30.6 km/h; 2.5%: 35.4 km/h; 1%: 38.6 km/h Ceiling Height: 2.6 m SENSIBLE LOADS Area m 2 U-value W/(m 2 ºC) Temp Diff ºC Energy W Exterior walls Wood Stud/No Insulation Wall to interior hallway Wood Stud/No Insulation Single Pane w/storm Windows Windows Exterior door Wood Door to interior hallway Wood Floor to heated unit Wood Stud/No Insulation Ceiling to unheated attic Wood Stud w/insulation Subtotal 9978 ρ Cp kj/(m 3 ºC) Volume Flow Rate m 3 /h Infiltration TOTAL Notes

5 Cooling Load Calculation Results Table [4] presents a table that summarizes the detailed room-by-room spreadsheet results for determining the cooling load of a first floor condominium unit located in an approximately 30 year old four story building that contains 22 condominium units. The ground floor is a parking garage. The Table [4] results are obtained from the procedures presented in Table 9 (Summary of Procedures for Residential Cooling Load Calculations) of Chapter 28 in ASHRAE [2]. In addition, a number of related tables and figures, their footnotes and detailed example calculations in Chapter 28 in ASHRAE [2] are used in the spreadsheet calculations for determining the various elements of the total sensible load. The areas, material properties are determined in the same manner as in the heating load calculations. Glass (windows and doors) are treated differently. A glass factor replaces the U-value and temperature difference to account for the peak solar and sensible heat load. This factor is a function of the type of glass; the orientation, inside and outside shading; the design outdoor temperature (the 0.4% value of 33.3 C given at the top portion of Figure [4] was used in the calculations) and the indoor design temperature (23.9 C was used). For exterior walls and floors, a cooling load temperature difference (CLTD) is determined by orientation; outside shading; the design outdoor temperature and the indoor design temperature. The air infiltration is not as important a factor as it is for heating situations. In Table [4], an ACH of 0.72 was used. It should be noted that the various temperatures have been rounded off to the nearest C, however the resulting energy values include the non-rounded off values in the detailed spreadsheet calculations. The critical cooling load element, the latent load, is estimated from the outdoor design humidity ratio and the type of construction (loose, medium, tight) in Figure 1 of Chapter 28 in ASHRAE [2] to determine a load factor (1.16 was used) which is multiplied by the total sensible load to arrive at the latent load as given near the bottom of Table [4]. A properly sized air-conditioning system has to be able to remove a significant amount of moisture from the air in sub-tropical climates.

6 Figure 4-RESIDENTIAL COOLING LOAD CALCULATION Location: Multifamily Residential Condominium-Tampa, Florida, FL Weather Data Location: Tampa: Indoor Design Temperature: 23.9 ºC Summer Cooling Design Temperature (Dry Bulb/Mean Coincident Wet-Bulb): 0.4%: 33.3/25 ºC; 1%: 32.8/25 ºC; 2%: 32.2/25 ºC Range of Dry Bulb Temperature: 8.3 ºC (Low Range-less than 8.9 ºC) Ceiling Height: 2.44m SENSIBLE LOADS GLASS Area m 2 Glass Load Factor W/(m 2 ) Energy W Window and Doors Patio Door U-value ENVELOPE W/(m 2 ºC) Cooling Load OUTSIDE FACING ΔT (ºC) NOTES West facing to Outside with Interior Blinds, and Exterior Shading North facing to Outside with Interior Blinds, and Exterior Shading Floor To Outside Fully Shaded Garage below Exterior wall North facing to Outside INSIDE FACING Temp Diff ºC Interior wall South facing to Air-Conditioned Unit Interior wall East facing to Air-Conditioned hallway Ceiling To Air-Conditioned Unit above INFILTRATION & VENTILATION ρ*cp kj/(m 3 K) Volume flow rate m 3 /h Infiltration Air Changes per Hour INTERNAL LOADS Lighting/Appliances 352 Value for Multifamily Unit People People for 2 Bedrooms SENSIBLE LOADS 5527 LATENT LOAD 6412 TOTAL LOADS Times Sensible Load for Medium Airtight Construction at 1% Design Humidity Ratio DISCUSSION The total heating energy required in the condominium in Albany, New York is W (in I- P units Btu/h). The oil-fired heating system has low pressure steam delivered to radiators in each of the heated rooms. This system also supplies some water vapor to the rooms to allow a comfortable winter relative humidity level to the occupants. Preliminary spreadsheet calculations indicated that insulating the ceiling/attic floor would reduce the energy to the unheated attic space enough to economically warrant the expense of having fiberglass blown into the cavity between the attic floor and 2 nd floor ceiling. The original heating system (boiler) was rated at Btu/h and the new higher efficiency boiler system was rated at Btu/h. Both ratings account for the heating system piping losses to the basement. The first season of oil consumption with the new heating system was 40% less than

7 the prior years. However the cost savings was much smaller due to the increase in heating fuel prices. In addition to the new heating systems for both of the Albany, New York condominium units, new 16 SEER (seasonal energy efficiency ratio), two-stage air-conditioning systems were also added to both units. See Dulley [4] for details of this type of modern high-efficiency, split system air-conditioning systems. Detailed cooling load calculations similar to that for the condominium in Tampa, Florida were performed. For the second floor unit studied here, the evaporator coil/air handler unit was located in the basement near the outdoor compressor/condenser unit. New high velocity smaller diameter duct work (supply and return) was snaked through the first floor unit walls to the second floor unit walls. See Dulley [5] for details of these types of mini-duct, pressurized central air-conditioning systems. The total cooling energy required in the condominium in Tampa, Florida is W (in I-P units Btu/h or 3.4 Tons). The original over-sized air conditioning system was rated at 4 tons or Btu/h. The new replacement high 16 SEER high-efficiency, heat pump airconditioning system was rated at 3 tons or Btu/h. The reason for selecting a lower rating than the calculated load was due to the fact the Unit Owner was away during the summer season and had recently installed hurricane shutters which significantly reduce the sensible cooling load and forced most of the replacement air to come from the air-conditioned hallway at a humidity ratio much lower than that of the outside air that normally came in through the west facing outside windows. In addition, a humidistat was added in parallel to the thermostat to insure that high relative humidity ratios did not occur while the Unit Owner was away. After two summers with the new air-conditioner reduced the Unit Owners electric bill by over 30% and the occurrence of mold growth was reduced significantly. During the short winter heating season, the electric costs were also reduced by using the new airconditioning system in the heat pump mode which is more efficient that the electric heating in the original system. In summary, it can be seen that significant energy savings can be achieved by properly sizing heating and cooling system without any reduction in user comfort. REFERENCES 1. Cochell, Robert, Florida Refrigeration and Air-Conditioning Contractors Association, presentation at Florida Building Commission Seminar in Tampa Florida, March 28, ASHRAE Handbook of Fundamentals, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. 3. ASHRAE Weather Data Viewer - CD Version 3.0, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. 4. Dulley, James Update Bulletin 921 Super-efficient 2007 central airconditioners buyers guide, ( 5. Dulley, James Update Bulletin 713, Mini-duct, pressurized central airconditioners for comfort, savings (