To illustrate the importance of load calculations and proper equipment sizing.

The students will be able to increase their awareness of energy waste in HVAC systems due to improper equipment sizing.

The most common problem in the HVAC industry is the tendency to oversize equipment. Contractors and customers alike are conditioned to believe bigger is better. Unfortunately, bigger leads to higher installation cost, higher operating cost and a more humid house. Yes, an oversized system will not run long enough to remove moisture from the house, and in turn, the customer will feel clammy due to a higher humidity level in the house. The customer in search of greater comfort, will now lower the thermostat to feel more comfortable and will keep lowering it until the unit runs enough to remove moisture.

The industry standard for residential load calculation is Manual J. The first paragraph on page eight of the introduction to the manual says it all. *"The residential heating and cooling system must be selected and designed to provide comfort conditions in all occupied spaces regardless of season. Temperature, humidity, air movement and ventilation must be controlled by the system. In addition, the system must perform these functions at maximum efficiency in order to minimize energy consumption."

*Information used by courtesy of The Air Conditioning Contractors of America (ACCA), publisher of Manual J, Seventh Edition, Load Calculation for Residential Winter and Summer Air Conditioning, 1986.

Many people believe a cooling load calculation is dividing the total square footage of a house by 500 and the answer is the amount of tons of A/C needed for the house. The heating load is based on the size heater needed to give the correct cubic feet per minute (cfm) for the A/C. The heating load method becomes more complex if there are two heating capacities available with the same cfm output, then flip a coin to choose which output fits the job. An example would be: 2000 square foot house divided by 500 would yield 4 tons. Based on 400 cfm per ton, we need a heater to produce 1600 cfm in the high speed operation. If we had an 80,000 Btu or 100,000 Btu input heater to yield 1600 cfm, we would just guess or blindly pick either unit. This is an example of wasting energy by design and will cause problems on most jobs. This calculation takes nothing into account except square footage. This type of calculation can be thrown off if the ceiling height varies from 8 feet or if the house is reasonably insulated or shaded by trees.

Manual J looks at lot location, the direction the house faces, glass area and type of glass, insulation, wall type, roof color, number of doors, number of appliances, occupants, etc. An effort is made to account for all aspects of the building and while they do not guarantee the load, their accuracy is remarkable. As equipment and houses become more complex, contractors are using Manual J in greater numbers.

A few recommendations to consider for a more energy efficient home are:
1. The air handler and ducts should be located in conditioned spaces.

2. Landscape your home to protect windows, walls and your compressor / condensing unit.

3. Use exhaust fans in the bath and kitchen areas.

4. Programmable thermostats should be used.

5. To diminish the heat caused by concrete, ground cover should be planted near the home.

6. Shade trees are an excellent way to protect the home from the direct sun.

Answer the following True / False questions. If you do not recall the answer, refer back to the text to locate it.
_____1. Most Americans tend to undersize their HVAC equipment in an effort to save money.

_____2. It is better to oversize the system rather than risk a humid house.

_____3. Oversized systems may lead to higher installation costs and higher operating costs.

_____4. A system that is too large will run a shorter time and leave excess humidity in the air.

_____5. There are only two basics to be controlled by the HVAC system: temperature and air movement.

To use the previous example of a house 2000 square feet with R-11 walls, R-19 ceiling, 1 1/2" duct wrap, 8' ceiling, on a slab in New Orleans, Louisiana, with a family of four, a basic speculation type house would require four tons of air conditioning by the square footage calculation. The same house figured with Manual J would actually only require 3 tons. The same house built today with R-19 walls, foam board insulation on walls, R-22 ceilings, light colored roof, double pane glass with Low-E coating, and 2" duct wrap would only require 2 tons. Now we are down to half the original air-conditioning required and this would be to maintain 75°F inside at 50% relative humidity indoors when it is 95°F outside.


III. FRAME WALL (2 x 4 Studs, R-13 Batt
1. Siding, Wood (1/2" x 8" Lapped) .80
2. Sheathing (1" Polystyrene Extruded) 5.00
3. Insulation Batt (R-13, 3-5/8") 13.00
4. Interior Finish (1/2" Sheetrock .45

By doing proper load calculations we can save our natural resources on installation and in operation. The customer will also see a quicker payback and be more inclined to buy higher efficiency equipment. This will further preserve our natural resources because the small more efficient system will use less than half the electricity the larger unit would have required.


Write a short paragraph to answer each of the following questions.
1. How was the old method of square footage used to determine cooling loads?

2. How does the Manual J method determine cooling loads?

Your HVAC system needs total replacement. List at least 10 things to consider, besides square footage, that you and your HVAC consultant should consider.

Remember house location, shading, climate, room location, insulation, window placement, etc.

a. I have large oak trees blocking the sun on the west side of my home.
b. I live in a climate that is usually between 50° and 80° year around.

Your Turn:










Activity 1:
1. False
2. False
3. True
4. True
5. False

Activity 2:
Paragraphs should resemble the following:
1. The old method of sizing air conditioning systems was to determine the square footage of the house and divide by 500. This gave the number of tons of cooling equipment needed. This method may have oversized many systems.

2. Manual J takes many factors into consideration to more accurately approximate the cooling load. Such elements as insulation, window placement, climate, shading, lot location, heat producing appliances, number of occupants, etc. It was found that this method was more accurate, was specific to the dwelling, usually determined a need for less tonnage, and made the home more comfortable.

Answers to Information Check:
Students' answers will vary. They should include information about some of the following:

1. lot location
2. placement of home on lot
3. number of windows
4. types of glass
5. number of doors
6. types and amount of insulation
7. roof type and color
8. house design
9. shading devices
10. other ventilation devices
11. appliances
12. number of occupants
13. climate

MANUAL J, 7th Edition Load Calculation for Residential Winter and Summer Air Conditioning. Air Conditioning Contractors of America.

SLEMCO'S Builder's Guide, Design One for Energy Efficient Construction.

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