Each safety factor applied to the indoor/outdoor design conditions, building components, ductwork conditions, or ventilation/infiltration conditions outlined above has its own impact on the resulting Manual J heating and cooling loads. But, a more significant impact occurs when the safety factors are combined. The compounding of safety factors will result in an inflated load that cannot be relied upon for the remaining steps of the residential HVAC design process.
"Manual J calculations should be aggressive, which means that the designer should take full advantage of legitimate opportunities to minimize the size of estimated loads. In this regard, the practice of manipulating the outdoor design temperature, not taking full credit for efficient construction features, ignoring external window shadings and then applying an arbitrary "safety factor" is indefensible."
"No additional safety factors are required when load estimates are based on accurate information pertaining to the envelope construction and duct system efficiency. Large errors are possible if there is uncertainty about insulation levels, fenestration performance, envelope tightness or the efficiency of the duct runs installed in the unconditioned space."
What happens when inaccurate adjustments are applied to the Manual J heating and cooling load calculation process? Something which seems quite minor such as changing the outdoor/indoor design conditions can result in exaggerated loads.
Making more than one adjustment only increases the inaccuracy of the heat loss and heat gain calculation results.
The results of the combined manipulations to outdoor/indoor design conditions, building
components, ductwork conditions, and ventilation/infiltration conditions produce significantly
oversized calculated loads. Not only will this oversizing impact the heating and cooling
equipment costs, but HVAC duct sizes and number of runs must also be increased for the
significantly increased system airflow.
Oversizing the HVAC system is detrimental to energy use, comfort, indoor air quality, building and equipment durability. All of these impacts means the system could be "short cycling" in both heating and cooling modes. To reach peak operational efficiency and effectiveness, a heating and cooling system should run for as long as possible to address the loads. Short cycling limits the total amount of air circulating through each room, and can lead to some rooms not receiving adequate duration of airflow.
In the cooling season in humid climates, cold clammy conditions can occur due to reduced dehumidification caused by the short cycling of the equipment. The system must run long enough for the coil to reach the temperature for condensation to occur and an oversized system that short cycles may not run long enough to sufficiently condense moisture from the air. Excess humidity in the conditioned air delivered to a space may lead to mold growth within the house.
WHAT DOES AN ACCA MANUAL J LOAD CALCULATION SHOW YOU?
The residential HVAC load calculation determines just the right size system for your specific home. The Manual J load calculation results in a recommendation for tonnage, which is how the HVAC industry determine size. If your home is well-insulated, has energy-efficient windows and has low infiltration rates, you won't need as large an air conditioner as you would in a structure that is poorly insulated or has a significant heat gain.
Many permit offices require an ACCA Manual J, S & D report to meet code requirements and to prove the equipment and ductwork are properly sized. The Manual J portion calculates the amount of heat that is loss through the building envelope (how much heat is needed) and the amount of heat that is gained (how much cooling is needed).
A Residential Manual J load analysis provides:
Individual Room x Room heating and cooling needs
How much heating does my home need for my family to be comfortable?
How much cooling does my home need for my family to be comfortable?
There are two parts to the cooling load:
How an ACCA certified Manual J load calculation ensures YOUR house is "Just Right" - Avoid a Goldilocks Home.
The Manual J calculation process determines the heating and cooling your home needs to stay "just right" - toasty warm in the cold frigid months and cool and comfortable in the hot steamy months. Decades ago HVAC engineers developed what is now called ACCA Certified Manual J load
Your house is probably a different size and shape and built with different construction materials than your neighbor's. One size HVAC really doesn't fit all homes. Before having a new heating and cooling system installed, you need a Manual J load calculation performed. The most proven method for sizing HVAC equipment is ACCA (Air Conditioning Contractors of America) Manual J load calculation. ACCA publication "Manual J Residential Load Calculation Eight Edition (ACCA MJ8)" also incorporates American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) rules in its methodology.
A residential HVAC load analysis determines the exact heating and cooling needs of your home, helping you to avoid issues such as oversizing which is quite common. "Just put in a bigger system" is the common misconception. But an oversized HVAC system costs a lot of $$$ to run, reduces efficiency, may break down more often and, because it is continuously running, your home may have big temperature differences. The Department of Energy estimates that "over 50% of HVAC contractors in the United States size heating and cooling systems incorrectly"
Upgrading an HVAC system is a big deal so maximize your investment value by getting an accurate Manual J heat gain and heat loss calculation. Heat gain is how much heat accumulates in your home on a hot summer day that your properly sized A/C unit should remove. If your HVAC is sized correctly, it will bring your home's temperature to a very comfortable level and be "just right"! Heat loss is how much cold air enters your home on a cold winter night and your properly sized furnace will supply the right amount of heating to compensate for the cold air.
Developed by Hank Rutkowski.
The first step of residential HVAC system design for a builder, HVAC contractor, and other trade partners is to understand the importance of properly calculating peak heating (heat loss) and cooling (heat gain) loads. While a complete HVAC design involves much more than just the Manual J load calculation, it is the first step for a successful heating and cooling load. The loads acquired by the heat gain and heat loss ACCA Manual J8 process dictates not only the equipment selection (ACCA Manual S), but also the duct design (ACCA Manual D) to deliver conditioned air (ACCA Manual T) to rooms of the house.
The three reasons properly sized HVAC equipment is critical are: Equipment Efficiency, Everyday Comfort and Indoor Air Quality.
HVAC Equipment Efficiency: Improperly sized comfort equipment will cost more $$$ to operate. If the installed HVAC is oversized, you will pay for the larger unit, larger installation costs, and higher energy costs to operate the oversized fans and compressors. In addition, the system will cycle on and off causing unusual wear and tear. In contrast, if the HVAC is undersized, it will run continuously. Your HVAC needs to be "just right". Properly sized heating and cooling equipment will run steadily and smoothly and will be more affordable to operate.
Need Manual J? A common practice is to just replace the existing A/C or furnace with the same size of a newer model. What if your existing HVAC wasn't properly sized originally? Then it's just a newer improperly sized system. Or what if your home has been updated with new windows or higher R-value insulation? And let's not forget that a 4.0 ton 1980's A/C is far less efficient than a higher efficiency SEER model today.
The first step in ensuring maximum HVAC energy efficiency and performance is an accurate Manual J load analysis using ACCA Certified software such as Wrightsoft.
What is Manual J? ACCA Manual J load calculations are used by homeowners and HVAC contractors to select HVAC equipment capacities (ACCA Manual S) based on the Manual J room by room heating and cooling load results. Manual J8 determines your specific home’s heating and cooling needs based on where your home is located (Weather location) , which direction your home faces (Orientation), the insulation R-values in your floor, ceiling and walls and how humid your climate is.
Manual J VIDEO: Please watch This Old House define "What is an ACCA Manual J load calculation?" towards bottom of page
VIDEO: What is a Manual J load calculation?
This Old House Season 40 Episode 3: This Old House Expert Richard Trethewey demonstrates heat loss and gain to design a balanced HVAC system. With a model of the new project house, he shows what sun exposure and ventilation can do to a building.
Click on “This Old House” picture above & watch video from 6:40-14:10 minutes
Everyday Comfort: Many HVAC installers believe "Bigger is Better" instead of "Just-Right" resulting in comfort issues for the people living in the home. An oversized air conditioner will not dehumidify the home. Because the A/C cycles on and off, the coil never has the opportunity to cool down. In a properly sized AC unit, the coil cools down producing condensation which in turn dehumidifies your home. So the thermostat set point is satisfied, but the occupants of the home certainly are not because they are cold and clammy.
Indoor Air Quality: If your contractor believes "Bigger is Better" and your system
is over-sized, the system will not properly dehumidify, resulting in moisture problems.
A moist atmosphere can result in mold and mildew, excessive dust mites, etc. Lack
of humidity is a major issue throughout the United States and properly sized HVAC
equipment by the proven industry standard of ACCA Certified Load Calculations is
the ONLY way to ensure YOUR house is "Just Right"!
See our video below. Many factors go into the HVAC load analysis including your geographical location (climate), building orientation (which direction does the front door face), r-values of your wall, roof & floor insulation, window size & type, and how many people and appliances are just a few. Residential Manual J load calculations have a proven track record over many decades as long as your HVAC designer puts in the correct parameters of your specific Thermal Envelope:
How Insulation R-values impact Manual J
For optimal energy efficiency, your home should be properly insulated from the roof down to its foundation. Your geographical location will determine the minimum insulation values for your walls, attic and floors based on current IECC, IRB & IRC code. A proper Manual J heat gain & heat loss must use correct r-values.
How Window types/size/orientation impact heat loss and heat gain
Whether you have single, double or triple-pane windows has a huge impact on the required cooling load. And the larger the window the more heat let into the home during the summer months. Overhangs reduce the cooling load. Finally, North facing windows let in less heat than W, S or SW windows.
How HVAC System Location and Duct Leakage can increase Manual J load
In an ideal world the best practice for HVAC design is to "keep all the ductwork within the conditioned space in order to eliminate the duct losses/gains to and from the outside conditions." But in the real world there are one-story slab-on-grade or houses with unconditioned attics. Sometimes it is impossible to keep all the ductwork inside conditioned space.
Typically an installer will put the HVAC system and ductwork completely in the attic in a slab-on-grade house. In a single-story house with a basement, the duct system is typically run in the basement, which is considered conditioned space provided the basement walls are insulated or there are supply registers in the basement. When ducts are placed outside conditioned space, the heating and cooling loads are impacted by the location of the unconditioned ducts, the R-values of the duct insulation and duct leakage.
How Occupants and Appliances add latent load to the Manual J results
ACCA Manual J specifies that the # of occupants in a home is equal to the # of bedrooms + 1. The number of occupants is calculated by accounting for two (2) per Master Suite and one (1) for each additional bedroom.
ACCA also recommends an additional whole house lighting and appliance load totaling 1,200 BTUh to be placed in the kitchen.
How Building Orientation impacts the Manual J sensible cooling load
Solar gains change depending on the time of day and the season. The orientation (N, NE, E, SE, S, SW, W, NW) of your house must be considered in the cooling load calculation.The sensible heat gain during the summer is impacted greatly by the orientation of the house, overhangs (shading from the sun) and window to wall ratio. While utilizing "worse-case" orientation may be tempting, most likely permit will reject for not meeting code requirements.
How using the correct ASHRAE Weather location closest to your home provides an accurate ACCAManual J: Why can't you use Worse Case Design Temperatures in your Manual J8 Residential Load Calculation?
The simple answer is "Because the Permit Office will reject your HVAC permit application"! Most HVAC contractors and homeowners want to design for the hottest day in August and a 70 F indoor design temperature. This creates a situation where your cooling equipment is oversized resulting in inadequate dehumidification, possible coil freeze and short cycling.
Hank Rutkowski developed ACCA Manual J decades ago and many programs such as EnergyStar and LEED have required Manual J load calculations for years. The track record has been proven to be accurate and reliable. But still there are those who insist on upsizing heating and cooling equipment for that one hot week two years ago...
So lets look at the numbers for wintertime. Table 1A in the ACCA Manual J book gives
the ASHRAE 99% design temperature for all ASHRAE city, airport and military weather locations.
Now in Englishthis is the 30-year average for your ASHRAE weather location 99% of the time.
So 1% of the time based on the 30-year average the outdoor temperature will drop below the
outdoor design temperature. 1% = 88 hours a year.
And looking at summertime: Table 1A in the ACCA Manual J8 book gives the ASHRAE 1% design temperature for all ASHRAE weather locations. This is the 30-year average for your ASHRAE weather location. Again 1% of the time (88 hours) based on the 30-year average the outdoor temperature will be warmer outside than the outdoor design temperature.
Size your HVAC for design loads not the most extreme load possible. By using the accurate ASHRAE design temperatures your HVAC equipment will be "Just Right" providing optimal performance and comfort for years to come!
Any Vaulted Ceilings?
Account for vaulted ceilings as these have higher ceilings, more room volume, different ceiling insulation and different duct losses.
How Ventilation and Infiltration Factors add to the heating and cooling needs of your home
Ventilation and infiltration impact both the heating and cooling Manual J loads by bringing outside air into the conditioned space. The target ventilation and infiltration rate must be accurately represented in the load calculation. In humid climates, the impact on the latent cooling load added by ventilation and infiltration can be significant.