The acronym HVAC stands for Heating/ Ventilating/ Air Conditioning. It’s used to refer to such systems for homes and businesses. In these systems, air moves between the inside and outside of a home or other building. That means heat is continually lost or gained depending on the climate and weather conditions. To install the correctly sized furnace or air conditioning unit, designers must apply calculations that factor in the amount of heat gained or lost. Contractors occasionally use “rules of thumb” to size equipment for houses and businesses.

One rule of thumb estimates the average heat loss or gain based on the square footage of the structure. For example, the builder may decide that it takes 1 ton (12,000 BTU) of air conditioning for each 800 square feet of space. The problem is that climates, house size, season, number of windows and which direction they face, insulation, and the number of sunny days, to name a few conditions, all make a difference. A heating/AC unit that’s too big not only wastes energy and costs more, it fails to maintain an even temperature, causes discomfort, and wreaks havoc on humidity levels.

Here are some things to consider when planning to install or replace heating and air conditioning:

  • Installing the correct size keeps costs down.
  • Correct humidity control prevents moisture damage from occurring.
  • Installing properly sized units costs less and will pay for itself in energy savings.
  • Cycling on and off is costly; if the furnace/air conditioner is sized correctly, it will maintain an even temperature.
  • Properly sized HVAC requires less ductwork, which means lower cost.
  • Correctly sized HVAC units help create a comfortable atmosphere for the building’s inhabitants.

With fuel costs on the rise and environmental concerns at the forefront, it makes sense to go beyond rule of thumb guesswork and carefully calculate our HVAC needs.

Installing a new HVAC System

If you are installing a new boiler, central air conditioner, furnace or heat pump one of the most important decisions is how large of a system to buy. (i.e.: what capacity of equipment is required to keep your home comfortable?) In the heating and cooling industry, determining the answer to that question is known as “sizing” your equipment, and the process is known as a load calculation or heat loss calculation.

The following will help you understand why sizing a system is important and what steps HVAC Contractors take to determine the proper capacity for your equipment.

Why Do I Need a Load Calculation?

A load calculation will help a contractor understand:

  • What your heating / cooling goals are for your home;
  • How much space needs to be cooled;
  • How hard the system will have to work to maintain that level of heating and cooling

The calculation will then be used to properly design the system, which includes estimating the number, size and location of air ducts, as well as properly sizing the equipment to best meet the cooling needs of the home in the most efficient manner possible.

Heat Load Calculation

According to Buck Taylor, Vice President of Roltay Inc. Energy Services, a consulting firm that specializes in the design of HVAC systems, one of the big challenges in the industry is getting contractors to stop trying to compete as if they were selling a commodity on price alone. “Unfortunately this is what the industry has become all about, selling a box without telling the customer whether it will work in their home and if it is properly sized,” he says. “They don’t care if the customer has an adequate distribution system, which more than likely the original system was never designed properly.”

When asked what a homeowner would expect to experience if the load calculation is not done or improperly done, Taylor says that when a system is installed properly the customer should NOT hear the system run or air moving through the ducts or grilles. If they do, the ducts are too small, air velocities are too high, and the fan is using more power than it should. The homeowner should also NOT experience any odors and the house should not get dusty faster than normal. If either occurs, it likely means the ducts are leaking.

The air temperature and comfort level should also be even throughout the house or the cooling zone within the house. If either changes from room to room or depending on where you are in the house, the air distribution system was not properly designed and/or installed and/or balanced.

Lastly, your utility bills should go down. After all, the system should be more efficient.

Therefore, the process of performing a load calculation will not only help avoid these issues, but ensure that the system you end up buying is the system that best matches your home. Properly functioning and sized equipment will also guarantee that the system operates at its peak efficiency, which saves you money.

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Why is Sizing Important?

  • A heating or cooling system that is too large will “short-cycle” — turn on and off too often. This makes the system less efficient, as the system is most efficient when it runs continuously when the outside temperature is equal to its target temperature.
  • On the other hand, if you install a system that is too small, it may not be capable of maintaining its target interior temperatures. Furthermore, (in the case of air conditioners or heat pumps) the system may not adequately dehumidify your house.

Beyond Sizing, what else will a good HVAC Contractor look for?

Studies have shown that more than 60 percent of central air conditioning systems have an inadequate charge of refrigerant (Freon or Puron) upon installation. Taylor says that in split systems (where the evaporator coil is indoors and the condenser coil and compressor are outdoors) the systems come from the factory with a pre-charge of refrigerant. This pre-charge is intended to be correct for a pre-defined length of piping between the indoor and outdoor equipment as well as matched to specific pieces of indoor and outdoor equipment.

During installation the contractor needs to adjust the cooling charge to compensate for varying lengths of connecting pipe and the compatibility of the indoor and outdoor equipment. To do this properly, the contractor must have read the installation documents provided by the equipment manufacturer. “It is a well-known fact that most males do not like to read such materials,” says Buck jokingly. “Because of this, most technicians ballpark the charge using rules of thumb that can leave the system improperly charged.”

The system will run inefficiently and with diminished cooling capacity, says Buck, even if the error in charge is minor.

Furthermore, on average, residential ductwork loses 30% of its heated or cooled air via leaks — often into uninsulated basements or attics. Taylor explains that if the system is designed to move 1,200 cubic feet per minute of conditioned air into your home, as much as 30 percent of that is lost due to gaps in the ducts and equipment cabinets. “So in this example,” he says, “400 CFM of air is lost in your attic, basement or other spaces. And more importantly, the same quantity of air that is lost must leak back into the system from the attic or basement where it will bring contaminants such as dust or radon or other things we don’t want in the house.

“The visual I like to throw out is; what if your plumbing leaked 30 percent? Given that this is an average in the industry, there are very few contractors that can really be proud of what they are doing.”

What is Cooling Load Calculation?

Given all that can go wrong with an improperly sized, designed and installed AC system, it is important to be sure your contractor performs a thorough load calculation. Some contractors will try to get away with not doing a calculation or will perform an overly simplified one, which is hardly better than no calculation at all. We spoke with two HVAC companies to get a sense of what their load calculations entail.

The simplified load calculation includes measuring:

  • The cubic area of the home (e.g. the air space inside the home that needs to be conditioned).
  • The insulation properties of the exterior walls and roof as well as all of their components (doors, windows, etc.).
  • How well insulated the building envelope is.

Additional elements that can and should be taken into account during a load calculation include:

  • Ceilings
  • Location and average external temperatures
  • Insulation
  • Shading (is the home in a wooded area?)
  • any other heat source that could affect cooling such as a wood stove or pellet stove, the oven and its venting, etc
  • How well insulated the building envelope is

Homeowners interested in better understanding how a load calculation should be performed can purchase a copy of the Air Conditioning Contractors of America (ACCA) Manual J. They should also make sure that any contractor hired to install the system follows the Quality Installation guidelines from the ACCA’s and the American National Standards Institute’s Standard 5.

A Note about Sizing your Heat Pump

Sizing a heat pump requires a bit more care than sizing a central AC system because the heating aspect of it must be taken into account. Every heat pump manufacturer provides specifications based on average outdoor temperatures for where the home is located and how much heat the heat pump will provide as the temperature drops.

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