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How to Size your Heating or Cooling System

By Chris Brooks
Updated: Aug 14, 2017

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.

Want to do a quick and dirty 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.

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 Does a Cooling Load Calculation Entail?

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 Air Conditioning Company of Chesapeake, VA

Bob Jackson, owner of The Air Conditioning Company based in Chesapeake, Virginia, says that he performs a preliminary computer-based calculation when preparing a bid rather than a calculation based on square footage as some contractors will do. This initial calculation takes less than 30 minutes and does not come with a fee. Once he has been given the job, he comes back and does a detailed, room-by-room calculation to determine the correct size of ducts, compressor and related equipment. The cost of this more thorough calculation is rolled into the overall bid.

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

At this early stage, says Jackson, he is not concerned with the design of the system of ducts or individual rooms and/ or zones

"After I'm awarded the job," says Jackson, "I'm concerned with duct design. Therefore, I need to include all the interior rooms with all of their components, which allows me to size the ducting for each space with the proper cubic feet per minute required to cool that area. If I'm not designing the ductwork because I'm just connecting to the existing system, then I can skip this step."

Harkening back to the leakage rate of ducts mentioned above, Jackson adds, "Homeowners need to be aware, though, that if we do not replace the ductwork with properly sealed and insulating ducting, then the efficiency of the system they just purchased will be something less than advertised, but it will still be far greater than what they currently have."

All American Heating and Air of Raleigh, NC

Stratton Lobdell, Manager of All American Heating and Air, a company that specializes in heat pump and central AC system installations based in Raleigh, North Carolina says: "We perform a load calculation while we are onsite to do the proposal for a potential customer. We spend about an hour to an hour-and-a-half with each customer to do the initial survey. We don't charge for this service as long as it is for a customer that is serious about replacing their system. The cost is simply something we expect to incur because we know we won't close 100 percent of our jobs...."

When asked what their load calculation entails, Lobdell says, "The first thing we do is talk with the homeowner about any concerns they may have with the current system's operation. If we walk into a two-story house and the downstairs is essentially a wide open space and they don't have any concerns with air flow or uneven temperatures, they just want a system that will heat and cool properly, we will do a block load, which is just the entire perimeter of the first floor and size the system based on that. If we are doing a full duct design, we will do a room-by-room calculation for every job and we actually size our duct system based on that. Not only does this include a room-by-room load calculation, but we will look at each room's cubic-feet-per-minute of air needed for heating and cooling."

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

According to Buck, 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. "In a colder climate you have to look much more closely at your heat load than your cooling load so that it is big enough to do the job," says Lobdell. "The result is that you may have a heat pump that is outsized on the cooling side in order to be big enough for heating. In North Carolina, we almost don't have to look at the heating because if we size the heat pump for cooling it usually will provide all of the heat that is needed."

He goes on to add that 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. "You have to pay very close attention to how much heat the heat pump will provide because when you get down to zero degrees you will need a backup heat source because a heat pump will provide very little heat," he says. "You will also have to figure out if you need the auxiliary heat source to carry the entire load or if it only needs to carry the gap."

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