Size Does Matter: The Importance of Heating & Cooling Load Calculations

Ensuring that you install the right size furnace, air conditioner, heat pump or complete HVAC system is critical to providing consistent indoor comfort for you and your family. It’s also important for saving energy, which can be wasted by either a too-large or too-small system. Since energy savings translate to monetary savings, in today’s economy you simply can’t afford to ignore system sizing when selecting your new HVAC equipment. Selecting an energy-efficient model is one facet of being a smart consumer, but buying the most energy-efficient system will do you far less good than it should if the system you buy is too large or too small for your home. The capacity of your equipment needs to match the size of the area being heated or cooled, and it must take other key factors into consideration as well.

What Happens When Your HVAC System is Too Large or Too Small?

A number of issues can occur when your heating or cooling system is the wrong size for your home. Keep these unpleasant consequences of poor system sizing in mind when you are considering which system to buy.

Too-Large System

A system that’s too large for your living space is highly inefficient, creating the following problems:

•It uses an unnecessary amount of energy simply because it takes more energy to run a larger system.

•It wastes energy through “short-cycling,” turning itself on and off repeatedly and running for only a short time.

•It can create unnecessary noise as excess air moves through the ductwork if the system is more powerful than needed for your home.

•It robs you and your family of the consistent indoor comfort a properly-sized system would provide by creating large temperature swings.

•It creates more wear and tear on the system.

•It fails to remove sufficient humidity from the air since it doesn’t run long enough to provide adequate humidity control.

Too-Small System

A system that’s too small for your living space is equally inefficient, though in a somewhat different way. A too-small system creates the following problems:

•It runs continuously yet can sometimes fail to maintain the proper comfort level — especially during extreme weather — potentially keeping your home too hot in summer and too cold in winter.

•It might require a supplemental form of heating or cooling to keep you comfortable. This raises costs and offsets the savings from the energy efficiency of the system because the system is switching on and off too frequently.

•It can create unnecessary noise as the air circulates through the air ducts if the ducts are smaller than required for serving your home’s HVAC needs.

•It’s somewhat more likely to malfunction since it runs continuously, which could mean extra repair bills. The good side to that issue is that by running longer the system should be able to provide adequate humidity control.

•It will require regular maintenance to keep it running optimally and prevent breakdowns since it will tend to be a bit overworked.

Correctly Sized System

A system that is correctly matched to your home’s heating or cooling requirements offers the following advantages:

•It operates at optimal efficiency, saving you money on energy costs.

•It keeps your indoor temperature and overall comfort level more consistent.

•It provides proper humidity control, increasing comfort and lowering the likelihood of mold or mildew problems.

•It operates without excessive noise, which could otherwise indicate an improperly sized system or another issue that needs addressing.

•It operates according to its intended specifications, rather than overburdening the system and shortening its life cycle.

System Capacity Ratings:

BTUs The initials BTU stand for British Thermal Unit. This rating is a measure of the system’s heating or cooling output per minute. HVAC products are labeled with a BTU rating to give consumers a better idea of the approximate area a given piece of HVAC equipment might be expected to heat or cool under ideal conditions. This rating, however, is not intended to replace the advice of an HVAC professional, who understands all the variables involved and can assess these variables in relation to your specific situation and your particular heating and cooling requirements. While BTUs per minute can get you in the ballpark on system sizing, this measurement can’t give you the specifics that will let you know exactly how all the zones in your home figure into the overall system-sizing equation. Therefore, BTU ratings should only be used to roughly estimate the system size you’ll be likely to need and should always be double-checked by performing official heating and cooling load calculations.

Key Factors that Determine the Size System You Need

The key factors that play a role in HVAC equipment sizing follow:

•The climate of the area in which you live.

•The size of your home, as well as its shape, design, orientation, number of floors and other features that make it unique.

•The building materials from which your home is made.

•The location, size, type and orientation of your home’s windows.

•The insulation and weatherization levels of your home.

•The lights and other heat-generating appliances and their efficiency levels.

•The amount of air leakage into the building from outside, known as air infiltration.

Should You Do the Calculations Yourself?

While the heating and cooling load calculations used for correctly sizing your HVAC equipment aren’t difficult, they are fairly involved and take time and patience to complete. While the average person would likely find the calculations more complex than they’d be interested in tackling, if you do decide to handle them on your own, you’ll need to make specific calculations for each zone in your home, figuring the area and insulation level of the space and estimating air infiltration. Help is available online if you’d like to try your hand at it. If not — or if your efforts are less than successful — you might want to consider letting a professional do the job for you.

Professional Heating and Cooling Calculations

Commissioning an HVAC specialist, such as us here at Weather Masters Inc., to handle your heating and cooling load calculations can save you the stress and hassle of doing the lengthy figuring yourself. It can also help avoid erroneous calculations, which can easily occur due to lack of experience. Your HVAC professional handles these heating and cooling load calculations on a regular basis. The contractor you choose should use the Air Conditioning Contractors of America (ACCA) Manual J, Residential Load Calculations. A specialized computer program and worksheets that are available online can also aid your professional contractor in arriving at the most accurate calculations.

Other ACCA Guidance for Your HVAC Professional

Once your Manual J load calculation is complete, the second step in the process will be selecting the appropriate equipment based on ACCA Manual S, Residential Equipment Selection. Your HVAC professional should refer to Manual S before making any specific equipment recommendations for your home comfort system. If you are purchasing a replacement system and your home is already equipped with ductwork, you won’t need the following step since selecting your equipment will complete the system-sizing and equipment-selection process. The only other major consideration will be scheduling your equipment installation with your trusted local HVAC professional.

If you plan to install a complete HVAC system, including ductwork, for a new home-construction project or total building refurbishment, your HVAC specialist will have one more step to follow before designing and installing your system. This industry professional’s calculations will include making an educated estimate of the size, number and location of the air ducts your system will require for operation. Getting your system’s air distribution system right will help your HVAC equipment perform better and bring your family greater indoor comfort. ACCA Manual D, Residential Duct Design, will provide guidance to your HVAC contractor in sizing and designing the ductwork for your system.

What’s Wrong with Rule-of-Thumb Calculations?

Rule-of-thumb calculations are a shortcut some HVAC companies take, rather than going to the trouble of working out the actual heating and cooling load calculations that are needed to ensure that the HVAC system you buy and install in your home is the optimal size for keeping your living space comfortable and energy efficient.

HVAC Equipment Sizing: An Inexact Science but a Science Nonetheless

While sizing your HVAC equipment is a process that’s based on scientific principles, it is, nevertheless, an inexact science — and that’s precisely why you need to be working with a company that has years of experience in the field and has certified HVAC design team in place. Because so many different factors combine to create the specific heating and cooling environment of every home, HVAC system requirements will vary widely. Cooling and heating load calculations are a key variable in the sizing equation — one that’s been carefully designed to determine the best system for your family’s needs. When used by an experienced and conscientious HVAC professional who takes a serious personal and professional interest in getting it right, this detailed approach to equipment sizing should succeed in helping you acquire a system that will serve you well for years to come, providing energy-efficient operation and optimal comfort for you and your family.

Let Weather Masters Inc. Size Your HVAC System

If you live in the Phoenix Valley, we can help you learn more about how to select the “right” size unit for your home. Contact us today by calling (480) 832-9659 or by visiting our website at www.acdudes.com to schedule someone to come to your home to begin your evaluation.

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