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When it Comes to HVAC, Size Really Matters

The Why and How of Sizing HVAC Equipment

Product Expert

One of the most important decisions to make when selecting a new heating or cooling system is how large a unit to buy. A properly sized unit will improve comfort and reduce energy, equipment, and maintenance costs. This holds true whether selecting an air conditioner, boiler, furnace or other type of home comfort equipment. By understanding what goes into sizing your equipment, you can make an educated decision on an important purchase.

How to Size Equipment
You may come across a number of simple methods that size heating and cooling equipment based primarily on the square footage of a structure. While these may be useful to provide an initial estimate of size for planning purposes, they should not be used to make a final selection of equipment. Determining how much heating or cooling capacity is required to keep a space comfortable depends on four factors:

  1. The first is the climate in which the building is located. A building located in a hot and humid climate will require a larger air conditioner than an identical building located in an area with cool, dry summers. Similarly, a building in South Dakota will require a larger furnace or boiler than an identical building in South Carolina since the winters are colder in South Dakota.

  2. The conditions required for comfort inside the building are also important. As the difference between the outdoor and indoor temperature increases, there will be more heat transfer through the walls of the building, as nature always seeks equilibrium. Therefore, you need even more energy than you might think to heat a building to 75° F compared to heating it to 68° F on the same day.

  3. To reduce the speed with which heat moves through a wall, insulation can be added. The quality of insulation and the tightness of the building envelope are the third factor that must be considered when sizing HVAC equipment. Insulation will not prevent heat from moving through a wall, but it can slow it significantly. Insulation is often the most cost-effective method to reduce a home’s energy costs. The quality of windows, doors, and weather stripping is also important.

  4. The final factor used in determining the amount of heating or cooling energy a building requires is the area of exterior surfaces. To illustrate this, imagine a room with all four walls, the floor, and the ceiling exposed to the exterior (like a cube floating in the air). This room will lose/gain heat through all six surfaces. Now imagine the same room placed on the second floor of a house with additional heated rooms sharing its left and right walls, plus a hallway connecting all three rooms at the back. It will now only be losing heat through one wall and the ceiling. Because the total area through which heat is escaping or entering is significantly smaller, the total load on the second room is much smaller.

As you can see, of the four factors that determine the heating and cooling requirements for a space, only one, exterior surface area, is even slightly related to the square footage of the space. The remaining 3 factors (climate, temperature preference, and build quality) are not included in any way when using rule-of-thumb-based methods. Many of these methods have been around for decades and do not reflect modern building techniques and the increased emphasis on efficiency seen in modern construction.

In the past, relying on these methods commonly resulted in equipment that was anywhere from 1.5 to 2 times larger than required. These days the results have only gotten worse. The results for cooling are often even worse than for heating, as a room’s sun exposure can contribute significantly to the cooling load. The best way to ensure the equipment you are purchasing is the correct size for your home is to have a professional perform a Manual J load calculation.

At eComfort, we are happy to have our engineers perform a professional sizing for you at no cost. Just download our sizing worksheet ahead of time and send us the completed version by email ( or by fax (1-866-554-4365) and we will get back to you shortly.

Alternatively, if you're just sizing for a single room (or multiple rooms to be conditioned independently) and want a good estimate without having to fill out our entire worksheet, you can use our online mini split sizing tool to give you a more accurate estimate than the simple calculators you may find elsewhere.

Mini Split Sizing Tool >

What Are the Effects of Having Over-Sized Heating and Cooling Equipment?
While having equipment that is too small to handle a home’s heating and cooling needs is annoying, the effects are only likely to show up during days of extreme heat or cold. The effects of having over-sized equipment are less widely understood, but have much larger drawbacks for a home owner.

High Initial Cost
The most obvious drawback of over-sized equipment is the initial cost. Buying a boiler, furnace, or air conditioner larger than needed is likely to increase the price a homeowner must pay for the equipment. This increase in price is usually modest and is often (falsely) justified on the grounds that it will keep the house more comfortable. In fact, this is just the beginning of the headache over-sized equipment will cause.

Short Cycling
When equipment delivers heating or cooling more quickly than it is needed, it will run briefly but often. This situation is called short-cycling. For heating equipment such as a boiler or furnace, this means that a large quantity of heat is quickly being added to the home. These injections of heat will often result in the home swinging from too hot to too cold and back again. Additionally, the heat does not have time to evenly distribute throughout the house, so any imbalances in the heating system will be magnified, resulting in rooms that are either warmer or cooler than the rest of the house. A properly sized heating system will deliver heat more gradually. This will extend the amount of time that the house spends at the desired temperature and allow the temperatures throughout the house to equalize somewhat.

Lack of Humidity Control
An over-sized cooling system will also result in short-cycling, temperature swings, and imbalances. However, the greatest drawback to an over-sized cooling system is the lack of humidity control it will provide. An air conditioning system removes moisture from the house by cooling the air across an evaporator coil and allowing the moisture in the air to condense. This moisture then drains off the coil and leaves through a house’s plumbing. When an air conditioner is over-sized and short cycles, the air temperature drops too quickly to give this process time to work. The result is a house that is cool but clammy.

High Operating Cost
If the lack of comfort was not enough, over-sized equipment will also cost more to operate than equipment that is sized correctly. The first ten minutes or so of operation are the least efficient for all types of heating and cooling equipment. When equipment is over-sized, it spends a far higher percentage of its time operating within this ten minute start-up phase and may never reach its full efficiency.

Premature Failure
The final major cost to an over-sized boiler, furnace, or air-conditioner is maintenance and repair. Short-cycling equipment will run more frequently than equipment that is properly-sized. This results in increased wear and tear on the equipment that will often cause it to fail prematurely.

Once the true costs of over-sizing equipment, whether to the wallet or to comfort, are considered, it becomes obvious that it is critical to select equipment that is properly sized for your home. Relying on outdated rules of thumb and other methods based on just the area of a home is likely to result in equipment that is grossly over-sized. To stay comfortable year round, and save money in the process, make sure to have your heating and cooling equipment sized by a professional using an approved method such as a Manual J Load Calculation.

Most companies will charge up to $500 for a professional sizing estimate. Since we want to make sure our customers always buy the product that’s right for them, we happily offer free professional sizing estimates by our in-house engineering department. If you’re ready to get a free sizing estimate, take advantage of our Unmatched Expertise and call us today at 866-554-HEAT (4328).

For a quicker, rougher estimate still grounded in the ideas discussed above, you can use our online mini split sizing tool. It will give you an estimate range of heating and cooling load for a single room, and is best used for sizing mini splits, unit heaters, PTAC, and other single room applications.

Mini Split Sizing Tool >

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