How to size ac for house?

The ideal solution to reduce electric costs related to air conditioning is to install a higher efficiency central air system (16 SEER or higher). However, high SEER central AC systems are expensive, and are typically the more advanced Central Heat Pump type (18 SEER+ systems).

On the down side, if you install this advanced, higher efficiency AC, you may not see the payoff for many years.

Getting the right size AC unit for your house is very important if you want to have the desired level of comfort and good energy efficiency. The smallest central air conditioning unit is 18,000 BTUs (1.5 tons), while the largest is 60,000 BTUs (5 tons).

Today electricity is very expensive (anywhere from $0.14 / kWh in most southern states to $0.25-$0.45 / kWh in CA, MA, NJ, and most of New England) and you don’t want to waste it on running central air that is too big. On the other hand, installing a central AC that is too small will not keep your house cool! So while you will be saving money, you will not have the optimal temperature control.

PRO TIP: Use our new AC Energy Cost calculator, to get an idea of how much it costs you to operate your Central Air Conditioner, based on your location, AC size, SEER rating, cooling days and local electric costs.

The calculator is using live power cost feed from US Department of Energy, which is updated every month, to give you most accurate estimates of power costs.

With that calculator, you can also estimate your energy savings from replacing your old 9-13 SEER AC with something like a 16-20 SEER two-stage AC / Heat Pumps.

Your average energy saving from replacing a 9 SEER AC with a 16 SEER ac will be $450-780 / year, plus you can get Energy Efficiency Rebates in most states!

NOTE: When calculating AC efficiency over 16 SEER, you will get replacement cost for Central Heat Pump systems, instead of regular Central AC. This is because regular Central AC will max out at 16 SEER.

All residential central systems with efficiency of 17 SEER or higher, available on the market today are HEAT PUMPs, which can also heat your home in moderately cold temperatures (above 32°F).

Heat Pumps are generally more efficient than traditional ACs, and with addition of a reversing valve, they can change the refrigerant flow when needed, and change from Cooling to Heating mode!

As far as efficiency ratings go, a 16 SEER Heat Pump is just as efficient as 16 SEER traditional AC, and the size in TONs, which is required to cool your house is the same. Again, the primary difference that is “visible” to homeowners, is the ability to heat the house.

For the most accurate heat load estimate and ac equipment recommendation, use our advanced Heat Load Calculator (BTUs)

If you are looking for the most accurate size calculation for a central air system, the best course of action is to get free estimates from licensed HVAC installers.

A pro will have a comprehensive, professionally designed HVAC calculator that can do a complete cooling load calculation. He will come in for an in-house estimate and tell you exactly what size central AC you need.

Lets take a look at what this cooling load calculation entails and why its important to get it. We will also show you how to do a DIY estimate to figure out the central AC unit size you need.

Before we get into sizing a residential central air unit, lets take a quick look at pricing. On average, homeowners report spending $3,900 – 5,000 to install a 2.5 ton central air system in a house that has functional ductwork.

Fixing or changing the ductwork to accommodate the AC system can add as much as $3,000-4,000 to the total price. If the house does not have a ductwork system, the cost of installing central air will skyrocket to as much as $8,000-10,000+

Prices for central air units vary primary based on size (tonnage).

The smallest AC unit 1 ton (12,000 BTUs) costs $1,300 – $1,700. The 1-ton units are very rarely installed as they are too small for a “CENTRAL AC” application.

If you need an AC that small, you are much better off going with a ductless mini-split system.

Here are unit prices for different tonnage (prices do not include installation):

In general, you can roughly add $400-$700 for each additional ton, as the units go up in size. Residential Central AC systems come in a maximum size of 5-ton. Anything bigger, requires a commercial roof-top AC, which are in most situations not suitable for residential use.

Final pricing for central AC also varies based on brand name, as well as other features. Its best to purchase a unit from a well known, reputable manufacturer that offers a very good warranty.

Top AC brands include: Amana, Day and Night, Goodman Air, Carrier, Bryant, American Standard, York, Daikin, Mitsubishi, Maytag, and Heil. Ask your HVAC installer to a recommend a couple of brands he trusts.

The term “size” of a central air unit does NOT refer to the physical dimensions. Rather, size is a measure of its capacity to generate cool air.

Air conditioning unit sizes are measured in tonnage as well as BTU (British Thermal Unit) per hour.

Consequently, AC sizing involves determining the home’s cooling load: the exact capacity in BTUs per hour and cubic feet per minute of airflow required to maintain a 78 degree temperature in the house.

Residential central air conditioners range in size from 1.5 to 5 tons. Unit sizes go up in half ton increments:

12,000 BTUs – 1 ton 18,000 BTUs – 1.5 tons 24,000 BTUs – 2 tons 30,000 BTUs – 2.5 tons 36,000 BTUs – 3 tons 48,000 BTUs – 4 tons 60,000 BTUs – 5 tons

A unit that is above 5 tons is considered commercial size and these large units are actually not available for home installation. If your house actually requires a unit that is over 5 tons, your HVAC contractor will install two units to match your home’s cooling needs.

The most popular central AC size that works for the majority of residential homes is either 2 or 2.5 ton unit.

If you want to get a sense of how much AC tonnage your home needs, take a look at this heat zone map and ac sizing chart.

First locate the zone that your state is in, based on color:

Next, look at the chart below, and find the appropriate zone as well as the square footage of your house. Then, look at the corresponding unit size.

Keep in mind that this is the least accurate way to determine the proper central air system size for your home. It does not take into consideration any of the specific factors that can really impact AC size needed for a particular house. We will get into these below, and show you a more accurate way to do the estimate.

Here is another very quick and basic way to figure out the size of an AC unit based on the number of square feet in your house.

Take the total amount of square feet in your home. Make sure to EXCLUDE the square footage of your attic/basement/garage, unless you are using any of them as living spaces. Take this number of square feet and divide it by 600.

For example: 2,400 sq.ft. : 600 = 4,tons

If your house is well insulated and energy efficient, you can estimate down. For example, a 2,200 sq.ft. house divided by 600 = 3.6666. So you would want to install a 3.5 ton unit.

If there is poor insulation/efficiency in your home, its best to estimate higher tonnage. In the example above, you would estimate a 4 ton unit for a poorly insulated 2,200 sq.ft. house.

House square footage/ AC unit size estimate

To get a truly accurate size for central air that will work most efficiently in your house, you need to do a professional load calculation, known as “the Manual J”.

Most HVAC installers include it for free as part of their estimate, when they give you the total cost to install central air in your home.

There are two types of Manual J calculations ( whole house and room by room) and your contractor will decide which one is best to do, depending on your particular situation.

Getting the right size AC unit for your house is very important if you want to have the desired level of comfort and good energy efficiency. The smallest unit is 18,000 BTUs (1.5 tons), while the largest is 60,000 BTUs (5 tons).

Today electricity is very expensive and you don’t want to waste it on running a unit that is too big. On the other hand, getting a system that is too small will not keep your house cool, so while you will save money, you will not have the optimal temperature control.

The best course of action is to hire a licensed installer who has a comprehensive, professionally designed HVAC calculator.

He will come in for an in-house estimate and tell you exactly what BTU capacity central air system you need.

Lets take a look at what this calculation entails and why its important to get it. We will also show you how to do a DIY estimate to figure out the right size for your central AC unit.

This estimate is used to determine the amount of cooling needed for your house as a whole. There are multiple factors included in this assessment, such as:

– heat transfer between walls – number of windows – insulation type and R-value – ceiling height – number of people in your home – presence of sunlight or natural shade – number of heat generating appliances – many other factors are also used in the software program

It is recommended to do a whole house Manual J if you are only planning to replace/install the AC unit itself and will not touch the ductwork.

Even if you get the most efficient ac unit, these savings will be nullified if you have a poorly functioning duct system. If you are not sure about whether or not your ducts are in good working order, ask the installer to inspect them.

It has the same inputs as the whole house one, except it is done for each room. Then they are all added together to get the final result.

Overall, a room by room cooling load calculation is more accurate than a whole house one. It is recommended to use this method if you are planning to replace the ductwork in addition to the AC unit.

If you want to get a sense of how big of a central air unit your house will need, it is possible to get a rough estimate on your own.

However, it is important to realize that because there are so many factors that may impact this calculation, you will most likely not be able to take them all into account properly. Thus, your final assessment may not be very accurate.

Here is a simple formula to do the estimate:

1. Use the following equation: ((House sq.ft. x 25) / 12,000) – 0.5) = required tons

For example: your house is 2,350 sq.ft. Plug it into the formula: ((2,350 x 25) / 12,000) – 0.5) = 4.39

This means that your house requires a central unit of 4 to 4.5 tons.

However, if you live in a hot climate, where temperatures are high most months out of the year, you will need to change the formula slightly, because otherwise your unit will be undersized.

Instead of subtracting 0.5, you will need to ADD it.

So the new formula looks like this: ((2,350 x 25) / 12,000) + 0.5) = 5.39

This means that your house requires a central air conditioning unit of 5 to 5.5 tons.

As you may have already guessed there are a number of important variables that will effect the size of the central air system your house requires.

Total number of square feet is just one of these factors.

In fact, you can have homes that are identical in size, but still call for completely different size units.

Here are the additional factors that need to be taken into consideration:

Many homeowners hold on to the misconception that it can’t hurt to get a bigger central air conditioner than needed, just in case.

If the HVAC contractor recommends a 2.5 ton unit, many are tempted to install a 3 ton or even 3.5, just to make sure that the house will be cool enough.

This would have been a good idea 20 years ago, when electricity rates were cheap, and AC units used A LOT of energy to quickly cool off the space. In those times, bigger really was better.

So what changed?

Today, new air conditioner units are energy efficient. This means that they save energy by running in cycles, which results in your home being cooled at a slow and steady, rather than a rapid pace. Because of this change in how the AC unit operates, its essential to get one that is just the right size for your house.

Here is a quick summary of the main reasons why it is not advisable to install the wrong size central air, whether too big or too small.

In addition to sizing, its important to consider other features of a central air unit, such as the SEER rating, 1 vs 2 cycle system, fancy extras that high-end brands like to include, etc.

AC units are not all created equal. There are significant differences in pricing, quality, durability and warranties offered by various HVAC manufacturers.

The importance of a quality install cannot be overstated enough. Over 40% of all HVAC systems, including central air conditioners, fail due to incompetent, hack job installation.

Especially if you are willing to spend extra money on a premium brand name AC unit, such as Carrier, don’t try to squeeze a few hundred dollars of savings by hiring a sub-par installer.

If you need to cool just a single room then a central AC may not be the best or most cost efficient system for you.

Ductless mini split systems provide targeting cooling (and heating) to a single room or a large area. If you get a multi-zone mini split system, then you can heat and cool multiple rooms.

Besides flexibility, mini split systems offer a far superior energy efficiency compared to central AC systems. Whereas modern central air units have an average rating of 16 SEER (seasonal energy efficiency rating), a typical ductless mini split has a SEER rating of 22, while high – end models are rated at 33-40 SEER, in single zone installations. Multi-zone mini split systems go as high as 24 SEER.

Another major benefit is that a mini split AC system can be installed DIY, which is not possible with central air. Doing a DIY install saves you at least a few thousand dollars on professional labor.

While your mini split equipment may not look as attractive as invisible central air, you need to consider that you have a significant loss energy in the ducts of a central air system.

Mini splits do not have this issue because the cold or hot air is produced right inside the room. Therefore, actual efficiency of a mini split is considerably higher compared to central AC.

Before purchasing an HVAC system, it is important to know right size air conditioner or heating system for your house. A common mistake homeowners make is to purchase the wrong air conditioner size, sometimes too large of a unit for their home. This results in the homeowner spending more money on energy and operating costs than necessary.

The most efficient way to determine HVAC system capacity will depend upon what type of heating or cooling you prefer.

When it comes to HVAC equipment, bigger isn't always better. You’ll see the best results, longest life and most energy efficiency from a system that's selected based off the proper air conditioner sizing for the square footage you want to cool and heat.

To help you identify the best fit for your home and budget, here are just a few of the factors you must consider when an HVAC replacement decision.

To determine what size HVAC unit works best for your home, it's important to know what type of heating or cooling system you currently have installed. If you are unable to identify what your current HVAC system is, then here are some general guidelines on what size unit is most often used for what type of heating or cooling.

Often, there will be a sticker on the outdoor compressor with information about what size unit it is. This sticker has all of the necessary information needed to determine what size system is currently in place.

If you are unable to locate this, then you can check what type of controls are being used for cooling or heating your home by checking what type of thermostat is being used as this will indicate what type of electrical system is currently in place.

Alternatively, you can also check what size system your home builder installed. Simply call your builder and ask what size cooling or heating systems were used for your new construction. Another option would be to contact a local HVAC contractor and have them inspect your home and what type of cooling or heating system is currently installed.

Once you have determined what size HVAC unit is currently in place, it will be easier to determine what size system you should install so as not to purchase too large a unit and lose out on energy savings.

Keep in mind that if your existing HVAC system is larger than what is needed, what size system you should install will be what is recommended by the manufacturer of your existing unit. It would not make sense to purchase an outdoor unit that is too small for what you currently have in place.

When shopping what size HVAC unit you should buy, what matters the most is what size house you have and what your desired temperature range is that you want to maintain throughout. Knowing how many BTUs are required for your home will allow you to make the best decision what size HVAC system you should purchase.

BTU stands for British Thermal Units. A BTU is what's required to heat or cool one pound of water by 1 degree Fahrenheit.

For example, your home's square footage total, including all floors and ceilings, is what determines what size what size HVAC unit you need. For every 500 BTUs required for your home, you will need approximately one ton of air conditioning or heating capacity (1 ton = 12,000 BTUs). Every degree above or below what the desired temperature is set at requires another 500 BTUs to increase or decrease.

There are two types of what size HVAC systems, single stage and two-stage. Single stage units are what you want for you home if your desired temperature does not go above what the normal setting is set at during summer or winter times.

Two-stage units are what you should get if there are times where temperatures get either hotter or colder than what the what size HVAC system you have will allow. If you live in an area where temperatures reach far extremes you need to add this into your residential load calculation.

The Manual J calculation is what HVAC professionals in the industry use to calculate what size unit you need. The Manual J sizing method includes measuring the cubic feet in your home, finding out what kind of attic insulation you have inside (R-11, R-13 etc.) and what your specific climate is like where you live. Here are the steps you should take to figure what Manual J sizing is perfect for your home:

1. Measure the outside air temperature of where you live and what the inside temperature averages during the winter.

2. Your manual J calculation requires knowing what your insulation is, what's included in your insulation and what climate zone you live in (hot or cold).

3. Look for what your insulation is and what the inside temperature averages on the Manual J calculation chart, and follow those steps to find what size unit you need.

4. Remember what features come with different sized units, so you can choose the perfect one that will fit all your needs.

When determining what size HVAC system is necessary for your home, it's important to get an accurate reading of what the square footage of your house will be. A square foot heating guide will make sure you're purchasing a unit that is big enough.

There are online calculators that can help you determine what size HVAC system you need based on what you input into them as well.

One of the most important things to consider is what size HVAC system your home uses now . It is generally most economical to replace an existing system, especially if it is old and inefficient. If you have a new home, however, it may be most cost-effective to install a much larger system.

What size HVAC unit you need for your home also depends on what climate zone it is in . The closer you are to the equator, the warmer it will be year-round. If your area has mostly mild temperatures, you do not need an incredibly powerful unit to cool your home. However, if there are frequent heat-waves and you live in an area near the equator, your unit is going to have to be able to keep up.

What size HVAC unit you need for your home is also based on what season it is . If it is winter and you live in a cold climate zone, choose a system that will easily heat your home without constantly turning on and off. Try to make sure it will be powerful enough to heat your house during what is expected to be the coldest day of the year. On hot days, however, you do not want a system that can cool your entire home all at once.

There are many other factors that affect what size HVAC unit you should purchase for your home. Before deciding what system to buy, calculate what size HVAC unit you need based on what season it is and what climate zone your house is in. Also consider what size system it currently has so that you do not waste money or energy trying to cool or heat the entire home at once.

So what size HVAC unit should you get for your home size? For a single what size HVAC unit, what matters the most is what area of your home to run it in and what temperature range you desire throughout.

For average sized homes between 1,500 to 3,000 square feet, what size HVAC system you should purchase is what's called a three-ton unit. Depending on what your desired temperature range is will depend what size what size HVAC system you decide to get.

For every degree above or below what the desired temperature is set at will require what size HVAC system you should purchase to be an additional 500 BTUs.

For example, if your desired temperature is set at 71 degrees and what size what size HVAC unit you should get for your home size is a three-ton air conditioning or heating capacity system, then this what size what size HVAC unit should get what size HVAC system your home requires for that temperature range is 6,000 BTUs.

Selecting a properly sized cooling system is essential for comfort and efficiency.

If you buy an undersized air conditioner, the equipment may fail to keep up with cooling demand on hot, humid summer afternoons, even when running nonstop. This also increases your energy bills, despite your efforts to save money by purchasing a smaller system

On the other hand, if you buy an oversized air conditioner, it will reach the target temperature too quickly. This short-cycling behavior increases wear and tear, leading to premature breakdowns. Cycling on and off frequently also makes it impossible for the AC to adequately dehumidify your home, leaving you feeling cool but clammy.

The cooling capacity of an air conditioner is measured in British thermal units (Btu). One Btu is about the amount of heat generated by a birthday candle. You’ll find window units with cooling capacities ranging from about 5,000 to 12,500 Btu, which indicates the amount of heat the unit can remove from a room per hour.

Central air conditioners are much larger, so they’re measured in tons, which is equivalent to 12,000 Btu. Most residential units range from 1.5 to 5 tons, or 18,000 to 60,000 Btu.

If you’ve ever wondered what size central air unit you need, or how to size a window AC unit, be aware that square footage is the number-one factor. Indeed, it’s often the only factor listed in rule-of-thumb guides. On average, an air conditioner needs 20 Btu of cooling power per square foot of living space. However, smaller areas tend to require more than 20 Btu per square foot, while larger rooms need less than 20 Btu per square foot.

But square footage is far from the only consideration you should make. These additional factors also affect what size central air or window unit you need:

The principles in this guide are merely rule-of-thumb estimates. To get the most accurately sized AC unit, you need to hire a professional. Cooling experts use Manual J, the industry standard for sizing HVAC systems established by the Air Conditioning Contractors of America (ACCA). This takes all of the factors outlined above and determines how they affect your home’s overall cooling load to calculate the ideal AC unit size.

Using a simple math equation, you can calculate the size of AC unit that will best fit your home. The equation looks like this: ((House square footage times 25, divided by 12,000) – 0.5) = required tons.