HVAC System Sizes

The sizing of your HVAC system directly affects how well it heats and cools your home. If your home heating and air conditioning needs are too much for your HVAC system, it will not heat and cool effectively. If the system is oversized, it can cause health and safety concerns, reduce your systems lifespan and waste energy. HVAC professionals use several factors to determine the appropriate size HVAC system for a home.

Interview With Residents

An integral part of correctly sizing an HVAC system is knowing not just the size of a home, but how it is used. To determine this, an HVAC professional will hold a quick interview with the residents of a home to answer questions such as:


How cool do you prefer to keep your home in the summer?


How warm do you prefer to keep your home in the winter?


Do you entertain large groups often that might require changing your home temperature?


Do you want to keep your entire home at the same temperature or keep only the most used areas perfectly conditioned?


Is the house vacant during significant parts of the day, such as when residents are at work or school?


Does the home need to remain at a certain temperature even when it is vacant during the day (such as if the needs of pets must be considered)?

These are just examples of some of the questions HVAC professionals may use to determine what size HVAC system would best serve the residents of a home. In addition to determining residents’ needs, an evaluation of the home itself must be completed.

Home Evaluation

Once the needs of residents are determined, HVAC professionals will evaluate the home to decide what size HVAC system it requires to meet those resident needs. This evaluation will make note of factors such as:

Type of foundation

Exterior sheathing & type of wall insulation

Fireplace (if any)

Window type, locations, and solar gain

Attic ventilation and insulation values

Size of the house

…and more

This information helps HVAC contractors to make a load calculation.

the roof of a house

What Is A Load Calculation?

Load is the amount of heating or cooling per unit time that a home needs to remain at the resident’s desired temperature. HVAC contractors take into account all of the information from the home evaluation, along with the size of the home, the local climate, and even potential changes to the home, to make a load calculation.

There are three different types of cooling and heating load.

a family in a kitchen

Design Load

The design load takes into account variables in the home that do not fluctuate. This can include the home layout, the orientation of the sun to the home, the home’s overall energy efficiency, etc. Since design load only considers non-changing variables, and many changing variables still affect a home’s load calculation, design load is never used as the sole factor in a load calculation.

a thermometer

Extreme Load

The extreme load takes into account the load required during extreme weather conditions, i.e. the hottest and coldest temperatures that the home’s location may experience. These extremes rarely occur, but are included as a buffer built into the load calculations.

a person sitting in a field

Part Load

The part load takes into account the many days throughout the year that heating and cooling loads will be less than the design load. During these days an HVAC system might need to cycle on and off, something that must be considered when determining system size.

electrical tools

Manual J Load Calculation

After the needs of residents are determined and the characteristics of the home are evaluated, HVAC contractors make a Manual J load calculation that utilizes all of the previous factors that have been considered.

The Manual J load calculation was developed by the Air Conditioning Contractors of America (ACCA). It is most often completed using software developed specifically for the task. The result determines how much air conditioning capacity your home needs, for heating and cooling.

This capacity is quantified in tons and BTUhs.

What Are Tons & BTUhs?

Tons, in relation to HVAC systems, are a measurement of capacity related to melting one ton of ice in a 24-hour period.

BTUhs are British Thermal Units per hour, which are a specific measurement of heat. One BTU (British Thermal Unit) is the amount of heat it takes to raise the temperature of 1 pound of water by 1 degree in Fahrenheit. One BTUh is how many BTUs of heat a cooling system can remove from a home in one hour.

There are 12,000 BTUs in 1 ton.
36,000 BTUs /
12,000 BTUs
= 3 tons

How HVAC System Sizes Work

HVAC sizes are based off of capacity. System capacity is included in the model number of a unit. Residential system sizes are usually between 18-60, representing unit sizes 1.5-5 tons.

For example, imagine that your system’s model number indicates it is a size 36 unit. The 36 represents the unit’s BTUs. A 36 means 36,000 BTUs. This means your system is a 3 ton system.

The Manual J load calculation results help HVAC contractors determine what ton size system your home requires in order to meet your heating and air conditioning needs without the unit under or overworking.

36,000 BTUs /
12,000 BTUs
= 3 tons

Types Of HVAC Systems

Once your Manual J load calculation has been completed, HVAC contractors will be able to recommend an HVAC system that will meet your needs. There are many different systems available. Below are the two most common we recommend for most homes:

a carrier ac unit

Traditional Split System

An HVAC split system is great for homes with plenty of indoor space for the metal housing and supply air ducts the unit requires.

In this unit, the condenser and compressor are normally set up outside the home. The evaporator coil is located in metal housing set up somewhere inside, often in an attic space. Copper tubing connects the indoor and outdoor components.

While installation of HVAC split system units can be more intensive than other units, homeowners often choose this system due to its high energy efficiency and low maintenance requirements.

Ductless Mini Split System

Ductless mini split HVAC systems are similar to traditional split systems. They are comprised of both outdoor and indoor components connected by refrigerant tubing. The indoor component is a wall-mounted unit that delivers conditioned air directly to the living space it is installed in.

Because the wall-mounted unit delivers air only to the space it is installed in, this system requires multiple indoor installations. The indoor units come in more than one style allowing a lot of versatility from a ductless mini split system.

Many homeowners prefer this system because it does not require any ducts to be installed, ensuring no loss of efficiency due to duct leakage or heat gain. The indoor components have a lot of flexibility in where they can be placed so it’s easy to minimize sound from this system as well.

a ductless mini split system

How System Efficiency Is Rated

HVAC efficiency is determined by how well a system cools a space and with how much electricity. The most efficient systems require less electricity to cool a given space. There are a variety of ratings used to measure system efficiency.


The ENERGY STAR® label is a great indicator of system efficiency. This label is only given to specific systems. These systems meet high performance standards set by the Environmental Protection Agency (EPA).

energy star logo
Cooling output / Overall Power Consumption

SEER Rating

The Seasonal Energy Efficiency Ratio (SEER) is a rating determined with a calculation using the system’s cooling output and the system’s overall power consumption during the cooling season. It is determined with the following equation:

In areas like Central Texas where the cooling season is longer, systems are required to have a minimum 14 SEER rating. However, SEER ratings currently can go as high as 27 and are always improving. Most variable stage air conditioners are rated 19 or above.

Cooling output / Overall Power Consumption

EER Rating

The Energy Efficiency Ratio (EER) is a similar measurement that looks at immediate efficiency, unlike the SEER rating which measures efficiency over an entire season.

EER is determined by a calculation similar to that used to get SEER ratings. The equation for getting EER ratings is:

An EER rating of 12 is considered a very efficient unit.

Capacity (BTU) / Power (Watts)
= EER.

When To Request An HVAC Size Recommendation

a technician working on an ac unit

As with most household necessities, it’s best to start looking for a new HVAC before your old system stops working. No one wants to be in a home without air conditioning in Hays County, especially during the summer. If your system is ten years old or more, it’s time to look into replacement options.

Replacing your HVAC system before the old one breaks will prevent uncomfortable time between the breakage and new installation. In addition, HVAC companies are constantly finding ways to improve energy efficiency, so if your system is older it’s likely that upgrading to a more efficient system could help lower utility bills as well.

If you’re ready to figure out what your next HVAC system should be, contact the HVAC experts at AirOne Heating & Air Conditioning, a San Marcos HVAC contractor, and we’ll schedule a time to meet with you right away. We’re also always happy to take questions, even if you aren’t ready for a new air conditioner just yet.