You need to understand a few critical ratings about HVAC design and installation when choosing or designing an HVAC system. Otherwise, you might end up with an HVAC system that doesn’t meet all your needs. Below are some ratings you should pay attention to.


The British Thermal Unit (BTU) is the energy necessary to raise the temperature of one pound of water by 1 degree Fahrenheit. In the HVAC context, BTU measures the amount of heat an AC extracts from the air inside your house per hour. The higher the BTU is, the more heat your AC can remove from your house.

However, don’t automatically get an AC with the highest possible BTU for your house. Your AC’s BTU should match your house’s cooling needs. Factors that determine the right BTU include:

  • Your local weather
  • The number of people in your household
  • The number of rooms in your house
  • Your house’s square footage

A system with a high BTU will cool your house faster than it should. That will lead to a short cycling AC, which is bad for your AC’s energy and cooling efficiency.


The Annual Fuel Utilization Efficiency (AFUE) measures the efficiency of heating systems that burn fuel. The AFUE measures the percentage of fuel consumed that the system uses to generate heat. AFUE is critical because not every unit of fuel that the heating system consumes turns into heat. A combustion system loses some of its fuel, for example, as exhaust.

Heating experts calculate AFUE as a percentage of the fuel consumed. For example, an AFUE of 90% means the system turns 90% of its fuel to heat and only loses 10%. The calculation involves dividing the amount of fuel consumed by the amount of heat (in BTU) produced. The higher the AFUE percentage is, the more efficient your heating system is.

High-efficiency systems can have AFUEs of over 90%. Low-efficiency systems have AFUEs lower than 70%. Apart from your systems’ initial AFUE rating, it’s likely to lose some of its efficiency as it ages.


HVAC systems work best with good airflow. Airflow is necessary to ensure every part of the house receives fresh air and maintains a set temperature. The HVAC industry measures HVAC airflow capabilities in cubic feet per minute (CFM). The CFM is the volume in cubic feet that the system forces to pass by a stationary point every minute.

The CFM you require primarily depends on your house’s volume (square footage and ceiling height). The relationship makes sense since larger homes require more airflow than smaller ones. Other critical factors for CFM calculations include:

  • Climate
  • Number of windows
  • Temperature preference

Again, a high CFM isn’t automatically good. For example, using an HVAC system with a higher CFM than you need can force some of the air to bypass the air filter. Bypassing the air filter can lead to poor indoor air quality.


If you use a heat pump, the Heating Seasonal Performance Factor (HSPF) is important. The HSPF measures the heat pump’s efficiency during the cold season when the system operates in the heating mode. The HSPF calculation involves dividing the total heat output of the system by the total electricity it consumes for the same period.

The higher the HSPF, the more efficient your heat pump is. The government requires a minimum of 8.2 HSPF — the standard came into effect in 2015. However, the government is set to increase the minimum to 8.8 HSPF beginning 2023.

You don’t need to do all the calculations and evaluations for your HVAC installation. An experienced HVAC technician will do that for you. Contact Apollo Heating & Air Conditioning for all your heating and cooling needs and benefit from our skills and 40+ years of experience in the industry.