How Much Does Your Heat and Cool Unit, Heat and Cool?

If You Purchased a 3 Ton, 14 SEER Heat Cool Unit, What Size Unit Did You Really Get? 

Your system can be tested to find out.

Under extreme conditions your new 3 ton unit may only have a 1.5 ton and 7 SEER efficiency.  Under normal conditions your unit might only measure up to only 90% of its nameplate rating. If your unit is over 12 years old it may have lost significant capacity and efficiency.

About 70% of your heat and cool air conditioner units capacity is used to lower the temperature of the air.  The remaining 30% capacity is used to remove humidity or moisture from the air.  As an example a 3 ton unit should have a total capacity of 12000 btu’s per ton or 36000 btu’s.  Of the 36000 btu’s, 24000 btu’s would lower the temperature of the air and 12000 btu’s would remove moisture from the air.  In order to measure total capacity, we will need to measure both. 

A normal thermometer will measure the air temperature entering the return of your AC and the temperature of the colder air can be measured at the first supply vent.  If you multiply the difference in temperature by 1.08 btu for each cubic foot of air flow produced by the Heat Cool system, you have the capacity measure that lowers the temperature.  This measure is called sensible heat.

Another thermometer called a psychrometer or hygrometer is needed to measure the humidity that is removed by the air conditioner.  A psychrometer is a normal thermometer with a wet cotton sock over the bulb.  The temperature reading called wet bulb temperature can be converted to a value indicating the temperature and grains of moisture in the air.   This value is called latent heat and enthalpy.  You can purchase or make a wet bulb thermometer.  A very accurate digital hygrometer that measures % relative humidity can be purchased for around $100.  Using an enthalpy chart or enthalpy calculator available online, you can convert your measurements to btu’s.  It is very important to measure wet bulb temperature within one tenth of a degree F.

The more difficult measure to obtain is the amount of air flow in cubic feet per minute (CFM).  An instrument called a vane anemometer can be used as can an instrument to measure static pressure.  Instruction videos for measuring air flow in CFM can be found on

The Formula

Calculating total Btu’s involves measuring airflow and wet bulb air temperatures in and into a conditioned space. Total Btu’s are calculated using the formula:

Total Btu = 4.5 X CFM X Enthalpy Change

The 4.5 factor represents the weight of 1 cubic foot/minute (CFM) of standard air per hour.

CFM is the total supply airflow entering the building from the system.

Enthalpy change is measured using a hygrometer and converting the readings to Enthalpy.

Latent eat = Total Heat – Sensible Heat


  • Air and heat can leak into the system between your temperature readings and the indoor cooling coil.  If this happens, your calculation will indicate the cooling delivered to the space and not the cooling produced by the AC unit.
  • You can also determine equipment airflow by carefully measuring fan static pressure, then plotting the fan CFM based on the manufacturer’s fan data
  • An enthalpy change of about 6 is normal.
  • Airflow has a major effect on system performance. It must be measured accurately with top quality instruments and procedures.
  • A humidity level of 90% plus from the supply side of the system is common. As the air is cooled is can hold less moisture.
  • Normal sensible/latent ratio is 70/30.  A significant return air duct leak from a hot attic, produce a 55/45 sensible/latent BTU ratio.

Air Conditioner / Furnace Installation Building Permits

The less you know about safe electrical and air conditioning installations, the more important the permit.  Nothing is more important than the safety of your family and property.

Some permits are simply sources of revenue for the town or city and have little value.  If the city sends a knowledgeable inspector out to inspect and approve the  installation, the value of a permit could be priceless.  In either case if a permit is required by law, you and the contractor are taking on significant liability if you don’t follow the law.

What is an Air Conditioner Installation Permit?

Although some types of air conditioners, like through-the-wall and window models, are easy to install, there are a number of safety and electrical concerns during an air conditioner installation project. For this reason, many city and state governments require homeowners to obtain a permit before beginning the installation process.

The specifications of your air conditioner permit will vary depending upon your location and the type of air conditioner that you are installing. However, most cities have requirements in some or all of the following categories:

•              Installation location relative to the building

•              Electrical safety during and after installation

•              Vent gas safety, carbon monoxide

•              Fire safety

•              Drainage from the condenser portion of the unit

•              Noise production

Why is an Air Conditioner Installation Permit Required?

The primary concern during an air conditioner installation project is safety. Because air conditioners require rewiring of electrical cables, it is necessary that you follow the instructions for installation carefully. It is generally required that you provide an electrical disconnect and an receptacle within a certain distance of the central air conditioner unit.

Flue gas vents must be installed properly to prevent fires or poisoning the occupants.

Some air conditioners produce a lot of noise or condensation waste. Most municipal governments require a permit in order to ensure that those elements are kept to a minimum, both during and after the installation process is complete.

How to Obtain an Air Conditioner Installation Permit

The method for obtaining an air conditioner installation permit in your city or state will vary depending upon the rules and requirements of your location. The type of permit may also vary according to the zoning district and type of building for which you are installing an air conditioning unit.

To learn more about the requirements for your city or state, consult a home improvement center or contact your local city hall. Some cities allow permits to be issued online, while others may require a fee. Additionally, it may be necessary to have your installation process inspected by a government official to ensure that you have followed the guidelines for installation properly.

Do not be discouraged by the prospect of having to obtain a permit in order to install your air conditioner. The permit application process is generally quite easy and inexpensive, and the purpose of requiring a permit is to ensure that you install your air conditioner safely and correctly.

In order to obtain a building permit, certain information must be given to the local building official. Intermediate and final inspections may need to be performed by their inspectors to verify that the work was performed in accordance with applicable building codes.


A building permit may be obtained by the owner or a licensed contractor after filling out a few forms and paying a small fee. Plans and specifications prepared by an HVAC contractor, architect or professional engineer describing new work or alterations are required for large projects where structural elements are involved, or when major electrical, air conditioning or plumbing systems are altered. Minor alterations may require a permit, but usually do not require plans and specifications.

In addition to installing air conditioning / heating units, these common alterations and improvements usually require a building permit:

  • Structural additions;
  • Installation of a new roof;
  • Adding or blocking off a door or window;
  • Adding or relocating electrical outlets;
  • Adding or relocating plumbing fixtures, such as showers, sinks or toilets; and 
  • Converting a garage or storage area to an air-conditioned, occupied area, or installation of central air-conditioning systems.

The penalty for non-compliance is usually double the fee for the permit. The permit can be applied for by the current property owner or by a contractor, but the problem often doesn’t end when the fee has been paid. If inspections and construction documents would have been required to satisfy the original permit, these items must now be obtained. Also, all work must now meet the current code — not the code that was applicable when the alteration was made.

Small Violations


Small violations usually are not detected by the local building department, as they are quite busy and don’t actively pry into peoples’ homes or buildings. Building owners often add electrical outlets and light fixtures, or convert a garage into an extra bedroom without a permit. Even if the building department discovered the violation, the owner can easily remove or change the alteration to its original condition. In most cases of small violations discovered by the building department, only a small fine or reprimand results.