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 www.youtube.com

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

Notes:

  • 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.