Cooling Capacity Dynamics

Estimating Cooling Capacity using the total heat formula

Cooling BTU’s = 4.5 X CFM X Change in enthalpy


  1. 4.527642 = (.075 X 60 = 4.5) Note: air density (.075 here) is a conditional constant.
  2. CFM = velocity of airflow in cubic feet per minute.
  3. Change in enthalpy = entering air enthalpy minus leaving air enthalpy.

Use enthalpy tables or a psychometric chart to convert Wet Bulb to enthalpy.  Other charts are available to adjust for altitude, humidity, and the specific heat of air.  The 4.5 conditional constant is based on sea level, dry air, and 70°F. These conditions are often referred to as “standard air”.

Detailed cooling capacity information is provided for some, but not all, system match-ups.  The chart below is based on 80°F air at sea level.


As you contemplate the breath and width of cooling system actual performance under real world field conditions keep in mind ….TPD compensates for these performance variations due to constantly changing operating conditions and reports system performance AS IF the equipment was operating at the Mfg. rated operating conditions.  Using this method, TPD not only produces an accurate system performance report, it communicates the technical information in language technicians and clients can both understand.

Total Performance uses this reporting method because technicians and homeowners find it more convenient and less confusing.  After all; when the cooling system was installed, the homeowner was told it was a 5 ton system.  No one ever mentioned that the systems performance would constantly be swinging between 5.8 and 3.6 tons dependent on a set of variable operating conditions.

Clients aren’t really very interested in how many Btu’s a system is producing under a random set of variable operating conditions.  What everyone really wants to know is how is the system operating in relationship to how it’s supposed to be operating.

The high degree of TPD reporting accuracy can be easily checked against any manufacturers detailed cooling performance chart.  Simply convert the EWB to enthalpy, calculate the LWB enthalpy, convert the LWB to WB.  Set the TPD altitude, and CFM preference settings to match those used in manufacturers detailed cooling performance chart and “run the numbers”.

Conventional psychrometric calculations would only report actual total cooling Btu’s, which require technicians consult the Mfg. detailed cooling performance chart in order to verify the system is operating at the correct Btu output … based on the current field test conditions.  Total Performance Diagnostics (using a series of algorithms and calculations) compensates for these field variables in order to communicate the findings as easily and clearly as possible.


This condensed version of the chart helps highlight the more relevant factors that will (for our purposes) demand some further consideration.

Cooling capacity in total Btu’s


Cooling capacity in total Btu’s


Let’s look at the chart another way …

Cooling capacity as a percentage of nominal tons.

Cooling capacity as a percentage of nominal tons.

The detailed cooling capacity chart clearly illustrates the extent to which cooling capacity is dependent on three factors:

1) Condenser Entering Air Temperature …

2 )CFM  …

3) Indoor Relative humidity…

Since the Mfg. has designated the indoor ambient temperature, each change in EWB represents a change in indoor relative humidity.  Total Performance Diagnostics harnesses the dynamic nature of this process in evaluating and estimating system cooling performance.

The process is so dynamic in fact, that it is impossible to alter one factor without significantly upsetting the outcome.  What is not readily obvious in these charts is that for every Entering Wet Bulb there is a corresponding Leaving Wet Bulb that can be calculated for each BTU Capacity.  Just remember the formula

Cooling BTU’s = 4.5 X CFM X Change in enthalpy

The 4.5 conditional constant will need adjusted for altitude and the indoor relative humidity.   TPD utilizes a nonstandard air model to compensate for humidity. A specific location altitude is set as a preference modification selection.

Change in enthalpy is relative simple to calculate based on entering and leaving wet bulb measurements.

The CFM must be measured for “Verification modeling.  (Verification modeling utilizes verified input data to calculate an unknown conclusion.) Total Performance Diagnostics utilizes the CFM Override field for known CFM.

TPD also utilizes an assumed CFM that is set as a preference modification selection.  Utilization of an assumed CFM allows TPD to employ Falsification” modeling.  (Falsification modeling utilizes a combination of verified and unverified data to calculate a “known conclusion”.)  In falsification modeling, any answer other than the “known conclusion” is understood to be false.

Both the verification and falsification models are still pretty useless unless you have the specific cooling capacity chart for the system match-up being serviced, or a method of utilizing an adjusting algorithm   (like the one used by TPD)  that consistently represents BTU’s in terms of ‘rated nominal tons’.

Consistently represent BTU’s in terms of ‘rated nominal tons’ has a number of communication advantages for both technicians and clients.  Performance raters and Building Performance specialists may be uncomfortable representing performance this way … but technicians and clients will love it for its simplicity and clarity.

TPD utilizes a number of techniques to balance the competing needs for speed, accuracy and clear communications because any Performance Diagnostic method that is not used … is useless.  This rather obvious statement may help explain why the methods and techniques employed by Building Performance and Performance Rating organization have not been widely adopted by field technician involved with Performance Diagnostics.

So what Performance Diagnostic methods do field technicians use?  …. And why do they choose them over what are obviously superior methods and techniques?

  1. “The beer can charging method.”  When the suction line reaches the same temperature as a can of beer ….  “Your system is working just fine.”
  2. When the low side gauge is at 75 PSI …  “Your system is working just fine.”
  3. When the low side gauge temperature is 45° or 50° or 55° …  “Your system is working just fine.”
  4. When the high side gauge is at outdoor ambient plus 30° or 25° or 20°  ….  “Your system is working just fine.”
  5. When the temperature drop across the evaporator is between 15 to 20 degrees … “Your system is working just fine.”
  6. When the air coming out of the condenser is blowing up instead of out … “Your system is working just fine.”
  7. When the superheat is close enough  …“Your system is working just fine.”
  8. When the subcooling is close enough … “Your system is working just fine.”
  9. When the supply registers have a “good cold blow” …  “Your system is working just fine.”
  10. When I’ve spent enough time checking things out …. “Your system is working just fine.”

Why are these techniques and practices popular?   Contrary to what some may think, they are not popular because technicians are inherently lazy or poorly educated.  They are popular because they balance the technicians and clients desire for some type of Performance Diagnostic, the cost/benefit reality of more time consuming Performance Diagnostic methods and the price clients are willing to pay for the Performance Diagnostic service.

Despite the rebate induced … System Performance Rating programs, Building Performance programs, and Performance Inspection programs …. most companies still live and operate in a more or less capitalistic system most of the time.  Without someone else “picking up the tab” things that don’t make economic sense for the client and the company are not likely to get done.  Total Performance Diagnostics was developed to make economic sense for both the contractor and the client.

Time saving methods of detecting latent performance problems is in the best economic interest of every HVAC company since the companies’ greatest expenses are:   1) getting a customer to call for service   2) getting the technician to the job  and then   3) retaining that one time customer as a regular client.