What size heat pump would you install in each  home? 

Smith ___tons

Brown ___  tons

Jones  ___  tons

Smith residence

Year built- 1962           location – Raleigh, NC     1880 square feet

    front facing south


average construction

walls- no insul

windows- storm

Floors – no insul. over crawl

Ceiling-  R-11

Ducts- R-4, not sealed, in attic

Brown residence

Year built- 1985    Location – Raleigh NC      1880 square feet

    front facing south


very tight construction

walls- R 11

windows- storm

Floors – R 11,  over crawl

Ceiling-  R-19

Ducts- R-4, unsealed, 

Jones residence 

Year built- 2021     Location – Raleigh NC      1880 square feet

    front facing south


very tight construction

walls- R15

windows- double pane, low-E

Floors – R19,  over crawl

Ceiling-  R-30

Ducts- R-8, sealed, 

 The solution

Many hvac contractors would use a Rule-of-Thumb, such as, “1 ton per 500 square feet of living area” (1880 Sq. Ft. / 500 = 3.8 tons). While some contractors will stand at the street, hold their hand a foot in front of them and count the number of fingers it takes to block out the house If three fingers do the trick, then, great, its a three ton house. A third, more accurate, method to size an ac unit is accomplished using the sizing tool as  shown below.


Please print this calculator and follow the instructions

Now, that I’ve covered the methods many hvac contractors use to size systems, let’s have a serious discusion on why and how a load calculation must be performed on every home needing a new hvac system.

 Let’s cover the “why” part first.  

An air conditioner has two functions, (1) cool the air and (2) dehumidify the air while simutaniously cooling.  Bare with me for a few minutes as I explain the properties of air using a psychrometric chart

Referring to the psych chart above, find the red star. You will see that it lies on a vertical line, corresponding to 90F outdoor temperature. The star also lies on a curved line indicating that the relative humidity is 60%. If, you were to place your curser on the star’s center and slide directly to the right, your curser will fall in the vicinity of 130 on the first vertical column of numbers. This is telling you that air at 90F with 60% RH holds about 130 grains of moisture per pound of air. In the field, you can place the star on the chart by locating the intersection point of the dry bulb temperature on the bottom line(90F)  and the wet bulb temperature (78). What is the RH with 85F dry bulb and 70F wet bulb? (you should get about 45% RH).

Now, let’s lower the temperature without removing any moisture. Place your curser on the red star and move directly to the left, staying on the 130 grain line, stop at 80F. Now the RH is more like 85%. If you were to move the curser to 70F without removing any moisture the RH would be over 100%, thus, it will begin raining.


The lesson learned here is, “If you do not remove moisture from the air as you cool it, the RH will increase, possibly causing mold and damaging the structure”. So, “how do you prevent the RH from climbing while air conditioning?” While an air conditioner is running, moisture in the air condenses on the coil and is carried outside via the drain pan and drain line. Therefore, an air conditioner cools and dehumidifies at the same time. If an air conditioner is too large for the home, it will cool the home fine, but will not run long enough to properly dehumidify. The indoor air will contain excess humidity, uncomfortable conditions and high operating costs.

Uncomfortable conditions?

The human body stays comfortable amongst a range of temperatures and relative humidity. For example; one may be perfectly comfortable with a temperature of 78F at 55% RH. On the otherhand, if the RH is 65% then the temperature may need to be 72F the obtain the same degree of comfort. “The lower the RH the higher the temperature we can tolerate”. It has been calculated that for every degree above 70F the thermostat is set there is an energy savings of 3%. If you opt to set your thermostat at 77 verses 72, you could realize a 15% savings on your electric bill.

Now, let’s cover the “how’ to properly size an hvac system.

Unfortunately, sizing an hvac system is not as easy as using the three methods I started this articel off with. To determine the air contioner size, a heat gain calculation must be performed and to determine the heating size a heat loss calculation must be performed. To simplify things, we will call these calculations “load calculations”. A load calculation will tell you how many BTUH ( British thermal units/hour) or KW it will require to heat or cool a home. The most accuaraate and widely accepted method for performing a load calculation is prescribed by American Society of Heating, Refrigeration Air Conditioning Engineers (ASHRAE) and ACCA’s Manual J. When surfing the net for AC sizing guides, you will find hundreds of methods, most are based on rules-of-thumb and generally grossly oversize the system, resulting in a system with pore humidity control

Many contractors shy away from performing load calculations because the math involved looks like the blackboard of a mad scientist. True. it may require a lot of math but its all easy math. basically a house is made up of walls, floors, ceilings, windows and doors. We need to determine the amount of BTUH’s that go through each component when the outdoor temperature is at its typical coldest or warmest, add the BTUH’s up, and “presto”, you have a cooling or heating load.

The EMS hvac load calculator was developed to make load calculations a painless as possible. The only math a user needs to learn is:

How to calculate square feet 

Easy:     length X width


How to calculate volume

Almost as easy:       length x width x height

Now, to answer the original question I posted at the top of this article. we will perform a load calculation on each home. I purposely chose three identical homes at identical locations to show the disparity of loads, hoping to show you old fashioned rules of thumb do not work.