How to perform an HVAC  load calculation

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World’s easiest HVAC load calculation software


HVAC load calculation software

“So easy, a dog can do it”

Also known as a btu calculator, heat loss or heat gain calculator, air conditioner sizing app, or furnace sizing app




Quickly and easily perform residential and commercial load calculations


  • Whole  house heat loss and heat gain calculation(block load) 
  •  room-by-room loads
  • Sizes the duct system
  • Selects equipment
  • Performs cost analysis and ROI
  • Compares system energy costs 
  • produces a printable report

Available as a 60 day trial or permanent, never expiring versions.

For 60  day trial. Good for DIY’s or a limited number of projects 

Perminent version (never expires)

Only $129


 “So easy, your dog can do it!” Want  a demo? Click on the dog.

Works on any device; anywhere there is an internet connection.

iOS compatible

PC’s, Mac’s, Tablets, I-pads, Androids and iPhones.

HVAC load calculation demo (Whole House)

This HVAC load calculation software has been developed using sound engineering principles as prescribed by ACCA’s Manual J, 8th edition, and ASHRAE’s “Handbook of Fundamentals”.

Loaded with Features

● Performs block (whole house) and room-by room load calculations
● Weather data for any locality may be entered (Fahrenheit or Celsius)
● Allows for R-values up to R-55
● Window manufacturer’s specs may be used
● Blower door results may be entered
● Includes equipment selection calculator according to Manual S guidelines
● Allows for basement calculations
● Prints a report for customers and/or permitting
● May be loaded on up to three computers at no extra charge
● Calculates minimum fresh air ventilation (CFM) requirement.

● Sizes duct work, metal or flex
● Calculates friction rate
● Gives velocity
● Converts round to rectangular duct
● Calculates whole house heat loss to size boiler
● Gives room-by-room btuh requirements for radiator sizing

  • Sizes the system to handle both sensible and latent loads
    ● Determines equipment capacity at any altitude
    ● Determines the correct CFM and TD for absolute humidity control

(View instructions)
Our commercial load calculator enables you to calculate loads for small to medium size buildings up to three stories in height. Example structures are: offices, retail stores, restaurants, warehouses or churches. This calculator is not designed for industrial or high rise buildings.
● Calculates infiltration according to building height
● Gives credit for refrigeration loads with remote compressors
● Calculates loads due to mechanical ventilation, hoods, and energy recovery systems
● Calculates load due to door traffic
● Calculates occupant loads according to various activity levels
● Calculates lighting loads
● Calculates motor loads
● Allows for appliance and other internal loads
● Adjusts fenestration load when most glass is facing either east or west

Why is our software different?

  • It’s just plain quick and easy and it sells more jobs and saves you $1000’s on equipment and operating cost
  •  We offer a 100% refund if you are not satisfied. 


Try getting that guarantee from other software developers.


 Works on all platforms  
● Estimates operating cost
● Compares operating cost of one system to another (using natural gas, oil, LP, heat pump or electric resistance heat)
● Gives return on investment (ROI)
● Shows payback
● Play “what if” when adding insulation, changing efficiencies or energy sources

NO RENEWAL FEES- includes automatic and free updates

We offer a 100% refund if you are not satisfied

Start using this software in the next few minutes
With the protection of our 100% guarantee


Pricing and Ordering

Energy Marketing Service

353 Winfield Lane, Pinetown, NC 27865


  • Once your purchase has been made, you will receive instructions to instantly access the software.
  • With the permanent package, you will have unlimited access to our residential and commercial load calculation software. NO YEARLY RENEWAL FEES
  • All updates are free and automatic.
  • Step-by-step instructions, with illustrations are available on each screen. Plus, we offer real live help if you should ever need it.
  • Your purchase will be processed via PayPal. However, a PayPal account is not necessary.                             

Purchase 60 day trial residential program

Perfect for one time or a limited number of HVAC projects.

 Includes residential load calculator, plus duct sizing, equipment selection and cost analysis software for one low price This software includes all the features and capabilities of the permanent version, however it will expire on your computer 60 days after purchase.

Only $49


Purchase permanent Residential and Commercial program

Includes both residential and commercial load calculators, plus duct sizing, equipment selection and cost analysis software for one low price. This software does not expire and includes free updates.

Only $129


What is a load calculation?

All structures either lose heat in the winter time or gain heat in the summer time. This heat loss or heat gain is caused by the fact that the transfer of heat, even in a super insulated house, cannot be completely stopped.

If we know how much heat is being transferred through its walls, ceilings, floors, windows and doors, ducts and through infiltration (air leakage) on an hourly basis, then we could calculate the precise size heater or air conditioner the house would need to maintain a comfortable temperature. This calculation is called a load calculation.  The load is measured in BTUH’s

Why perform a load calculation?

The obvious reason is to prevent installing a system that is too small to do the job. However, if this were the only reason, why wouldn’t we just put a 5-ton air conditioner and a 140,000 BTUH furnace in a 1200 sq. ft. house and never worry about it again? The real reason for a load calculation is to size the equipment in order to assure comfort, economy, and good indoor air quality.

When heating, it is important to size the system as close to the heat loss calculation as possible to prevent (1) drafts, (2) hot and cold spots, and (3) short cycling of equipment. When a furnace is grossly oversized, the unit will constantly shut off and on. It may satisfy the thermostat but leave other parts of the home either over or under heated thus, leaving the occupants uncomfortable. A correctly sized unit runs longer, resulting in a better distribution of air and reduced short cycling. Short cycling also leads to higher energy costs. Each time a furnace fires up, it must heat up the heat exchanger before the indoor fan comes on. This heat is wasted up the chimney. Short cycling (short on-off periods) increases the amount of heat wasted up the chimney. In addition, if the occupant is cold in the area he is sitting in (a cold spot), he will turn up the thermostat, which wastes fuel as other areas are over heated.

Sizing rules for heating (ACCA):

  • Fossil fuel furnaces – Do not exceed 125% load calculation (may be twice the size required).
  • Electric resistance heat – Do not exceed 110% of load calculation.
  • Heat pumps (used for heating and cooling) – Do not exceed 125% of cooling load
  • Heat pumps (used for heating only) – Do not exceed 115% of heating
  • Auxiliary heat (electric resistance) – Install only enough KW to make up for the heat pump’s deficit. If more heat is desired, the additional heat must be controlled to remain off during normal heat pump operation.

Sizing an air conditioner correctly is even more important than sizing heat. Aside from causing hot and cold spots, over-sizing an air conditioner can result in causing high humidity and the problems associated with it. When an air conditioner runs, it is not only cooling, it is also dehumidifying. An oversized air conditioner will cool the house but, will not run long enough to dehumidify. High relative humidity can have two detrimental effects: (1) higher energy bills because higher humidity requires lower thermostat settings to remain comfortable, and (2) promote mold, mildew, moisture, and possibly health related problems.

NOTE: Even a correctly sized air conditioner is oversized most of the time. For example, a load calculation may call for a three-ton unit at 95-degree outdoor temperature, however, 97% of the time it is less than 95 degrees outdoors. A practical solution is to slightly undersize the unit but, talk this over with the owner.

Sizing rules for air conditioners (ACCA):

  • Air conditioner – May be sized up to 115% of calculation.
  • Heat pump – May be sized up to 125% of cooling calculation if needed to supply extra heating capacity.

Energy Marketing Service
374 Cattlelot Lane
Belhaven NC

252-943-1028 (M-F, 9am-6pm, Eastern Time)

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