Wise Energy Efficiency Solutions 

Affordable Home Energy solutions that save you money 

ACCA residential load calculation, manual J


The heating system add heat due to heat loss in the winter and the A/C removes the heat out of the house in the summer due to heat gain.

There is no such a thing as "cooling", anything above -459°F (absolute zero) has heat in it. In order to cool something, the heat must be removed out of it.

Before any home can be fitted with a heating and cooling system, it must first be determined how much heat the building loses in the winter and gains in the summer. The process that calculate heat gain and heat loss is called a load calculation or manual J.

Heat escapes the house in the winter (heat loss) and enters the house in the summer (heat gain). How much heat is lost or gained in impossible to determine without a load calculation. Many factors will affect the heat loss and heat gain, including:

  • Local Climate
  • Orientation of the house
  • Type of construction
  • Windows
  • Insulation
  • Air infiltration
  • Location of the ductwork
  • Number of people
  • equipment, appliance, etc...

There are no benefits to an oversized heating and cooling system.

Just like a 8 cylinder engine use more gasoline than a 6 cylinder engine, a 4 ton compressor uses 30% more energy than a 3 ton compressor (16 Amps VS. 12 Amps). Despite using more energy an oversized heating and cooling system is also making the house uncomfortable.

Your A/C is also a dehumidifier

In the summer, the A/C unit is also a dehumidifier, but in order to removes the humidity out of the house the unit needs to run a minimum of 15 minutes. An oversized unit will cool the house too quickly, satisfying the thermostat before any humidity is removed; the consequence is an uncomfortable house. This can be a major problem when the outside temperature is fairly mild (high 70's-low 80's) and the humidity is fairly high (70-80%).  Never spend money on a dehumidifier before doing a load calculation to size your unit properly. A small whole house dehumidifier uses as much electricity as 2 refrigerators.

How to find out if your A/C is oversized?

The easiest way to find out if your A/C is oversized it to time it when it is running. If your new system runs for less than 15 minutes when the outdoor temperature is 82°F, it is oversized.

We can help if you have humidity problems

If you just installed a new A/C unit and it is oversized, we can help you with your humidity issue by increasing the runtime of your unit without replacing the existing equipment or adding a dehumidifier.

Keeping the humidity in your house around 45-50% in the summer can also help reduce your electric bill by allowing you to increase the temperature inside the house while keeping the same comfort level.

Did you know that a house at 72°F and 60% humidity contains more heat than a house at 75°F and 50% humidity.

Know the heat loss and heat gain before doing any home improvement will save you money

You want to make some improvements in your house to reduce your energy bill but you are not quite sure where to start; should you replace the windows, add insulation, replace your HVAC system or seal your house air tight? Without knowing the house heating and cooling load there is no way to know which improvement will have the best impact on the load and save on the energy bill.

A load calculation measure heat loss and heat gain and help homeowners and contractors identify which home improvement will be most beneficial.

See an example of a load calculation for a house with single pane aluminum windows compare to the same house with vinyl windows below.

Example of a load calculation for a house with single pane aluminum windows. Click on the picture

Compare the Load calculation for the same house using vinyl windows. Click on the picture

In the load calculation example above, replacing the existing single pane aluminum windows with vinyl windows will cut the heat loss and the heat gain by 25%. Without a load calculation there is no way to calculate this.

Benefit to doing a load calculation

A load calculation helps homeowner decide on the best investment when It comes to energy savings; for both new construction and existing home. 

There are a lot of options when it comes to building a high efficient house or making your existing house more efficient; knowing where to best spend your money when its time to make a decision will save a lot of time, headache and money.

The recipe for making a house more efficient or even net zero is simple once you have all the ingredients.

A load calculation also help size the ductwork; the most important part of the HVAC system.

The ductwork is the most misunderstood (and often dismissed) part of a HVAC system and yet it often is the primary reason why homes are uncomfortable. In the summer, air velocity is needed to mix the air and remove the heat. the lack of air velocity at the registers create stale air and it is the primary cause for hot spots and cold spots in a home. 

Ductwork must always be sized based on the room heating and cooling load; to remove BTUs require CFM not FT²; a 200 FT² room full of windows facing south may need 600 CFM while a 400 FT² room with no windows may only need 150 CFM.

Ductwork should NEVER ever be sized based on a room FT²

Do not spend money on a high efficient HVAC unit if you cannot afford a proper duct job; the equipment will never achieve its rated efficiency.

Sizing the ducts without a load calculation is a sure path to bad air flow leading to an uncomfortable house and high energy bills.

There are also unforeseen ductwork cost associated with oversized HVAC system. Larger HVAC units produce more CFM requiring larger ducts (both on the supply side and on the return side); if you are building a 2 story house this is an important factor that must be taken into account; you may need to increase the size of the trusses adding cost to the framing job.

Never replace an existing A/C unit for a new one larger without resizing the ductwork and always make sure that there is enough returns. After all a HVAC is a closed loop system, the blower can only put out what it takes in. Not enough return air affect the efficiency of the system as well as the comfort of the house.

Do it right the first time, ductwork is the most difficult and the most expensive part of the HVAC system to replace (when it can be replaced).

To see the relationship between load calculation and duct size calculation click on the picture below.

Benefits to a properly sized HVAC system

  • Increase comfort
  • reduce cold spot in the house in the winter and hot spot in the summer
  • reduce humidity level inside the home in the summer
  • increase indoor air quality
  • reduce heating and cooling bills
  • last longer (less wear and tear due to short cycling)



contact us, phone

Contact Wise Energy Efficiency Solutions today to order your house load calculation and start saving money.