Manual J load calculation, explained

What a load calculation is

A load is the heat a system has to add or remove to keep a house comfortable. We measure it in BTU per hour. The heating load is the heat the house loses on a very cold night. The cooling load is the heat the house gains on a very hot afternoon. You pick equipment to match those two numbers.

The weather used is called design weather. It is a cold night and a hot afternoon that happen only a few hours a year. We do not size for the worst day ever. We size for a hard day that is still normal.

What goes into it

• The size of the house: floor area, ceiling height and stories.
• The weather where the house stands.
• The walls, the ceiling and the floor, and how well each one holds heat back.
• The windows: how big, which way they face, what glass they have and how much shade they get.
• Air leaks, and fresh air from a fan.
• The ducts: where they run, how well they are insulated and how much they leak.
• The people and the appliances inside.

How heat gets in and out

Heat moves through walls, the roof and the floor from the warm side to the cool side. How fast depends on the U-value. A low U-value means good insulation. U-value is one divided by R-value.

In summer the sun adds more. It warms the outside of the walls and roof, and it shines through the windows. A west window in the late afternoon can add a lot of heat. That is why the tool checks the sun hour by hour.

Ducts in a hot attic gain heat, and their leaks pull in hot air. People give off heat and moisture. Ovens, lights and TVs add heat too.

Sensible and latent heat

Sensible heat is what a thermometer feels. Latent heat is the moisture in the air. In summer an air conditioner must cool the air and also dry it. Air leaks and people bring in moisture. The cooling load is the sensible load plus the latent load.

One ton of cooling is 12,000 BTU/h, so the tons you need are the cooling load divided by 12,000.

Why a unit that is too big causes trouble

A big air conditioner cools the air very fast and then shuts off. The coil does not run long enough to pull out the moisture. The house feels cold and clammy. The unit starts and stops often, which wears it out and wastes power. A big furnace does the same with heat. A unit that is too small cannot hold the temperature on the hardest day. The goal is a close match.

How to read the result

The two big numbers are the heating load and the cooling load in BTU per hour. The bars show where each one comes from, so you can see what to fix first. Big bars for ducts or air leaks often mean sealing will help more than a bigger unit. Big bars for windows and sun mean shade or better glass will help.

The supply air figures tell you how much air the system must move to carry the load. Use them with the ductulator to size the ducts.

Common mistakes

• Sizing from square feet alone, with no weather or sun.
• Adding a safety margin on top of a margin that is already built in.
• Guessing the insulation. When you cannot see it, use the house age to fill in typical values, then check what you can.
• Forgetting the ducts. Ducts in a hot attic can add a lot to the load.

What a full Manual J does that this page does not

A full Manual J is a method published by ACCA, the Air Conditioning Contractors of America. It goes room by room. It measures every wall, window and door, uses the exact glass data, and counts the shade from each overhang. Its results feed duct design and equipment choice.

This page does a whole-house estimate in the same style. It does not split the house into rooms, and it does not take every detail of the glass. It is a fast check, a first number. The iPhone app adds a split by room, an equipment check and a report. A certified room by room design is still the final word. Check your local code for what it asks for. To learn more about the method, visit acca.org.