RULE OF THUMB HVAC SIZING DOES NOT WORK because it depends upon a number of factors, all of which must be calculated.

Example 1:

A 1000 square foot home has 8 foot ceilings, three single pane windows, no insulation in the ceilings and very little insulation in the walls. It has one large window directly facing the Sun most of the day.

Another 1000 square foot home has 10 foot ceilings, three triple pane windows, lots and lots of insulation in the ceilings and walls. It has no windows directly facing the Sun.

Both have the same square foot area. Do they both lose heat at the same rate? Do they both need the same size heating system? No & no.

EXAMPLE 1 LOAD CALCULATIONS

Are you going to sell them both the same size system?

I just gave you a few of the variables involved. There are still more, but you get the point, right?

Here is another example:

Example 2:

Two buildings both have 1000 square feet of floor area. Both are identical in construction, have the same windows, doors, insulation, etc...

However, structure #1 is 31.62 feet wide by 31.62 feet long. That gives it its 1000 square feet.

Structure #2 is 10 feet wide by 100 feet long so it also has 1000 square feet of internal area.

But do they both have the same wall surface area for heat to escape from the inside to the outside? Not at all!

Structure #2 has nearly twice the outside wall area as structure #1. The 1000 square foot rectangular shaped structure will lose up to TWICE the heat compared to the square structure. That could mean it will take a furnace twice the size to keep it heated to the same temperature.

EXAMPLE 2 LOAD CALCULATIONS

Estimating heating and cooling equipment sizes by square footage DOES NOT work. Never use rules of thumb to size a system.