System Sizing

Sizing your system

Deciding on what type of heating and/or cooling system is right for you is only a portion of your decision making process, i.e. furnace vs. boiler or ductless split system vs. central air. The other important part of installing one of these systems is something called system sizing. In order to best explain how to size your system, we have included a few frequently asked questions about system sizing.

What is system sizing?

Simply put, it is the process of calculating the size your heating and cooling system unit(s) should be to efficiently keep your home temperature comfortable year-round.

Is it really a problem if I get a system that is too big?

Professional System Sizing Contractor
Installing the wrong size system will cost you more money every month.

Yes. An over-sized system will heat or cool the home quickly, but then shut off. This is often referred to as short cycling. This causes a few problems. 1) The moisture in your home will not be removed effectively. 2) The stress of too many starts and stops will shorten the life of your equipment. 3) The constant starting and stopping requires more energy, therefore, you will have higher monthly bills. 4) You will never be as comfortable as you could be in your own home.

What happens if my system is too small?

Most systems today are designed to heat and cool your home gradually, so if you system is too small, it runs the risk of running all day to try to catch up with where the thermostat says it should be. This will cost you more money each month.

How do I calculate the size my system should be?

Having a professional complete a load calculation is our strongest recommendation. This will be a more detailed room by room heat loss/heat gain analysis. Our professionals use the industry standard of using Manual J to make our calculations.

There is a do-it-yourself way that can help you calculate a ballpark figure so you can know what to expect.

((House square footage times 30, divided by 12,000) – 1.0) = Required Tonnage.

So for an average 1,500 sq. ft. American home in an average climate the equation would look like this:

  1. 1,500 X 30 = 45,000
  2. 45,000 / 12,000 = 3.75
  3. 3.75 – 1.0 = 2.75   So you’d need a 2.5 or a 3-ton sized central air conditioning unit.

But we have included a list of items that need to be considered before making a more accurate estimate.

  • Your areas climate
  • Number of windows and type of window treatments
  • Insulation and ductwork
  • Building materials and roof construction
  • Skylights and fireplaces
  • Ceiling heights
  • The number of people in your home
  • In existing homes, any additions, or modifications to a basement or attic since the home was first sized could change the size unit needed


To properly calculate what size system is right for your home there are many variables to consider. We strongly recommend that you have a trained professional perform a complete load calculation before making the final decision on a heating or cooling system.

Realize you need your system sized by a qualified contractor? Call or contact us to set up an appointment today!