Now, on our recent post, How Often Do I Need to Change my HVAC Filter we created a guide for you to figure out for yourself an estimate of how often you should change the filter in your system based on how many people and pets live in your house, etc.

We like to help you, the homeowner, to be able to take care of as much of your machine as you can on your own. In the case of today’s question, however, it is best to refer to a State Certified HVAC contractor to help you find the correct answer for your home. This is because there is only one method approved by the EPA and the Department of Energy for determining the proper size HVAC to install: a residential load calculation, also known as a Manual J survey.

If you Google, “What size air conditioner do I need”, the suggestions will pop up: “How big of an air conditioner do I need for a 3000 square foot house?” “What type of a/c will cool a three bedroom house” and “How many square feet does a 3-ton air conditioner cover?”As many questions as there are out there, there really is no way to answer this online! The size HVAC you need for your house is so specialized, down the direction your house is facing and how many people are in your family. So, you really don’t want to guess on this!

What is a residential load calculation?

A residential load calculation, also known as a Manual J calculation, or even sometimes referred to as a heat loss/heat gain calculation, is the test that determines the heating and cooling load of your home.

It may seem as if all four bedroom houses of about the same size would need the same air conditioner, but this test takes into account a slew of variables in your house, including:

• • Size of the home
• • Number of windows
• • Location of windows
• • Type of windows
• • Climate of the home’s location
• • Type of insulation
• • Modifications to insulation
• • Appliances generating heat in the home
• • Color and type of roof
• • Number of residents in the home

Can’t I just skip this and get the biggest one?

When purchasing a new HVAC (heating, ventilation, and air conditioning) system, as a customer it can be a daunting task. There are so many factors when it comes to what size you will need. A unit that is too small will obviously not be able to keep up with the demands of the home, resulting in higher bills, and a system that will be quickly overloaded. You may think just to add some extra power for good measure, take a guess and get the biggest one. Aside from the unnecessary expense, a bigger HVAC system won’t necessarily result in a more comfortable home. A system that is too large, doesn’t need to run as long to reach the desired temperature, which normally would be great, right? But, in this case, the system doesn’t end up running long enough to remove humidity from the air. So, then you have a house that is cold, but also damp! Not cozy.

What happens if I get the wrong sized HVAC?

In addition to the above concerns, Indoor Air Quality concerns grow in an environment with improper airflow. In stagnant areas, especially with increased humidity, there is increased growth of mold, mildew, and dust mites! Allergies and other health issues are greatly improved as Indoor Air Quality goes up.

Improperly sized units can also cause:

• • Hot and or cold spots in the house due to improper circulation
• • Higher operating costs
• • Increased or decreased humidity

Your Manual J calculation will give you the number of BTU’s needed in an HVAC system to efficiently cool and heat your home. When researching this topic, you may see the term BTU’s used a lot. Let’s define this quickly so you know what is being discussed.

What are BTU’s? 

British Thermal Unit – a unit used to measure energy, in particular, thermal heat.

The amount of energy required to raise one pound of water by 1-degree Fahrenheit.

So, once you know how many BTU’s you need to run your system, you can successfully choose a unit. However many of these BTU’s (how much energy) it takes to heat your house will be your key to what size HVAC system you should purchase. It usually varies from about 30,000 BTU’s for a small house to more than 60,000 for larger homes.

What if my calculation falls in between sizes?

When selecting the proper system for your home, should you fall in between sizes, you will nearly always want to size up. So, if your home requires 34,000 BTU’s and your choices are between a system producing 33,000 and one producing 36,000, choose the one that can produce 36,000. This difference of 2,000 BTU’s will not result in an oversized air conditioning system but going a size down could result in a system that could struggle to meet your home’s requirements.

Think of it this way: on days with extremely high and low temperatures, the little bit of extra power in a slightly larger system will help make sure the system runs smoothly. On the other hand, on those temperature spiking days with a slightly smaller machine, it would be struggling to keep up with the needs of the family.

How much does a Manual J load calculation cost?

Manual J calculations typically range in cost from $250 should you provide the necessary measurements and details to upwards of $1000 depending on the size and complexity of the HVAC system. This may sound costly to find out the correct size air conditioner, but it will pay dividends in energy savings and comfort versus using one of the incorrect size. If you are purchasing a new HVAC system this is often included in the cost. If your contractor does not perform a Manual J audit, you should not proceed with that company.

A word of caution, some utility companies will perform free energy audits but these are not Manual J calculations and rules of thumb should not be relied upon for system sizing. Please feel free to call us anytime should you have any questions!

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