Whether you are building a new home, recently moved into a home that is new to you, or if the HVAC unit in your existing home is on its last legs, you don’t want to spend any more money than you need to, right? 

HVAC units (Heating, ventilation, and air conditioning) systems are a bit complicated for the average homeowner, but they are an extremely important part of keeping your home comfortable. 

If you’ve ever experienced a time where the air conditioning unit was running constantly but the room was still quite warm, you will understand how important it is to have the right-sized HVAC unit. 

Let’s take a look at this complicated process and find out not only how you can determine the correct units you need, but how having a home insulated with spray foam can greatly reduce the size of the unit required. 

The Good News and The Bad News

Let’s get the bad news out of the way first. 

To find the correct size HVAC unit you need, you must determine the load calculations of how much tonnage your home requires so the AC unit won’t be working constantly. This is the same way you calculate the BTU’s you need for heating. 

Load calculations need to meet something called the ASHRAE Manual J specifications (American Society of Heating, Refrigerating, and Air conditioning Engineers). The Manual J is fairly complicated, and it takes into consideration the size, construction, and type and number of windows, etc. so it can give you a precise number for the tonnage and BTU’s your home requires. 

The good news is that you don’t have to be a mathematician to figure this all out. You can purchase software that will make this process extremely easy for you. Some of the best software includes names like Elite Software and Wright Soft. 

What Does Spray Foam Have to Do with This?

Glad you asked! Spray foam is an important calculation when it comes to discovering the size of the HVAC unit you will need. 

Since spray foam is both insulation and an air sealant, it works double duty, providing your home with the best insulation possible. 

The better insulated your home is, the smaller the HVAC unit you will need. This will not only save you money when purchasing the HVAC unit itself but over time, you will save on your heating and cooling costs. 

I’ve actually seen clients who were able to cut the size of the unit they needed much as 90% by using  spray foam insulation to insulate their home. 

The Manual J calculation does not figure in spray foam insulation, but I will be happy to work with you to obtain the correct tonnage/BTU figures your home would require if it were insulated with spray foam. 

Why Does the Size of the HVAC Unit Matter So Much?

You can think of an HVAC unit as a pair of pants. 

You wouldn’t want to walk around every day with a pair of pants that were two sizes too large, would you? You would forever be hitching up your pants, have cold legs in the winter, etc. 

On the other hand, you wouldn’t want to have pants that were two sizes too small, either. 

Your HVAC unit isn’t that much different. 

An HVAC unit that is larger than you need will not only cost more money to buy outright, but it will also use more energy than is needed. Every time the unit turns on, it must heat up the ducts and the furnace itself. It stops quickly because it fulfills its job, but it doesn’t run awfully long. HVAC units are not designed to operate for such short periods, which means the unit isn’t operating as it should. 

An HVAC device that is too small, however, will run frequently, perhaps even constantly, trying to meet the demands of the thermostat setting. This will use too much energy, as well as wear the device out more quickly. 

Most air conditioning sizing results in systems being used that are as much as 50 percent oversized (the same with heating systems, by the way). Contractors by far prefer to have you purchase a unit that is too large, then too small. 

Of course, the Manual J calculations are important, but they should be only part of the overall formula. You also need to consider room sizing, the house layout, the ductwork used and the return ductwork that is needed for proper airflow, and the type of insulation, if any, that the home has. 

Very few contractors or HVAC salespersons will take all these items into consideration.